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1 Introduction

BFD is a package which allows applications to use the same routines to operate on object files whatever the object file format. A new object file format can be supported simply by creating a new BFD back end and adding it to the library.

BFD is split into two parts: the front end, and the back ends (one for each object file format).

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1.1 History

One spur behind BFD was the desire, on the part of the GNU 960 team at Intel Oregon, for interoperability of applications on their COFF and b.out file formats. Cygnus was providing GNU support for the team, and was contracted to provide the required functionality.

The name came from a conversation David Wallace was having with Richard Stallman about the library: RMS said that it would be quite hard—David said “BFD”. Stallman was right, but the name stuck.

At the same time, Ready Systems wanted much the same thing, but for different object file formats: IEEE-695, Oasys, Srecords, a.out and 68k coff.

BFD was first implemented by members of Cygnus Support; Steve Chamberlain (, John Gilmore (, K. Richard Pixley ( and David Henkel-Wallace (

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1.2 How To Use BFD

To use the library, include bfd.h and link with libbfd.a.

BFD provides a common interface to the parts of an object file for a calling application.

When an application sucessfully opens a target file (object, archive, or whatever), a pointer to an internal structure is returned. This pointer points to a structure called bfd, described in bfd.h. Our convention is to call this pointer a BFD, and instances of it within code abfd. All operations on the target object file are applied as methods to the BFD. The mapping is defined within bfd.h in a set of macros, all beginning with `bfd_' to reduce namespace pollution.

For example, this sequence does what you would probably expect: return the number of sections in an object file attached to a BFD abfd.

     #include "bfd.h"
     unsigned int number_of_sections (abfd)
     bfd *abfd;
       return bfd_count_sections (abfd);

The abstraction used within BFD is that an object file has:

Also, BFDs opened for archives have the additional attribute of an index and contain subordinate BFDs. This approach is fine for a.out and coff, but loses efficiency when applied to formats such as S-records and IEEE-695.

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1.3 What BFD Version 2 Can Do

When an object file is opened, BFD subroutines automatically determine the format of the input object file. They then build a descriptor in memory with pointers to routines that will be used to access elements of the object file's data structures.

As different information from the object files is required, BFD reads from different sections of the file and processes them. For example, a very common operation for the linker is processing symbol tables. Each BFD back end provides a routine for converting between the object file's representation of symbols and an internal canonical format. When the linker asks for the symbol table of an object file, it calls through a memory pointer to the routine from the relevant BFD back end which reads and converts the table into a canonical form. The linker then operates upon the canonical form. When the link is finished and the linker writes the output file's symbol table, another BFD back end routine is called to take the newly created symbol table and convert it into the chosen output format.

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1.3.1 Information Loss

Information can be lost during output. The output formats supported by BFD do not provide identical facilities, and information which can be described in one form has nowhere to go in another format. One example of this is alignment information in b.out. There is nowhere in an a.out format file to store alignment information on the contained data, so when a file is linked from b.out and an a.out image is produced, alignment information will not propagate to the output file. (The linker will still use the alignment information internally, so the link is performed correctly).

Another example is COFF section names. COFF files may contain an unlimited number of sections, each one with a textual section name. If the target of the link is a format which does not have many sections (e.g., a.out) or has sections without names (e.g., the Oasys format), the link cannot be done simply. You can circumvent this problem by describing the desired input-to-output section mapping with the linker command language.

Information can be lost during canonicalization. The BFD internal canonical form of the external formats is not exhaustive; there are structures in input formats for which there is no direct representation internally. This means that the BFD back ends cannot maintain all possible data richness through the transformation between external to internal and back to external formats.

This limitation is only a problem when an application reads one format and writes another. Each BFD back end is responsible for maintaining as much data as possible, and the internal BFD canonical form has structures which are opaque to the BFD core, and exported only to the back ends. When a file is read in one format, the canonical form is generated for BFD and the application. At the same time, the back end saves away any information which may otherwise be lost. If the data is then written back in the same format, the back end routine will be able to use the canonical form provided by the BFD core as well as the information it prepared earlier. Since there is a great deal of commonality between back ends, there is no information lost when linking or copying big endian COFF to little endian COFF, or a.out to b.out. When a mixture of formats is linked, the information is only lost from the files whose format differs from the destination.

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1.3.2 The BFD canonical object-file format

The greatest potential for loss of information occurs when there is the least overlap between the information provided by the source format, that stored by the canonical format, and that needed by the destination format. A brief description of the canonical form may help you understand which kinds of data you can count on preserving across conversions.

Information stored on a per-file basis includes target machine architecture, particular implementation format type, a demand pageable bit, and a write protected bit. Information like Unix magic numbers is not stored here—only the magic numbers' meaning, so a ZMAGIC file would have both the demand pageable bit and the write protected text bit set. The byte order of the target is stored on a per-file basis, so that big- and little-endian object files may be used with one another.
Each section in the input file contains the name of the section, the section's original address in the object file, size and alignment information, various flags, and pointers into other BFD data structures.
Each symbol contains a pointer to the information for the object file which originally defined it, its name, its value, and various flag bits. When a BFD back end reads in a symbol table, it relocates all symbols to make them relative to the base of the section where they were defined. Doing this ensures that each symbol points to its containing section. Each symbol also has a varying amount of hidden private data for the BFD back end. Since the symbol points to the original file, the private data format for that symbol is accessible. ld can operate on a collection of symbols of wildly different formats without problems.

Normal global and simple local symbols are maintained on output, so an output file (no matter its format) will retain symbols pointing to functions and to global, static, and common variables. Some symbol information is not worth retaining; in a.out, type information is stored in the symbol table as long symbol names. This information would be useless to most COFF debuggers; the linker has command line switches to allow users to throw it away.

There is one word of type information within the symbol, so if the format supports symbol type information within symbols (for example, COFF, IEEE, Oasys) and the type is simple enough to fit within one word (nearly everything but aggregates), the information will be preserved.

relocation level
Each canonical BFD relocation record contains a pointer to the symbol to relocate to, the offset of the data to relocate, the section the data is in, and a pointer to a relocation type descriptor. Relocation is performed by passing messages through the relocation type descriptor and the symbol pointer. Therefore, relocations can be performed on output data using a relocation method that is only available in one of the input formats. For instance, Oasys provides a byte relocation format. A relocation record requesting this relocation type would point indirectly to a routine to perform this, so the relocation may be performed on a byte being written to a 68k COFF file, even though 68k COFF has no such relocation type.
line numbers
Object formats can contain, for debugging purposes, some form of mapping between symbols, source line numbers, and addresses in the output file. These addresses have to be relocated along with the symbol information. Each symbol with an associated list of line number records points to the first record of the list. The head of a line number list consists of a pointer to the symbol, which allows finding out the address of the function whose line number is being described. The rest of the list is made up of pairs: offsets into the section and line numbers. Any format which can simply derive this information can pass it successfully between formats (COFF, IEEE and Oasys).

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2 BFD Front End

2.1 typedef bfd

A BFD has type bfd; objects of this type are the cornerstone of any application using BFD. Using BFD consists of making references though the BFD and to data in the BFD.

Here is the structure that defines the type bfd. It contains the major data about the file and pointers to the rest of the data.

     struct bfd
       /* A unique identifier of the BFD  */
       unsigned int id;
       /* The filename the application opened the BFD with.  */
       const char *filename;
       /* A pointer to the target jump table.  */
       const struct bfd_target *xvec;
       /* The IOSTREAM, and corresponding IO vector that provide access
          to the file backing the BFD.  */
       void *iostream;
       const struct bfd_iovec *iovec;
       /* Is the file descriptor being cached?  That is, can it be closed as
          needed, and re-opened when accessed later?  */
       bfd_boolean cacheable;
       /* Marks whether there was a default target specified when the
          BFD was opened. This is used to select which matching algorithm
          to use to choose the back end.  */
       bfd_boolean target_defaulted;
       /* The caching routines use these to maintain a
          least-recently-used list of BFDs.  */
       struct bfd *lru_prev, *lru_next;
       /* When a file is closed by the caching routines, BFD retains
          state information on the file here...  */
       ufile_ptr where;
       /* ... and here: (``once'' means at least once).  */
       bfd_boolean opened_once;
       /* Set if we have a locally maintained mtime value, rather than
          getting it from the file each time.  */
       bfd_boolean mtime_set;
       /* File modified time, if mtime_set is TRUE.  */
       long mtime;
       /* Reserved for an unimplemented file locking extension.  */
       int ifd;
       /* The format which belongs to the BFD. (object, core, etc.)  */
       bfd_format format;
       /* The direction with which the BFD was opened.  */
       enum bfd_direction
           no_direction = 0,
           read_direction = 1,
           write_direction = 2,
           both_direction = 3
       /* Format_specific flags.  */
       flagword flags;
       /* Currently my_archive is tested before adding origin to
          anything. I believe that this can become always an add of
          origin, with origin set to 0 for non archive files.  */
       ufile_ptr origin;
       /* Remember when output has begun, to stop strange things
          from happening.  */
       bfd_boolean output_has_begun;
       /* A hash table for section names.  */
       struct bfd_hash_table section_htab;
       /* Pointer to linked list of sections.  */
       struct bfd_section *sections;
       /* The last section on the section list.  */
       struct bfd_section *section_last;
       /* The number of sections.  */
       unsigned int section_count;
       /* Stuff only useful for object files:
          The start address.  */
       bfd_vma start_address;
       /* Used for input and output.  */
       unsigned int symcount;
       /* Symbol table for output BFD (with symcount entries).  */
       struct bfd_symbol  **outsymbols;
       /* Used for slurped dynamic symbol tables.  */
       unsigned int dynsymcount;
       /* Pointer to structure which contains architecture information.  */
       const struct bfd_arch_info *arch_info;
       /* Flag set if symbols from this BFD should not be exported.  */
       bfd_boolean no_export;
       /* Stuff only useful for archives.  */
       void *arelt_data;
       struct bfd *my_archive;      /* The containing archive BFD.  */
       struct bfd *next;            /* The next BFD in the archive.  */
       struct bfd *archive_head;    /* The first BFD in the archive.  */
       bfd_boolean has_armap;
       /* A chain of BFD structures involved in a link.  */
       struct bfd *link_next;
       /* A field used by _bfd_generic_link_add_archive_symbols.  This will
          be used only for archive elements.  */
       int archive_pass;
       /* Used by the back end to hold private data.  */
           struct aout_data_struct *aout_data;
           struct artdata *aout_ar_data;
           struct _oasys_data *oasys_obj_data;
           struct _oasys_ar_data *oasys_ar_data;
           struct coff_tdata *coff_obj_data;
           struct pe_tdata *pe_obj_data;
           struct xcoff_tdata *xcoff_obj_data;
           struct ecoff_tdata *ecoff_obj_data;
           struct ieee_data_struct *ieee_data;
           struct ieee_ar_data_struct *ieee_ar_data;
           struct srec_data_struct *srec_data;
           struct ihex_data_struct *ihex_data;
           struct tekhex_data_struct *tekhex_data;
           struct elf_obj_tdata *elf_obj_data;
           struct nlm_obj_tdata *nlm_obj_data;
           struct bout_data_struct *bout_data;
           struct mmo_data_struct *mmo_data;
           struct sun_core_struct *sun_core_data;
           struct sco5_core_struct *sco5_core_data;
           struct trad_core_struct *trad_core_data;
           struct som_data_struct *som_data;
           struct hpux_core_struct *hpux_core_data;
           struct hppabsd_core_struct *hppabsd_core_data;
           struct sgi_core_struct *sgi_core_data;
           struct lynx_core_struct *lynx_core_data;
           struct osf_core_struct *osf_core_data;
           struct cisco_core_struct *cisco_core_data;
           struct versados_data_struct *versados_data;
           struct netbsd_core_struct *netbsd_core_data;
           struct mach_o_data_struct *mach_o_data;
           struct mach_o_fat_data_struct *mach_o_fat_data;
           struct bfd_pef_data_struct *pef_data;
           struct bfd_pef_xlib_data_struct *pef_xlib_data;
           struct bfd_sym_data_struct *sym_data;
           void *any;
       /* Used by the application to hold private data.  */
       void *usrdata;
       /* Where all the allocated stuff under this BFD goes.  This is a
          struct objalloc *, but we use void * to avoid requiring the inclusion
          of objalloc.h.  */
       void *memory;

2.2 Error reporting

Most BFD functions return nonzero on success (check their individual documentation for precise semantics). On an error, they call bfd_set_error to set an error condition that callers can check by calling bfd_get_error. If that returns bfd_error_system_call, then check errno.

The easiest way to report a BFD error to the user is to use bfd_perror.

2.2.1 Type bfd_error_type

The values returned by bfd_get_error are defined by the enumerated type bfd_error_type.

     typedef enum bfd_error
       bfd_error_no_error = 0,
     bfd_error_type; bfd_get_error


     bfd_error_type bfd_get_error (void);

Return the current BFD error condition. bfd_set_error


     void bfd_set_error (bfd_error_type error_tag);

Set the BFD error condition to be error_tag. bfd_errmsg


     const char *bfd_errmsg (bfd_error_type error_tag);

Return a string describing the error error_tag, or the system error if error_tag is bfd_error_system_call. bfd_perror


     void bfd_perror (const char *message);

Print to the standard error stream a string describing the last BFD error that occurred, or the last system error if the last BFD error was a system call failure. If message is non-NULL and non-empty, the error string printed is preceded by message, a colon, and a space. It is followed by a newline.

2.2.2 BFD error handler

Some BFD functions want to print messages describing the problem. They call a BFD error handler function. This function may be overridden by the program.

The BFD error handler acts like printf.

     typedef void (*bfd_error_handler_type) (const char *, ...); bfd_set_error_handler


     bfd_error_handler_type bfd_set_error_handler (bfd_error_handler_type);

Set the BFD error handler function. Returns the previous function. bfd_set_error_program_name


     void bfd_set_error_program_name (const char *);

Set the program name to use when printing a BFD error. This is printed before the error message followed by a colon and space. The string must not be changed after it is passed to this function. bfd_get_error_handler


     bfd_error_handler_type bfd_get_error_handler (void);

Return the BFD error handler function.

2.3 Symbols bfd_get_reloc_upper_bound


     long bfd_get_reloc_upper_bound (bfd *abfd, asection *sect);

Return the number of bytes required to store the relocation information associated with section sect attached to bfd abfd. If an error occurs, return -1. bfd_canonicalize_reloc


     long bfd_canonicalize_reloc
        (bfd *abfd, asection *sec, arelent **loc, asymbol **syms);

Call the back end associated with the open BFD abfd and translate the external form of the relocation information attached to sec into the internal canonical form. Place the table into memory at loc, which has been preallocated, usually by a call to bfd_get_reloc_upper_bound. Returns the number of relocs, or -1 on error.

The syms table is also needed for horrible internal magic reasons. bfd_set_reloc


     void bfd_set_reloc
        (bfd *abfd, asection *sec, arelent **rel, unsigned int count);

Set the relocation pointer and count within section sec to the values rel and count. The argument abfd is ignored. bfd_set_file_flags


     bfd_boolean bfd_set_file_flags (bfd *abfd, flagword flags);

Set the flag word in the BFD abfd to the value flags.

Possible errors are: bfd_get_arch_size


     int bfd_get_arch_size (bfd *abfd);

Returns the architecture address size, in bits, as determined by the object file's format. For ELF, this information is included in the header.

Returns the arch size in bits if known, -1 otherwise. bfd_get_sign_extend_vma


     int bfd_get_sign_extend_vma (bfd *abfd);

Indicates if the target architecture "naturally" sign extends an address. Some architectures implicitly sign extend address values when they are converted to types larger than the size of an address. For instance, bfd_get_start_address() will return an address sign extended to fill a bfd_vma when this is the case.

Returns 1 if the target architecture is known to sign extend addresses, 0 if the target architecture is known to not sign extend addresses, and -1 otherwise. bfd_set_start_address


     bfd_boolean bfd_set_start_address (bfd *abfd, bfd_vma vma);

Make vma the entry point of output BFD abfd.

Returns TRUE on success, FALSE otherwise. bfd_get_gp_size


     unsigned int bfd_get_gp_size (bfd *abfd);

Return the maximum size of objects to be optimized using the GP register under MIPS ECOFF. This is typically set by the -G argument to the compiler, assembler or linker. bfd_set_gp_size


     void bfd_set_gp_size (bfd *abfd, unsigned int i);

Set the maximum size of objects to be optimized using the GP register under ECOFF or MIPS ELF. This is typically set by the -G argument to the compiler, assembler or linker. bfd_scan_vma


     bfd_vma bfd_scan_vma (const char *string, const char **end, int base);

Convert, like strtoul, a numerical expression string into a bfd_vma integer, and return that integer. (Though without as many bells and whistles as strtoul.) The expression is assumed to be unsigned (i.e., positive). If given a base, it is used as the base for conversion. A base of 0 causes the function to interpret the string in hex if a leading "0x" or "0X" is found, otherwise in octal if a leading zero is found, otherwise in decimal.

If the value would overflow, the maximum bfd_vma value is returned. bfd_copy_private_header_data


     bfd_boolean bfd_copy_private_header_data (bfd *ibfd, bfd *obfd);

Copy private BFD header information from the BFD ibfd to the the BFD obfd. This copies information that may require sections to exist, but does not require symbol tables. Return true on success, false on error. Possible error returns are:

     #define bfd_copy_private_header_data(ibfd, obfd) \
          BFD_SEND (obfd, _bfd_copy_private_header_data, \
                    (ibfd, obfd)) bfd_copy_private_bfd_data


     bfd_boolean bfd_copy_private_bfd_data (bfd *ibfd, bfd *obfd);

Copy private BFD information from the BFD ibfd to the the BFD obfd. Return TRUE on success, FALSE on error. Possible error returns are:

     #define bfd_copy_private_bfd_data(ibfd, obfd) \
          BFD_SEND (obfd, _bfd_copy_private_bfd_data, \
                    (ibfd, obfd)) bfd_merge_private_bfd_data


     bfd_boolean bfd_merge_private_bfd_data (bfd *ibfd, bfd *obfd);

Merge private BFD information from the BFD ibfd to the the output file BFD obfd when linking. Return TRUE on success, FALSE on error. Possible error returns are:

     #define bfd_merge_private_bfd_data(ibfd, obfd) \
          BFD_SEND (obfd, _bfd_merge_private_bfd_data, \
                    (ibfd, obfd)) bfd_set_private_flags


     bfd_boolean bfd_set_private_flags (bfd *abfd, flagword flags);

Set private BFD flag information in the BFD abfd. Return TRUE on success, FALSE on error. Possible error returns are:

     #define bfd_set_private_flags(abfd, flags) \
          BFD_SEND (abfd, _bfd_set_private_flags, (abfd, flags)) Other functions

The following functions exist but have not yet been documented.

     #define bfd_sizeof_headers(abfd, reloc) \
            BFD_SEND (abfd, _bfd_sizeof_headers, (abfd, reloc))
     #define bfd_find_nearest_line(abfd, sec, syms, off, file, func, line) \
            BFD_SEND (abfd, _bfd_find_nearest_line, \
                      (abfd, sec, syms, off, file, func, line))
     #define bfd_find_line(abfd, syms, sym, file, line) \
            BFD_SEND (abfd, _bfd_find_line, \
                      (abfd, syms, sym, file, line))
     #define bfd_find_inliner_info(abfd, file, func, line) \
            BFD_SEND (abfd, _bfd_find_inliner_info, \
                      (abfd, file, func, line))
     #define bfd_debug_info_start(abfd) \
            BFD_SEND (abfd, _bfd_debug_info_start, (abfd))
     #define bfd_debug_info_end(abfd) \
            BFD_SEND (abfd, _bfd_debug_info_end, (abfd))
     #define bfd_debug_info_accumulate(abfd, section) \
            BFD_SEND (abfd, _bfd_debug_info_accumulate, (abfd, section))
     #define bfd_stat_arch_elt(abfd, stat) \
            BFD_SEND (abfd, _bfd_stat_arch_elt,(abfd, stat))
     #define bfd_update_armap_timestamp(abfd) \
            BFD_SEND (abfd, _bfd_update_armap_timestamp, (abfd))
     #define bfd_set_arch_mach(abfd, arch, mach)\
            BFD_SEND ( abfd, _bfd_set_arch_mach, (abfd, arch, mach))
     #define bfd_relax_section(abfd, section, link_info, again) \
            BFD_SEND (abfd, _bfd_relax_section, (abfd, section, link_info, again))
     #define bfd_gc_sections(abfd, link_info) \
            BFD_SEND (abfd, _bfd_gc_sections, (abfd, link_info))
     #define bfd_merge_sections(abfd, link_info) \
            BFD_SEND (abfd, _bfd_merge_sections, (abfd, link_info))
     #define bfd_is_group_section(abfd, sec) \
            BFD_SEND (abfd, _bfd_is_group_section, (abfd, sec))
     #define bfd_discard_group(abfd, sec) \
            BFD_SEND (abfd, _bfd_discard_group, (abfd, sec))
     #define bfd_link_hash_table_create(abfd) \
            BFD_SEND (abfd, _bfd_link_hash_table_create, (abfd))
     #define bfd_link_hash_table_free(abfd, hash) \
            BFD_SEND (abfd, _bfd_link_hash_table_free, (hash))
     #define bfd_link_add_symbols(abfd, info) \
            BFD_SEND (abfd, _bfd_link_add_symbols, (abfd, info))
     #define bfd_link_just_syms(abfd, sec, info) \
            BFD_SEND (abfd, _bfd_link_just_syms, (sec, info))
     #define bfd_final_link(abfd, info) \
            BFD_SEND (abfd, _bfd_final_link, (abfd, info))
     #define bfd_free_cached_info(abfd) \
            BFD_SEND (abfd, _bfd_free_cached_info, (abfd))
     #define bfd_get_dynamic_symtab_upper_bound(abfd) \
            BFD_SEND (abfd, _bfd_get_dynamic_symtab_upper_bound, (abfd))
     #define bfd_print_private_bfd_data(abfd, file)\
            BFD_SEND (abfd, _bfd_print_private_bfd_data, (abfd, file))
     #define bfd_canonicalize_dynamic_symtab(abfd, asymbols) \
            BFD_SEND (abfd, _bfd_canonicalize_dynamic_symtab, (abfd, asymbols))
     #define bfd_get_synthetic_symtab(abfd, count, syms, dyncount, dynsyms, ret) \
            BFD_SEND (abfd, _bfd_get_synthetic_symtab, (abfd, count, syms, \
                                                        dyncount, dynsyms, ret))
     #define bfd_get_dynamic_reloc_upper_bound(abfd) \
            BFD_SEND (abfd, _bfd_get_dynamic_reloc_upper_bound, (abfd))
     #define bfd_canonicalize_dynamic_reloc(abfd, arels, asyms) \
            BFD_SEND (abfd, _bfd_canonicalize_dynamic_reloc, (abfd, arels, asyms))
     extern bfd_byte *bfd_get_relocated_section_contents
       (bfd *, struct bfd_link_info *, struct bfd_link_order *, bfd_byte *,
        bfd_boolean, asymbol **); bfd_alt_mach_code


     bfd_boolean bfd_alt_mach_code (bfd *abfd, int alternative);

When more than one machine code number is available for the same machine type, this function can be used to switch between the preferred one (alternative == 0) and any others. Currently, only ELF supports this feature, with up to two alternate machine codes.

     struct bfd_preserve
       void *marker;
       void *tdata;
       flagword flags;
       const struct bfd_arch_info *arch_info;
       struct bfd_section *sections;
       struct bfd_section *section_last;
       unsigned int section_count;
       struct bfd_hash_table section_htab;
     }; bfd_preserve_save


     bfd_boolean bfd_preserve_save (bfd *, struct bfd_preserve *);

When testing an object for compatibility with a particular target back-end, the back-end object_p function needs to set up certain fields in the bfd on successfully recognizing the object. This typically happens in a piecemeal fashion, with failures possible at many points. On failure, the bfd is supposed to be restored to its initial state, which is virtually impossible. However, restoring a subset of the bfd state works in practice. This function stores the subset and reinitializes the bfd. bfd_preserve_restore


     void bfd_preserve_restore (bfd *, struct bfd_preserve *);

This function restores bfd state saved by bfd_preserve_save. If MARKER is non-NULL in struct bfd_preserve then that block and all subsequently bfd_alloc'd memory is freed. bfd_preserve_finish


     void bfd_preserve_finish (bfd *, struct bfd_preserve *);

This function should be called when the bfd state saved by bfd_preserve_save is no longer needed. ie. when the back-end object_p function returns with success. struct bfd_iovec

The struct bfd_iovec contains the internal file I/O class. Each BFD has an instance of this class and all file I/O is routed through it (it is assumed that the instance implements all methods listed below).

     struct bfd_iovec
       /* To avoid problems with macros, a "b" rather than "f"
          prefix is prepended to each method name.  */
       /* Attempt to read/write NBYTES on ABFD's IOSTREAM storing/fetching
          bytes starting at PTR.  Return the number of bytes actually
          transfered (a read past end-of-file returns less than NBYTES),
          or -1 (setting bfd_error) if an error occurs.  */
       file_ptr (*bread) (struct bfd *abfd, void *ptr, file_ptr nbytes);
       file_ptr (*bwrite) (struct bfd *abfd, const void *ptr,
                           file_ptr nbytes);
       /* Return the current IOSTREAM file offset, or -1 (setting bfd_error
          if an error occurs.  */
       file_ptr (*btell) (struct bfd *abfd);
       /* For the following, on successful completion a value of 0 is returned.
          Otherwise, a value of -1 is returned (and  bfd_error is set).  */
       int (*bseek) (struct bfd *abfd, file_ptr offset, int whence);
       int (*bclose) (struct bfd *abfd);
       int (*bflush) (struct bfd *abfd);
       int (*bstat) (struct bfd *abfd, struct stat *sb);
     }; bfd_get_mtime


     long bfd_get_mtime (bfd *abfd);

Return the file modification time (as read from the file system, or from the archive header for archive members). bfd_get_size


     long bfd_get_size (bfd *abfd);

Return the file size (as read from file system) for the file associated with BFD abfd.

The initial motivation for, and use of, this routine is not so we can get the exact size of the object the BFD applies to, since that might not be generally possible (archive members for example). It would be ideal if someone could eventually modify it so that such results were guaranteed.

Instead, we want to ask questions like "is this NNN byte sized object I'm about to try read from file offset YYY reasonable?" As as example of where we might do this, some object formats use string tables for which the first sizeof (long) bytes of the table contain the size of the table itself, including the size bytes. If an application tries to read what it thinks is one of these string tables, without some way to validate the size, and for some reason the size is wrong (byte swapping error, wrong location for the string table, etc.), the only clue is likely to be a read error when it tries to read the table, or a "virtual memory exhausted" error when it tries to allocate 15 bazillon bytes of space for the 15 bazillon byte table it is about to read. This function at least allows us to answer the question, "is the size reasonable?".

Next: , Previous: BFD front end, Up: BFD front end

2.4 Memory Usage

BFD keeps all of its internal structures in obstacks. There is one obstack per open BFD file, into which the current state is stored. When a BFD is closed, the obstack is deleted, and so everything which has been allocated by BFD for the closing file is thrown away.

BFD does not free anything created by an application, but pointers into bfd structures become invalid on a bfd_close; for example, after a bfd_close the vector passed to bfd_canonicalize_symtab is still around, since it has been allocated by the application, but the data that it pointed to are lost.

The general rule is to not close a BFD until all operations dependent upon data from the BFD have been completed, or all the data from within the file has been copied. To help with the management of memory, there is a function (bfd_alloc_size) which returns the number of bytes in obstacks associated with the supplied BFD. This could be used to select the greediest open BFD, close it to reclaim the memory, perform some operation and reopen the BFD again, to get a fresh copy of the data structures.

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2.5 Initialization

These are the functions that handle initializing a BFD. bfd_init


     void bfd_init (void);

This routine must be called before any other BFD function to initialize magical internal data structures.

Next: , Previous: Initialization, Up: BFD front end

2.6 Sections

The raw data contained within a BFD is maintained through the section abstraction. A single BFD may have any number of sections. It keeps hold of them by pointing to the first; each one points to the next in the list.

Sections are supported in BFD in section.c.

Next: , Previous: Sections, Up: Sections

2.6.1 Section input

When a BFD is opened for reading, the section structures are created and attached to the BFD.

Each section has a name which describes the section in the outside world—for example, a.out would contain at least three sections, called .text, .data and .bss.

Names need not be unique; for example a COFF file may have several sections named .data.

Sometimes a BFD will contain more than the “natural” number of sections. A back end may attach other sections containing constructor data, or an application may add a section (using bfd_make_section) to the sections attached to an already open BFD. For example, the linker creates an extra section COMMON for each input file's BFD to hold information about common storage.

The raw data is not necessarily read in when the section descriptor is created. Some targets may leave the data in place until a bfd_get_section_contents call is made. Other back ends may read in all the data at once. For example, an S-record file has to be read once to determine the size of the data. An IEEE-695 file doesn't contain raw data in sections, but data and relocation expressions intermixed, so the data area has to be parsed to get out the data and relocations.

Next: , Previous: Section Input, Up: Sections

2.6.2 Section output

To write a new object style BFD, the various sections to be written have to be created. They are attached to the BFD in the same way as input sections; data is written to the sections using bfd_set_section_contents.

Any program that creates or combines sections (e.g., the assembler and linker) must use the asection fields output_section and output_offset to indicate the file sections to which each section must be written. (If the section is being created from scratch, output_section should probably point to the section itself and output_offset should probably be zero.)

The data to be written comes from input sections attached (via output_section pointers) to the output sections. The output section structure can be considered a filter for the input section: the output section determines the vma of the output data and the name, but the input section determines the offset into the output section of the data to be written.

E.g., to create a section "O", starting at 0x100, 0x123 long, containing two subsections, "A" at offset 0x0 (i.e., at vma 0x100) and "B" at offset 0x20 (i.e., at vma 0x120) the asection structures would look like:

        section name          "A"
          output_offset   0x00
          size            0x20
          output_section ----------->  section name    "O"
                                  |    vma             0x100
        section name          "B" |    size            0x123
          output_offset   0x20    |
          size            0x103   |
          output_section  --------|

2.6.3 Link orders

The data within a section is stored in a link_order. These are much like the fixups in gas. The link_order abstraction allows a section to grow and shrink within itself.

A link_order knows how big it is, and which is the next link_order and where the raw data for it is; it also points to a list of relocations which apply to it.

The link_order is used by the linker to perform relaxing on final code. The compiler creates code which is as big as necessary to make it work without relaxing, and the user can select whether to relax. Sometimes relaxing takes a lot of time. The linker runs around the relocations to see if any are attached to data which can be shrunk, if so it does it on a link_order by link_order basis.

Next: , Previous: Section Output, Up: Sections

2.6.4 typedef asection

Here is the section structure:

     typedef struct bfd_section
       /* The name of the section; the name isn't a copy, the pointer is
          the same as that passed to bfd_make_section.  */
       const char *name;
       /* A unique sequence number.  */
       int id;
       /* Which section in the bfd; 0..n-1 as sections are created in a bfd.  */
       int index;
       /* The next section in the list belonging to the BFD, or NULL.  */
       struct bfd_section *next;
       /* The previous section in the list belonging to the BFD, or NULL.  */
       struct bfd_section *prev;
       /* The field flags contains attributes of the section. Some
          flags are read in from the object file, and some are
          synthesized from other information.  */
       flagword flags;
     #define SEC_NO_FLAGS   0x000
       /* Tells the OS to allocate space for this section when loading.
          This is clear for a section containing debug information only.  */
     #define SEC_ALLOC      0x001
       /* Tells the OS to load the section from the file when loading.
          This is clear for a .bss section.  */
     #define SEC_LOAD       0x002
       /* The section contains data still to be relocated, so there is
          some relocation information too.  */
     #define SEC_RELOC      0x004
       /* A signal to the OS that the section contains read only data.  */
     #define SEC_READONLY   0x008
       /* The section contains code only.  */
     #define SEC_CODE       0x010
       /* The section contains data only.  */
     #define SEC_DATA       0x020
       /* The section will reside in ROM.  */
     #define SEC_ROM        0x040
       /* The section contains constructor information. This section
          type is used by the linker to create lists of constructors and
          destructors used by g++. When a back end sees a symbol
          which should be used in a constructor list, it creates a new
          section for the type of name (e.g., __CTOR_LIST__), attaches
          the symbol to it, and builds a relocation. To build the lists
          of constructors, all the linker has to do is catenate all the
          sections called __CTOR_LIST__ and relocate the data
          contained within - exactly the operations it would peform on
          standard data.  */
     #define SEC_CONSTRUCTOR 0x080
       /* The section has contents - a data section could be
          SEC_ALLOC | SEC_HAS_CONTENTS; a debug section could be
          SEC_HAS_CONTENTS  */
     #define SEC_HAS_CONTENTS 0x100
       /* An instruction to the linker to not output the section
          even if it has information which would normally be written.  */
     #define SEC_NEVER_LOAD 0x200
       /* The section contains thread local data.  */
     #define SEC_THREAD_LOCAL 0x400
       /* The section has GOT references.  This flag is only for the
          linker, and is currently only used by the elf32-hppa back end.
          It will be set if global offset table references were detected
          in this section, which indicate to the linker that the section
          contains PIC code, and must be handled specially when doing a
          static link.  */
     #define SEC_HAS_GOT_REF 0x800
       /* The section contains common symbols (symbols may be defined
          multiple times, the value of a symbol is the amount of
          space it requires, and the largest symbol value is the one
          used).  Most targets have exactly one of these (which we
          translate to bfd_com_section_ptr), but ECOFF has two.  */
     #define SEC_IS_COMMON 0x1000
       /* The section contains only debugging information.  For
          example, this is set for ELF .debug and .stab sections.
          strip tests this flag to see if a section can be
          discarded.  */
     #define SEC_DEBUGGING 0x2000
       /* The contents of this section are held in memory pointed to
          by the contents field.  This is checked by bfd_get_section_contents,
          and the data is retrieved from memory if appropriate.  */
     #define SEC_IN_MEMORY 0x4000
       /* The contents of this section are to be excluded by the
          linker for executable and shared objects unless those
          objects are to be further relocated.  */
     #define SEC_EXCLUDE 0x8000
       /* The contents of this section are to be sorted based on the sum of
          the symbol and addend values specified by the associated relocation
          entries.  Entries without associated relocation entries will be
          appended to the end of the section in an unspecified order.  */
     #define SEC_SORT_ENTRIES 0x10000
       /* When linking, duplicate sections of the same name should be
          discarded, rather than being combined into a single section as
          is usually done.  This is similar to how common symbols are
          handled.  See SEC_LINK_DUPLICATES below.  */
     #define SEC_LINK_ONCE 0x20000
       /* If SEC_LINK_ONCE is set, this bitfield describes how the linker
          should handle duplicate sections.  */
     #define SEC_LINK_DUPLICATES 0x40000
       /* This value for SEC_LINK_DUPLICATES means that duplicate
          sections with the same name should simply be discarded.  */
       /* This value for SEC_LINK_DUPLICATES means that the linker
          should warn if there are any duplicate sections, although
          it should still only link one copy.  */
     #define SEC_LINK_DUPLICATES_ONE_ONLY 0x80000
       /* This value for SEC_LINK_DUPLICATES means that the linker
          should warn if any duplicate sections are a different size.  */
     #define SEC_LINK_DUPLICATES_SAME_SIZE 0x100000
       /* This value for SEC_LINK_DUPLICATES means that the linker
          should warn if any duplicate sections contain different
          contents.  */
       /* This section was created by the linker as part of dynamic
          relocation or other arcane processing.  It is skipped when
          going through the first-pass output, trusting that someone
          else up the line will take care of it later.  */
     #define SEC_LINKER_CREATED 0x200000
       /* This section should not be subject to garbage collection.  */
     #define SEC_KEEP 0x400000
       /* This section contains "short" data, and should be placed
          "near" the GP.  */
     #define SEC_SMALL_DATA 0x800000
       /* Attempt to merge identical entities in the section.
          Entity size is given in the entsize field.  */
     #define SEC_MERGE 0x1000000
       /* If given with SEC_MERGE, entities to merge are zero terminated
          strings where entsize specifies character size instead of fixed
          size entries.  */
     #define SEC_STRINGS 0x2000000
       /* This section contains data about section groups.  */
     #define SEC_GROUP 0x4000000
       /* The section is a COFF shared library section.  This flag is
          only for the linker.  If this type of section appears in
          the input file, the linker must copy it to the output file
          without changing the vma or size.  FIXME: Although this
          was originally intended to be general, it really is COFF
          specific (and the flag was renamed to indicate this).  It
          might be cleaner to have some more general mechanism to
          allow the back end to control what the linker does with
          sections.  */
     #define SEC_COFF_SHARED_LIBRARY 0x10000000
       /* This section contains data which may be shared with other
          executables or shared objects. This is for COFF only.  */
     #define SEC_COFF_SHARED 0x20000000
       /* When a section with this flag is being linked, then if the size of
          the input section is less than a page, it should not cross a page
          boundary.  If the size of the input section is one page or more,
          it should be aligned on a page boundary.  This is for TI
          TMS320C54X only.  */
     #define SEC_TIC54X_BLOCK 0x40000000
       /* Conditionally link this section; do not link if there are no
          references found to any symbol in the section.  This is for TI
          TMS320C54X only.  */
     #define SEC_TIC54X_CLINK 0x80000000
       /*  End of section flags.  */
       /* Some internal packed boolean fields.  */
       /* See the vma field.  */
       unsigned int user_set_vma : 1;
       /* A mark flag used by some of the linker backends.  */
       unsigned int linker_mark : 1;
       /* Another mark flag used by some of the linker backends.  Set for
          output sections that have an input section.  */
       unsigned int linker_has_input : 1;
       /* Mark flags used by some linker backends for garbage collection.  */
       unsigned int gc_mark : 1;
       unsigned int gc_mark_from_eh : 1;
       /* The following flags are used by the ELF linker. */
       /* Mark sections which have been allocated to segments.  */
       unsigned int segment_mark : 1;
       /* Type of sec_info information.  */
       unsigned int sec_info_type:3;
     #define ELF_INFO_TYPE_NONE      0
     #define ELF_INFO_TYPE_STABS     1
     #define ELF_INFO_TYPE_MERGE     2
     #define ELF_INFO_TYPE_EH_FRAME  3
     #define ELF_INFO_TYPE_JUST_SYMS 4
       /* Nonzero if this section uses RELA relocations, rather than REL.  */
       unsigned int use_rela_p:1;
       /* Bits used by various backends.  The generic code doesn't touch
          these fields.  */
       /* Nonzero if this section has TLS related relocations.  */
       unsigned int has_tls_reloc:1;
       /* Nonzero if this section has a gp reloc.  */
       unsigned int has_gp_reloc:1;
       /* Nonzero if this section needs the relax finalize pass.  */
       unsigned int need_finalize_relax:1;
       /* Whether relocations have been processed.  */
       unsigned int reloc_done : 1;
       /* End of internal packed boolean fields.  */
       /*  The virtual memory address of the section - where it will be
           at run time.  The symbols are relocated against this.  The
           user_set_vma flag is maintained by bfd; if it's not set, the
           backend can assign addresses (for example, in a.out, where
           the default address for .data is dependent on the specific
           target and various flags).  */
       bfd_vma vma;
       /*  The load address of the section - where it would be in a
           rom image; really only used for writing section header
           information.  */
       bfd_vma lma;
       /* The size of the section in octets, as it will be output.
          Contains a value even if the section has no contents (e.g., the
          size of .bss).  */
       bfd_size_type size;
       /* For input sections, the original size on disk of the section, in
          octets.  This field is used by the linker relaxation code.  It is
          currently only set for sections where the linker relaxation scheme
          doesn't cache altered section and reloc contents (stabs, eh_frame,
          SEC_MERGE, some coff relaxing targets), and thus the original size
          needs to be kept to read the section multiple times.
          For output sections, rawsize holds the section size calculated on
          a previous linker relaxation pass.  */
       bfd_size_type rawsize;
       /* If this section is going to be output, then this value is the
          offset in *bytes* into the output section of the first byte in the
          input section (byte ==> smallest addressable unit on the
          target).  In most cases, if this was going to start at the
          100th octet (8-bit quantity) in the output section, this value
          would be 100.  However, if the target byte size is 16 bits
          (bfd_octets_per_byte is "2"), this value would be 50.  */
       bfd_vma output_offset;
       /* The output section through which to map on output.  */
       struct bfd_section *output_section;
       /* The alignment requirement of the section, as an exponent of 2 -
          e.g., 3 aligns to 2^3 (or 8).  */
       unsigned int alignment_power;
       /* If an input section, a pointer to a vector of relocation
          records for the data in this section.  */
       struct reloc_cache_entry *relocation;
       /* If an output section, a pointer to a vector of pointers to
          relocation records for the data in this section.  */
       struct reloc_cache_entry **orelocation;
       /* The number of relocation records in one of the above.  */
       unsigned reloc_count;
       /* Information below is back end specific - and not always used
          or updated.  */
       /* File position of section data.  */
       file_ptr filepos;
       /* File position of relocation info.  */
       file_ptr rel_filepos;
       /* File position of line data.  */
       file_ptr line_filepos;
       /* Pointer to data for applications.  */
       void *userdata;
       /* If the SEC_IN_MEMORY flag is set, this points to the actual
          contents.  */
       unsigned char *contents;
       /* Attached line number information.  */
       alent *lineno;
       /* Number of line number records.  */
       unsigned int lineno_count;
       /* Entity size for merging purposes.  */
       unsigned int entsize;
       /* Points to the kept section if this section is a link-once section,
          and is discarded.  */
       struct bfd_section *kept_section;
       /* When a section is being output, this value changes as more
          linenumbers are written out.  */
       file_ptr moving_line_filepos;
       /* What the section number is in the target world.  */
       int target_index;
       void *used_by_bfd;
       /* If this is a constructor section then here is a list of the
          relocations created to relocate items within it.  */
       struct relent_chain *constructor_chain;
       /* The BFD which owns the section.  */
       bfd *owner;
       /* A symbol which points at this section only.  */
       struct bfd_symbol *symbol;
       struct bfd_symbol **symbol_ptr_ptr;
       /* Early in the link process, map_head and map_tail are used to build
          a list of input sections attached to an output section.  Later,
          output sections use these fields for a list of bfd_link_order
          structs.  */
       union {
         struct bfd_link_order *link_order;
         struct bfd_section *s;
       } map_head, map_tail;
     } asection;
     /* These sections are global, and are managed by BFD.  The application
        and target back end are not permitted to change the values in
        these sections.  New code should use the section_ptr macros rather
        than referring directly to the const sections.  The const sections
        may eventually vanish.  */
     #define BFD_ABS_SECTION_NAME "*ABS*"
     #define BFD_UND_SECTION_NAME "*UND*"
     #define BFD_COM_SECTION_NAME "*COM*"
     #define BFD_IND_SECTION_NAME "*IND*"
     /* The absolute section.  */
     extern asection bfd_abs_section;
     #define bfd_abs_section_ptr ((asection *) &bfd_abs_section)
     #define bfd_is_abs_section(sec) ((sec) == bfd_abs_section_ptr)
     /* Pointer to the undefined section.  */
     extern asection bfd_und_section;
     #define bfd_und_section_ptr ((asection *) &bfd_und_section)
     #define bfd_is_und_section(sec) ((sec) == bfd_und_section_ptr)
     /* Pointer to the common section.  */
     extern asection bfd_com_section;
     #define bfd_com_section_ptr ((asection *) &bfd_com_section)
     /* Pointer to the indirect section.  */
     extern asection bfd_ind_section;
     #define bfd_ind_section_ptr ((asection *) &bfd_ind_section)
     #define bfd_is_ind_section(sec) ((sec) == bfd_ind_section_ptr)
     #define bfd_is_const_section(SEC)              \
      (   ((SEC) == bfd_abs_section_ptr)            \
       || ((SEC) == bfd_und_section_ptr)            \
       || ((SEC) == bfd_com_section_ptr)            \
       || ((SEC) == bfd_ind_section_ptr))
     extern const struct bfd_symbol * const bfd_abs_symbol;
     extern const struct bfd_symbol * const bfd_com_symbol;
     extern const struct bfd_symbol * const bfd_und_symbol;
     extern const struct bfd_symbol * const bfd_ind_symbol;
     /* Macros to handle insertion and deletion of a bfd's sections.  These
        only handle the list pointers, ie. do not adjust section_count,
        target_index etc.  */
     #define bfd_section_list_remove(ABFD, S) \
       do                                                   \
         {                                                  \
           asection *_s = S;                                \
           asection *_next = _s->next;                      \
           asection *_prev = _s->prev;                      \
           if (_prev)                                       \
             _prev->next = _next;                           \
           else                                             \
             (ABFD)->sections = _next;                      \
           if (_next)                                       \
             _next->prev = _prev;                           \
           else                                             \
             (ABFD)->section_last = _prev;                  \
         }                                                  \
       while (0)
     #define bfd_section_list_append(ABFD, S) \
       do                                                   \
         {                                                  \
           asection *_s = S;                                \
           bfd *_abfd = ABFD;                               \
           _s->next = NULL;                                 \
           if (_abfd->section_last)                         \
             {                                              \
               _s->prev = _abfd->section_last;              \
               _abfd->section_last->next = _s;              \
             }                                              \
           else                                             \
             {                                              \
               _s->prev = NULL;                             \
               _abfd->sections = _s;                        \
             }                                              \
           _abfd->section_last = _s;                        \
         }                                                  \
       while (0)
     #define bfd_section_list_prepend(ABFD, S) \
       do                                                   \
         {                                                  \
           asection *_s = S;                                \
           bfd *_abfd = ABFD;                               \
           _s->prev = NULL;                                 \
           if (_abfd->sections)                             \
             {                                              \
               _s->next = _abfd->sections;                  \
               _abfd->sections->prev = _s;                  \
             }                                              \
           else                                             \
             {                                              \
               _s->next = NULL;                             \
               _abfd->section_last = _s;                    \
             }                                              \
           _abfd->sections = _s;                            \
         }                                                  \
       while (0)
     #define bfd_section_list_insert_after(ABFD, A, S) \
       do                                                   \
         {                                                  \
           asection *_a = A;                                \
           asection *_s = S;                                \
           asection *_next = _a->next;                      \
           _s->next = _next;                                \
           _s->prev = _a;                                   \
           _a->next = _s;                                   \
           if (_next)                                       \
             _next->prev = _s;                              \
           else                                             \
             (ABFD)->section_last = _s;                     \
         }                                                  \
       while (0)
     #define bfd_section_list_insert_before(ABFD, B, S) \
       do                                                   \
         {                                                  \
           asection *_b = B;                                \
           asection *_s = S;                                \
           asection *_prev = _b->prev;                      \
           _s->prev = _prev;                                \
           _s->next = _b;                                   \
           _b->prev = _s;                                   \
           if (_prev)                                       \
             _prev->next = _s;                              \
           else                                             \
             (ABFD)->sections = _s;                         \
         }                                                  \
       while (0)
     #define bfd_section_removed_from_list(ABFD, S) \
       ((S)->next == NULL ? (ABFD)->section_last != (S) : (S)->next->prev != (S))

Previous: typedef asection, Up: Sections

2.6.5 Section prototypes

These are the functions exported by the section handling part of BFD. bfd_section_list_clear


     void bfd_section_list_clear (bfd *);

Clears the section list, and also resets the section count and hash table entries. bfd_get_section_by_name


     asection *bfd_get_section_by_name (bfd *abfd, const char *name);

Run through abfd and return the one of the asections whose name matches name, otherwise NULL. See Sections, for more information.

This should only be used in special cases; the normal way to process all sections of a given name is to use bfd_map_over_sections and strcmp on the name (or better yet, base it on the section flags or something else) for each section. bfd_get_section_by_name_if


     asection *bfd_get_section_by_name_if
        (bfd *abfd,
         const char *name,
         bfd_boolean (*func) (bfd *abfd, asection *sect, void *obj),
         void *obj);

Call the provided function func for each section attached to the BFD abfd whose name matches name, passing obj as an argument. The function will be called as if by

            func (abfd, the_section, obj);

It returns the first section for which func returns true, otherwise NULL. bfd_get_unique_section_name


     char *bfd_get_unique_section_name
        (bfd *abfd, const char *templat, int *count);

Invent a section name that is unique in abfd by tacking a dot and a digit suffix onto the original templat. If count is non-NULL, then it specifies the first number tried as a suffix to generate a unique name. The value pointed to by count will be incremented in this case. bfd_make_section_old_way


     asection *bfd_make_section_old_way (bfd *abfd, const char *name);

Create a new empty section called name and attach it to the end of the chain of sections for the BFD abfd. An attempt to create a section with a name which is already in use returns its pointer without changing the section chain.

It has the funny name since this is the way it used to be before it was rewritten....

Possible errors are: bfd_make_section_anyway_with_flags


     asection *bfd_make_section_anyway_with_flags
        (bfd *abfd, const char *name, flagword flags);

Create a new empty section called name and attach it to the end of the chain of sections for abfd. Create a new section even if there is already a section with that name. Also set the attributes of the new section to the value flags.

Return NULL and set bfd_error on error; possible errors are: bfd_make_section_anyway


     asection *bfd_make_section_anyway (bfd *abfd, const char *name);

Create a new empty section called name and attach it to the end of the chain of sections for abfd. Create a new section even if there is already a section with that name.

Return NULL and set bfd_error on error; possible errors are: bfd_make_section_with_flags


     asection *bfd_make_section_with_flags
        (bfd *, const char *name, flagword flags);

Like bfd_make_section_anyway, but return NULL (without calling bfd_set_error ()) without changing the section chain if there is already a section named name. Also set the attributes of the new section to the value flags. If there is an error, return NULL and set bfd_error. bfd_make_section


     asection *bfd_make_section (bfd *, const char *name);

Like bfd_make_section_anyway, but return NULL (without calling bfd_set_error ()) without changing the section chain if there is already a section named name. If there is an error, return NULL and set bfd_error. bfd_set_section_flags


     bfd_boolean bfd_set_section_flags
        (bfd *abfd, asection *sec, flagword flags);

Set the attributes of the section sec in the BFD abfd to the value flags. Return TRUE on success, FALSE on error. Possible error returns are: bfd_map_over_sections


     void bfd_map_over_sections
        (bfd *abfd,
         void (*func) (bfd *abfd, asection *sect, void *obj),
         void *obj);

Call the provided function func for each section attached to the BFD abfd, passing obj as an argument. The function will be called as if by

            func (abfd, the_section, obj);

This is the preferred method for iterating over sections; an alternative would be to use a loop:

               section *p;
               for (p = abfd->sections; p != NULL; p = p->next)
                  func (abfd, p, ...) bfd_sections_find_if


     asection *bfd_sections_find_if
        (bfd *abfd,
         bfd_boolean (*operation) (bfd *abfd, asection *sect, void *obj),
         void *obj);

Call the provided function operation for each section attached to the BFD abfd, passing obj as an argument. The function will be called as if by

            operation (abfd, the_section, obj);

It returns the first section for which operation returns true. bfd_set_section_size


     bfd_boolean bfd_set_section_size
        (bfd *abfd, asection *sec, bfd_size_type val);

Set sec to the size val. If the operation is ok, then TRUE is returned, else FALSE.

Possible error returns: bfd_set_section_contents


     bfd_boolean bfd_set_section_contents
        (bfd *abfd, asection *section, const void *data,
         file_ptr offset, bfd_size_type count);

Sets the contents of the section section in BFD abfd to the data starting in memory at data. The data is written to the output section starting at offset offset for count octets.

Normally TRUE is returned, else FALSE. Possible error returns are:

This routine is front end to the back end function _bfd_set_section_contents. bfd_get_section_contents


     bfd_boolean bfd_get_section_contents
        (bfd *abfd, asection *section, void *location, file_ptr offset,
         bfd_size_type count);

Read data from section in BFD abfd into memory starting at location. The data is read at an offset of offset from the start of the input section, and is read for count bytes.

If the contents of a constructor with the SEC_CONSTRUCTOR flag set are requested or if the section does not have the SEC_HAS_CONTENTS flag set, then the location is filled with zeroes. If no errors occur, TRUE is returned, else FALSE. bfd_malloc_and_get_section


     bfd_boolean bfd_malloc_and_get_section
        (bfd *abfd, asection *section, bfd_byte **buf);

Read all data from section in BFD abfd into a buffer, *buf, malloc'd by this function. bfd_copy_private_section_data


     bfd_boolean bfd_copy_private_section_data
        (bfd *ibfd, asection *isec, bfd *obfd, asection *osec);

Copy private section information from isec in the BFD ibfd to the section osec in the BFD obfd. Return TRUE on success, FALSE on error. Possible error returns are:

     #define bfd_copy_private_section_data(ibfd, isection, obfd, osection) \
          BFD_SEND (obfd, _bfd_copy_private_section_data, \
                    (ibfd, isection, obfd, osection)) bfd_generic_is_group_section


     bfd_boolean bfd_generic_is_group_section (bfd *, const asection *sec);

Returns TRUE if sec is a member of a group. bfd_generic_discard_group


     bfd_boolean bfd_generic_discard_group (bfd *abfd, asection *group);

Remove all members of group from the output.

Next: , Previous: Sections, Up: BFD front end

2.7 Symbols

BFD tries to maintain as much symbol information as it can when it moves information from file to file. BFD passes information to applications though the asymbol structure. When the application requests the symbol table, BFD reads the table in the native form and translates parts of it into the internal format. To maintain more than the information passed to applications, some targets keep some information “behind the scenes” in a structure only the particular back end knows about. For example, the coff back end keeps the original symbol table structure as well as the canonical structure when a BFD is read in. On output, the coff back end can reconstruct the output symbol table so that no information is lost, even information unique to coff which BFD doesn't know or understand. If a coff symbol table were read, but were written through an a.out back end, all the coff specific information would be lost. The symbol table of a BFD is not necessarily read in until a canonicalize request is made. Then the BFD back end fills in a table provided by the application with pointers to the canonical information. To output symbols, the application provides BFD with a table of pointers to pointers to asymbols. This allows applications like the linker to output a symbol as it was read, since the “behind the scenes” information will be still available.

Next: , Previous: Symbols, Up: Symbols

2.7.1 Reading symbols

There are two stages to reading a symbol table from a BFD: allocating storage, and the actual reading process. This is an excerpt from an application which reads the symbol table:

              long storage_needed;
              asymbol **symbol_table;
              long number_of_symbols;
              long i;
              storage_needed = bfd_get_symtab_upper_bound (abfd);
              if (storage_needed < 0)
              if (storage_needed == 0)
              symbol_table = xmalloc (storage_needed);
              number_of_symbols =
                 bfd_canonicalize_symtab (abfd, symbol_table);
              if (number_of_symbols < 0)
              for (i = 0; i < number_of_symbols; i++)
                process_symbol (symbol_table[i]);

All storage for the symbols themselves is in an objalloc connected to the BFD; it is freed when the BFD is closed.

Next: , Previous: Reading Symbols, Up: Symbols

2.7.2 Writing symbols

Writing of a symbol table is automatic when a BFD open for writing is closed. The application attaches a vector of pointers to pointers to symbols to the BFD being written, and fills in the symbol count. The close and cleanup code reads through the table provided and performs all the necessary operations. The BFD output code must always be provided with an “owned” symbol: one which has come from another BFD, or one which has been created using bfd_make_empty_symbol. Here is an example showing the creation of a symbol table with only one element:

            #include "bfd.h"
            int main (void)
              bfd *abfd;
              asymbol *ptrs[2];
              asymbol *new;
              abfd = bfd_openw ("foo","a.out-sunos-big");
              bfd_set_format (abfd, bfd_object);
              new = bfd_make_empty_symbol (abfd);
              new->name = "dummy_symbol";
              new->section = bfd_make_section_old_way (abfd, ".text");
              new->flags = BSF_GLOBAL;
              new->value = 0x12345;
              ptrs[0] = new;
              ptrs[1] = 0;
              bfd_set_symtab (abfd, ptrs, 1);
              bfd_close (abfd);
              return 0;
            nm foo
            00012345 A dummy_symbol

Many formats cannot represent arbitrary symbol information; for instance, the a.out object format does not allow an arbitrary number of sections. A symbol pointing to a section which is not one of .text, .data or .bss cannot be described.

Next: , Previous: Writing Symbols, Up: Symbols

2.7.3 Mini Symbols

Mini symbols provide read-only access to the symbol table. They use less memory space, but require more time to access. They can be useful for tools like nm or objdump, which may have to handle symbol tables of extremely large executables.

The bfd_read_minisymbols function will read the symbols into memory in an internal form. It will return a void * pointer to a block of memory, a symbol count, and the size of each symbol. The pointer is allocated using malloc, and should be freed by the caller when it is no longer needed.

The function bfd_minisymbol_to_symbol will take a pointer to a minisymbol, and a pointer to a structure returned by bfd_make_empty_symbol, and return a asymbol structure. The return value may or may not be the same as the value from bfd_make_empty_symbol which was passed in.

Next: , Previous: Mini Symbols, Up: Symbols

2.7.4 typedef asymbol

An asymbol has the form:

     typedef struct bfd_symbol
       /* A pointer to the BFD which owns the symbol. This information
          is necessary so that a back end can work out what additional
          information (invisible to the application writer) is carried
          with the symbol.
          This field is *almost* redundant, since you can use section->owner
          instead, except that some symbols point to the global sections
          bfd_{abs,com,und}_section.  This could be fixed by making
          these globals be per-bfd (or per-target-flavor).  FIXME.  */
       struct bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field.  */
       /* The text of the symbol. The name is left alone, and not copied; the
          application may not alter it.  */
       const char *name;
       /* The value of the symbol.  This really should be a union of a
          numeric value with a pointer, since some flags indicate that
          a pointer to another symbol is stored here.  */
       symvalue value;
       /* Attributes of a symbol.  */
     #define BSF_NO_FLAGS    0x00
       /* The symbol has local scope; static in C. The value
          is the offset into the section of the data.  */
     #define BSF_LOCAL      0x01
       /* The symbol has global scope; initialized data in C. The
          value is the offset into the section of the data.  */
     #define BSF_GLOBAL     0x02
       /* The symbol has global scope and is exported. The value is
          the offset into the section of the data.  */
     #define BSF_EXPORT     BSF_GLOBAL /* No real difference.  */
       /* A normal C symbol would be one of:
          BSF_GLOBAL.  */
       /* The symbol is a debugging record. The value has an arbitrary
          meaning, unless BSF_DEBUGGING_RELOC is also set.  */
     #define BSF_DEBUGGING  0x08
       /* The symbol denotes a function entry point.  Used in ELF,
          perhaps others someday.  */
     #define BSF_FUNCTION    0x10
       /* Used by the linker.  */
     #define BSF_KEEP        0x20
     #define BSF_KEEP_G      0x40
       /* A weak global symbol, overridable without warnings by
          a regular global symbol of the same name.  */
     #define BSF_WEAK        0x80
       /* This symbol was created to point to a section, e.g. ELF's
          STT_SECTION symbols.  */
     #define BSF_SECTION_SYM 0x100
       /* The symbol used to be a common symbol, but now it is
          allocated.  */
     #define BSF_OLD_COMMON  0x200
       /* The default value for common data.  */
       /* In some files the type of a symbol sometimes alters its
          location in an output file - ie in coff a ISFCN symbol
          which is also C_EXT symbol appears where it was
          declared and not at the end of a section.  This bit is set
          by the target BFD part to convey this information.  */
     #define BSF_NOT_AT_END    0x400
       /* Signal that the symbol is the label of constructor section.  */
     #define BSF_CONSTRUCTOR   0x800
       /* Signal that the symbol is a warning symbol.  The name is a
          warning.  The name of the next symbol is the one to warn about;
          if a reference is made to a symbol with the same name as the next
          symbol, a warning is issued by the linker.  */
     #define BSF_WARNING       0x1000
       /* Signal that the symbol is indirect.  This symbol is an indirect
          pointer to the symbol with the same name as the next symbol.  */
     #define BSF_INDIRECT      0x2000
       /* BSF_FILE marks symbols that contain a file name.  This is used
          for ELF STT_FILE symbols.  */
     #define BSF_FILE          0x4000
       /* Symbol is from dynamic linking information.  */
     #define BSF_DYNAMIC       0x8000
       /* The symbol denotes a data object.  Used in ELF, and perhaps
          others someday.  */
     #define BSF_OBJECT        0x10000
       /* This symbol is a debugging symbol.  The value is the offset
          into the section of the data.  BSF_DEBUGGING should be set
          as well.  */
     #define BSF_DEBUGGING_RELOC 0x20000
       /* This symbol is thread local.  Used in ELF.  */
     #define BSF_THREAD_LOCAL  0x40000
       flagword flags;
       /* A pointer to the section to which this symbol is
          relative.  This will always be non NULL, there are special
          sections for undefined and absolute symbols.  */
       struct bfd_section *section;
       /* Back end special data.  */
           void *p;
           bfd_vma i;

Previous: typedef asymbol, Up: Symbols

2.7.5 Symbol handling functions bfd_get_symtab_upper_bound

Return the number of bytes required to store a vector of pointers to asymbols for all the symbols in the BFD abfd, including a terminal NULL pointer. If there are no symbols in the BFD, then return 0. If an error occurs, return -1.

     #define bfd_get_symtab_upper_bound(abfd) \
          BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd)) bfd_is_local_label


     bfd_boolean bfd_is_local_label (bfd *abfd, asymbol *sym);

Return TRUE if the given symbol sym in the BFD abfd is a compiler generated local label, else return FALSE. bfd_is_local_label_name


     bfd_boolean bfd_is_local_label_name (bfd *abfd, const char *name);

Return TRUE if a symbol with the name name in the BFD abfd is a compiler generated local label, else return FALSE. This just checks whether the name has the form of a local label.

     #define bfd_is_local_label_name(abfd, name) \
       BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name)) bfd_is_target_special_symbol


     bfd_boolean bfd_is_target_special_symbol (bfd *abfd, asymbol *sym);

Return TRUE iff a symbol sym in the BFD abfd is something special to the particular target represented by the BFD. Such symbols should normally not be mentioned to the user.

     #define bfd_is_target_special_symbol(abfd, sym) \
       BFD_SEND (abfd, _bfd_is_target_special_symbol, (abfd, sym)) bfd_canonicalize_symtab

Read the symbols from the BFD abfd, and fills in the vector location with pointers to the symbols and a trailing NULL. Return the actual number of symbol pointers, not including the NULL.

     #define bfd_canonicalize_symtab(abfd, location) \
       BFD_SEND (abfd, _bfd_canonicalize_symtab, (abfd, location)) bfd_set_symtab


     bfd_boolean bfd_set_symtab
        (bfd *abfd, asymbol **location, unsigned int count);

Arrange that when the output BFD abfd is closed, the table location of count pointers to symbols will be written. bfd_print_symbol_vandf


     void bfd_print_symbol_vandf (bfd *abfd, void *file, asymbol *symbol);

Print the value and flags of the symbol supplied to the stream file. bfd_make_empty_symbol

Create a new asymbol structure for the BFD abfd and return a pointer to it.

This routine is necessary because each back end has private information surrounding the asymbol. Building your own asymbol and pointing to it will not create the private information, and will cause problems later on.

     #define bfd_make_empty_symbol(abfd) \
       BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd)) _bfd_generic_make_empty_symbol


     asymbol *_bfd_generic_make_empty_symbol (bfd *);

Create a new asymbol structure for the BFD abfd and return a pointer to it. Used by core file routines, binary back-end and anywhere else where no private info is needed. bfd_make_debug_symbol

Create a new asymbol structure for the BFD abfd, to be used as a debugging symbol. Further details of its use have yet to be worked out.

     #define bfd_make_debug_symbol(abfd,ptr,size) \
       BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size)) bfd_decode_symclass

Return a character corresponding to the symbol class of symbol, or '?' for an unknown class.


     int bfd_decode_symclass (asymbol *symbol); bfd_is_undefined_symclass

Returns non-zero if the class symbol returned by bfd_decode_symclass represents an undefined symbol. Returns zero otherwise.


     bfd_boolean bfd_is_undefined_symclass (int symclass); bfd_symbol_info

Fill in the basic info about symbol that nm needs. Additional info may be added by the back-ends after calling this function.


     void bfd_symbol_info (asymbol *symbol, symbol_info *ret); bfd_copy_private_symbol_data


     bfd_boolean bfd_copy_private_symbol_data
        (bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym);

Copy private symbol information from isym in the BFD ibfd to the symbol osym in the BFD obfd. Return TRUE on success, FALSE on error. Possible error returns are:

     #define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \
       BFD_SEND (obfd, _bfd_copy_private_symbol_data, \
                 (ibfd, isymbol, obfd, osymbol))

Next: , Previous: Symbols, Up: BFD front end

2.8 Archives

An archive (or library) is just another BFD. It has a symbol table, although there's not much a user program will do with it.

The big difference between an archive BFD and an ordinary BFD is that the archive doesn't have sections. Instead it has a chain of BFDs that are considered its contents. These BFDs can be manipulated like any other. The BFDs contained in an archive opened for reading will all be opened for reading. You may put either input or output BFDs into an archive opened for output; they will be handled correctly when the archive is closed.

Use bfd_openr_next_archived_file to step through the contents of an archive opened for input. You don't have to read the entire archive if you don't want to! Read it until you find what you want.

Archive contents of output BFDs are chained through the next pointer in a BFD. The first one is findable through the archive_head slot of the archive. Set it with bfd_set_archive_head (q.v.). A given BFD may be in only one open output archive at a time.

As expected, the BFD archive code is more general than the archive code of any given environment. BFD archives may contain files of different formats (e.g., a.out and coff) and even different architectures. You may even place archives recursively into archives!

This can cause unexpected confusion, since some archive formats are more expressive than others. For instance, Intel COFF archives can preserve long filenames; SunOS a.out archives cannot. If you move a file from the first to the second format and back again, the filename may be truncated. Likewise, different a.out environments have different conventions as to how they truncate filenames, whether they preserve directory names in filenames, etc. When interoperating with native tools, be sure your files are homogeneous.

Beware: most of these formats do not react well to the presence of spaces in filenames. We do the best we can, but can't always handle this case due to restrictions in the format of archives. Many Unix utilities are braindead in regards to spaces and such in filenames anyway, so this shouldn't be much of a restriction.

Archives are supported in BFD in archive.c. bfd_get_next_mapent


     symindex bfd_get_next_mapent
        (bfd *abfd, symindex previous, carsym **sym);

Step through archive abfd's symbol table (if it has one). Successively update sym with the next symbol's information, returning that symbol's (internal) index into the symbol table.

Supply BFD_NO_MORE_SYMBOLS as the previous entry to get the first one; returns BFD_NO_MORE_SYMBOLS when you've already got the last one.

A carsym is a canonical archive symbol. The only user-visible element is its name, a null-terminated string. bfd_set_archive_head


     bfd_boolean bfd_set_archive_head (bfd *output, bfd *new_head);

Set the head of the chain of BFDs contained in the archive output to new_head. bfd_openr_next_archived_file


     bfd *bfd_openr_next_archived_file (bfd *archive, bfd *previous);

Provided a BFD, archive, containing an archive and NULL, open an input BFD on the first contained element and returns that. Subsequent calls should pass the archive and the previous return value to return a created BFD to the next contained element. NULL is returned when there are no more.

Next: , Previous: Archives, Up: BFD front end

2.9 File formats

A format is a BFD concept of high level file contents type. The formats supported by BFD are:

The BFD may contain data, symbols, relocations and debug info. The BFD contains other BFDs and an optional index. The BFD contains the result of an executable core dump. bfd_check_format


     bfd_boolean bfd_check_format (bfd *abfd, bfd_format format);

Verify if the file attached to the BFD abfd is compatible with the format format (i.e., one of bfd_object, bfd_archive or bfd_core).

If the BFD has been set to a specific target before the call, only the named target and format combination is checked. If the target has not been set, or has been set to default, then all the known target backends is interrogated to determine a match. If the default target matches, it is used. If not, exactly one target must recognize the file, or an error results.

The function returns TRUE on success, otherwise FALSE with one of the following error codes: bfd_check_format_matches


     bfd_boolean bfd_check_format_matches
        (bfd *abfd, bfd_format format, char ***matching);

Like bfd_check_format, except when it returns FALSE with bfd_errno set to bfd_error_file_ambiguously_recognized. In that case, if matching is not NULL, it will be filled in with a NULL-terminated list of the names of the formats that matched, allocated with malloc. Then the user may choose a format and try again.

When done with the list that matching points to, the caller should free it. bfd_set_format


     bfd_boolean bfd_set_format (bfd *abfd, bfd_format format);

This function sets the file format of the BFD abfd to the format format. If the target set in the BFD does not support the format requested, the format is invalid, or the BFD is not open for writing, then an error occurs. bfd_format_string


     const char *bfd_format_string (bfd_format format);

Return a pointer to a const string invalid, object, archive, core, or unknown, depending upon the value of format.

Next: , Previous: Formats, Up: BFD front end

2.10 Relocations

BFD maintains relocations in much the same way it maintains symbols: they are left alone until required, then read in en-masse and translated into an internal form. A common routine bfd_perform_relocation acts upon the canonical form to do the fixup.

Relocations are maintained on a per section basis, while symbols are maintained on a per BFD basis.

All that a back end has to do to fit the BFD interface is to create a struct reloc_cache_entry for each relocation in a particular section, and fill in the right bits of the structures.

Next: , Previous: Relocations, Up: Relocations

2.10.1 typedef arelent

This is the structure of a relocation entry:

     typedef enum bfd_reloc_status
       /* No errors detected.  */
       /* The relocation was performed, but there was an overflow.  */
       /* The address to relocate was not within the section supplied.  */
       /* Used by special functions.  */
       /* Unsupported relocation size requested.  */
       /* Unused.  */
       /* The symbol to relocate against was undefined.  */
       /* The relocation was performed, but may not be ok - presently
          generated only when linking i960 coff files with i960 b.out
          symbols.  If this type is returned, the error_message argument
          to bfd_perform_relocation will be set.  */
     typedef struct reloc_cache_entry
       /* A pointer into the canonical table of pointers.  */
       struct bfd_symbol **sym_ptr_ptr;
       /* offset in section.  */
       bfd_size_type address;
       /* addend for relocation value.  */
       bfd_vma addend;
       /* Pointer to how to perform the required relocation.  */
       reloc_howto_type *howto;

Here is a description of each of the fields within an arelent:

The symbol table pointer points to a pointer to the symbol associated with the relocation request. It is the pointer into the table returned by the back end's canonicalize_symtab action. See Symbols. The symbol is referenced through a pointer to a pointer so that tools like the linker can fix up all the symbols of the same name by modifying only one pointer. The relocation routine looks in the symbol and uses the base of the section the symbol is attached to and the value of the symbol as the initial relocation offset. If the symbol pointer is zero, then the section provided is looked up. The address field gives the offset in bytes from the base of the section data which owns the relocation record to the first byte of relocatable information. The actual data relocated will be relative to this point; for example, a relocation type which modifies the bottom two bytes of a four byte word would not touch the first byte pointed to in a big endian world. The addend is a value provided by the back end to be added (!) to the relocation offset. Its interpretation is dependent upon the howto. For example, on the 68k the code:
             char foo[];
                     return foo[0x12345678];

Could be compiled into:

             linkw fp,#-4
             moveb @#12345678,d0
             extbl d0
             unlk fp

This could create a reloc pointing to foo, but leave the offset in the data, something like:

     offset   type      value
     00000006 32        _foo
     00000000 4e56 fffc          ; linkw fp,#-4
     00000004 1039 1234 5678     ; moveb @#12345678,d0
     0000000a 49c0               ; extbl d0
     0000000c 4e5e               ; unlk fp
     0000000e 4e75               ; rts

Using coff and an 88k, some instructions don't have enough space in them to represent the full address range, and pointers have to be loaded in two parts. So you'd get something like:

             or.u     r13,r0,hi16(_foo+0x12345678)
             ld.b     r2,r13,lo16(_foo+0x12345678)
             jmp      r1

This should create two relocs, both pointing to _foo, and with 0x12340000 in their addend field. The data would consist of:

     offset   type      value
     00000002 HVRT16    _foo+0x12340000
     00000006 LVRT16    _foo+0x12340000
     00000000 5da05678           ; or.u r13,r0,0x5678
     00000004 1c4d5678           ; ld.b r2,r13,0x5678
     00000008 f400c001           ; jmp r1

The relocation routine digs out the value from the data, adds it to the addend to get the original offset, and then adds the value of _foo. Note that all 32 bits have to be kept around somewhere, to cope with carry from bit 15 to bit 16.

One further example is the sparc and the a.out format. The sparc has a similar problem to the 88k, in that some instructions don't have room for an entire offset, but on the sparc the parts are created in odd sized lumps. The designers of the a.out format chose to not use the data within the section for storing part of the offset; all the offset is kept within the reloc. Anything in the data should be ignored.

             save %sp,-112,%sp
             sethi %hi(_foo+0x12345678),%g2
             ldsb [%g2+%lo(_foo+0x12345678)],%i0

Both relocs contain a pointer to foo, and the offsets contain junk.

     offset   type      value
     00000004 HI22      _foo+0x12345678
     00000008 LO10      _foo+0x12345678
     00000000 9de3bf90     ; save %sp,-112,%sp
     00000004 05000000     ; sethi %hi(_foo+0),%g2
     00000008 f048a000     ; ldsb [%g2+%lo(_foo+0)],%i0
     0000000c 81c7e008     ; ret
     00000010 81e80000     ; restore
The howto field can be imagined as a relocation instruction. It is a pointer to a structure which contains information on what to do with all of the other information in the reloc record and data section. A back end would normally have a relocation instruction set and turn relocations into pointers to the correct structure on input - but it would be possible to create each howto field on demand. enum complain_overflow

Indicates what sort of overflow checking should be done when performing a relocation.

     enum complain_overflow
       /* Do not complain on overflow.  */
       /* Complain if the bitfield overflows, whether it is considered
          as signed or unsigned.  */
       /* Complain if the value overflows when considered as signed
          number.  */
       /* Complain if the value overflows when considered as an
          unsigned number.  */
     }; reloc_howto_type

The reloc_howto_type is a structure which contains all the information that libbfd needs to know to tie up a back end's data.

     struct bfd_symbol;             /* Forward declaration.  */
     struct reloc_howto_struct
       /*  The type field has mainly a documentary use - the back end can
           do what it wants with it, though normally the back end's
           external idea of what a reloc number is stored
           in this field.  For example, a PC relative word relocation
           in a coff environment has the type 023 - because that's
           what the outside world calls a R_PCRWORD reloc.  */
       unsigned int type;
       /*  The value the final relocation is shifted right by.  This drops
           unwanted data from the relocation.  */
       unsigned int rightshift;
       /*  The size of the item to be relocated.  This is *not* a
           power-of-two measure.  To get the number of bytes operated
           on by a type of relocation, use bfd_get_reloc_size.  */
       int size;
       /*  The number of bits in the item to be relocated.  This is used
           when doing overflow checking.  */
       unsigned int bitsize;
       /*  Notes that the relocation is relative to the location in the
           data section of the addend.  The relocation function will
           subtract from the relocation value the address of the location
           being relocated.  */
       bfd_boolean pc_relative;
       /*  The bit position of the reloc value in the destination.
           The relocated value is left shifted by this amount.  */
       unsigned int bitpos;
       /* What type of overflow error should be checked for when
          relocating.  */
       enum complain_overflow complain_on_overflow;
       /* If this field is non null, then the supplied function is
          called rather than the normal function.  This allows really
          strange relocation methods to be accommodated (e.g., i960 callj
          instructions).  */
       bfd_reloc_status_type (*special_function)
         (bfd *, arelent *, struct bfd_symbol *, void *, asection *,
          bfd *, char **);
       /* The textual name of the relocation type.  */
       char *name;
       /* Some formats record a relocation addend in the section contents
          rather than with the relocation.  For ELF formats this is the
          distinction between USE_REL and USE_RELA (though the code checks
          for USE_REL == 1/0).  The value of this field is TRUE if the
          addend is recorded with the section contents; when performing a
          partial link (ld -r) the section contents (the data) will be
          modified.  The value of this field is FALSE if addends are
          recorded with the relocation (in arelent.addend); when performing
          a partial link the relocation will be modified.
          All relocations for all ELF USE_RELA targets should set this field
          to FALSE (values of TRUE should be looked on with suspicion).
          However, the converse is not true: not all relocations of all ELF
          USE_REL targets set this field to TRUE.  Why this is so is peculiar
          to each particular target.  For relocs that aren't used in partial
          links (e.g. GOT stuff) it doesn't matter what this is set to.  */
       bfd_boolean partial_inplace;
       /* src_mask selects the part of the instruction (or data) to be used
          in the relocation sum.  If the target relocations don't have an
          addend in the reloc, eg. ELF USE_REL, src_mask will normally equal
          dst_mask to extract the addend from the section contents.  If
          relocations do have an addend in the reloc, eg. ELF USE_RELA, this
          field should be zero.  Non-zero values for ELF USE_RELA targets are
          bogus as in those cases the value in the dst_mask part of the
          section contents should be treated as garbage.  */
       bfd_vma src_mask;
       /* dst_mask selects which parts of the instruction (or data) are
          replaced with a relocated value.  */
       bfd_vma dst_mask;
       /* When some formats create PC relative instructions, they leave
          the value of the pc of the place being relocated in the offset
          slot of the instruction, so that a PC relative relocation can
          be made just by adding in an ordinary offset (e.g., sun3 a.out).
          Some formats leave the displacement part of an instruction
          empty (e.g., m88k bcs); this flag signals the fact.  */
       bfd_boolean pcrel_offset;
     }; The HOWTO Macro

The HOWTO define is horrible and will go away.

       { (unsigned) C, R, S, B, P, BI, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC }

And will be replaced with the totally magic way. But for the moment, we are compatible, so do it this way.

       HOWTO (0, 0, SIZE, 0, REL, 0, complain_overflow_dont, FUNCTION, \
              NAME, FALSE, 0, 0, IN)

This is used to fill in an empty howto entry in an array.

     #define EMPTY_HOWTO(C) \
       HOWTO ((C), 0, 0, 0, FALSE, 0, complain_overflow_dont, NULL, \
              NULL, FALSE, 0, 0, FALSE)

Helper routine to turn a symbol into a relocation value.

     #define HOWTO_PREPARE(relocation, symbol)               \
       {                                                     \
         if (symbol != NULL)                                 \
           {                                                 \
             if (bfd_is_com_section (symbol->section))       \
               {                                             \
                 relocation = 0;                             \
               }                                             \
             else                                            \
               {                                             \
                 relocation = symbol->value;                 \
               }                                             \
           }                                                 \
       } bfd_get_reloc_size


     unsigned int bfd_get_reloc_size (reloc_howto_type *);

For a reloc_howto_type that operates on a fixed number of bytes, this returns the number of bytes operated on. arelent_chain

How relocs are tied together in an asection:

     typedef struct relent_chain
       arelent relent;
       struct relent_chain *next;
     arelent_chain; bfd_check_overflow


     bfd_reloc_status_type bfd_check_overflow
        (enum complain_overflow how,
         unsigned int bitsize,
         unsigned int rightshift,
         unsigned int addrsize,
         bfd_vma relocation);

Perform overflow checking on relocation which has bitsize significant bits and will be shifted right by rightshift bits, on a machine with addresses containing addrsize significant bits. The result is either of bfd_reloc_ok or bfd_reloc_overflow. bfd_perform_relocation


     bfd_reloc_status_type bfd_perform_relocation
        (bfd *abfd,
         arelent *reloc_entry,
         void *data,
         asection *input_section,
         bfd *output_bfd,
         char **error_message);

If output_bfd is supplied to this function, the generated image will be relocatable; the relocations are copied to the output file after they have been changed to reflect the new state of the world. There are two ways of reflecting the results of partial linkage in an output file: by modifying the output data in place, and by modifying the relocation record. Some native formats (e.g., basic a.out and basic coff) have no way of specifying an addend in the relocation type, so the addend has to go in the output data. This is no big deal since in these formats the output data slot will always be big enough for the addend. Complex reloc types with addends were invented to solve just this problem. The error_message argument is set to an error message if this return bfd_reloc_dangerous. bfd_install_relocation


     bfd_reloc_status_type bfd_install_relocation
        (bfd *abfd,
         arelent *reloc_entry,
         void *data, bfd_vma data_start,
         asection *input_section,
         char **error_message);

This looks remarkably like bfd_perform_relocation, except it does not expect that the section contents have been filled in. I.e., it's suitable for use when creating, rather than applying a relocation.

For now, this function should be considered reserved for the assembler.

Previous: typedef arelent, Up: Relocations

2.11 The howto manager

When an application wants to create a relocation, but doesn't know what the target machine might call it, it can find out by using this bit of code. bfd_reloc_code_type

The insides of a reloc code. The idea is that, eventually, there will be one enumerator for every type of relocation we ever do. Pass one of these values to bfd_reloc_type_lookup, and it'll return a howto pointer.

This does mean that the application must determine the correct enumerator value; you can't get a howto pointer from a random set of attributes.

Here are the possible values for enum bfd_reloc_code_real:

— : BFD_RELOC_64
— : BFD_RELOC_32
— : BFD_RELOC_26
— : BFD_RELOC_24
— : BFD_RELOC_16
— : BFD_RELOC_14

Basic absolute relocations of N bits.


PC-relative relocations. Sometimes these are relative to the address of the relocation itself; sometimes they are relative to the start of the section containing the relocation. It depends on the specific target.

The 24-bit relocation is used in some Intel 960 configurations.


Section relative relocations. Some targets need this for DWARF2.


For ELF.


Relocations used by 68K ELF.


Linkage-table relative.

— : BFD_RELOC_8_FFnn

Absolute 8-bit relocation, but used to form an address like 0xFFnn.


These PC-relative relocations are stored as word displacements – i.e., byte displacements shifted right two bits. The 30-bit word displacement (<<32_PCREL_S2>> – 32 bits, shifted 2) is used on the SPARC. (SPARC tools generally refer to this as <<WDISP30>>.) The signed 16-bit displacement is used on the MIPS, and the 23-bit displacement is used on the Alpha.


High 22 bits and low 10 bits of 32-bit value, placed into lower bits of the target word. These are used on the SPARC.


For systems that allocate a Global Pointer register, these are displacements off that register. These relocation types are handled specially, because the value the register will have is decided relatively late.


Reloc types used for i960/b.out.


SPARC ELF relocations. There is probably some overlap with other relocation types already defined.


I think these are specific to SPARC a.out (e.g., Sun 4).


SPARC64 relocations


SPARC little endian relocation


SPARC TLS relocations


Alpha ECOFF and ELF relocations. Some of these treat the symbol or "addend" in some special way. For GPDISP_HI16 ("gpdisp") relocations, the symbol is ignored when writing; when reading, it will be the absolute section symbol. The addend is the displacement in bytes of the "lda" instruction from the "ldah" instruction (which is at the address of this reloc).


For GPDISP_LO16 ("ignore") relocations, the symbol is handled as with GPDISP_HI16 relocs. The addend is ignored when writing the relocations out, and is filled in with the file's GP value on reading, for convenience.


The ELF GPDISP relocation is exactly the same as the GPDISP_HI16 relocation except that there is no accompanying GPDISP_LO16 relocation.


The Alpha LITERAL/LITUSE relocs are produced by a symbol reference; the assembler turns it into a LDQ instruction to load the address of the symbol, and then fills in a register in the real instruction.

The LITERAL reloc, at the LDQ instruction, refers to the .lita section symbol. The addend is ignored when writing, but is filled in with the file's GP value on reading, for convenience, as with the GPDISP_LO16 reloc.

The ELF_LITERAL reloc is somewhere between 16_GOTOFF and GPDISP_LO16. It should refer to the symbol to be referenced, as with 16_GOTOFF, but it generates output not based on the position within the .got section, but relative to the GP value chosen for the file during the final link stage.

The LITUSE reloc, on the instruction using the loaded address, gives information to the linker that it might be able to use to optimize away some literal section references. The symbol is ignored (read as the absolute section symbol), and the "addend" indicates the type of instruction using the register: 1 - "memory" fmt insn 2 - byte-manipulation (byte offset reg) 3 - jsr (target of branch)


The HINT relocation indicates a value that should be filled into the "hint" field of a jmp/jsr/ret instruction, for possible branch- prediction logic which may be provided on some processors.


The LINKAGE relocation outputs a linkage pair in the object file, which is filled by the linker.


The CODEADDR relocation outputs a STO_CA in the object file, which is filled by the linker.


The GPREL_HI/LO relocations together form a 32-bit offset from the GP register.


Like BFD_RELOC_23_PCREL_S2, except that the source and target must share a common GP, and the target address is adjusted for STO_ALPHA_STD_GPLOAD.


Alpha thread-local storage relocations.


Bits 27..2 of the relocation address shifted right 2 bits; simple reloc otherwise.


The MIPS16 jump instruction.


MIPS16 GP relative reloc.


High 16 bits of 32-bit value; simple reloc.


High 16 bits of 32-bit value but the low 16 bits will be sign extended and added to form the final result. If the low 16 bits form a negative number, we need to add one to the high value to compensate for the borrow when the low bits are added.


Low 16 bits.


High 16 bits of 32-bit pc-relative value


High 16 bits of 32-bit pc-relative value, adjusted


Low 16 bits of pc-relative value


MIPS16 high 16 bits of 32-bit value.


MIPS16 high 16 bits of 32-bit value but the low 16 bits will be sign extended and added to form the final result. If the low 16 bits form a negative number, we need to add one to the high value to compensate for the borrow when the low bits are added.


MIPS16 low 16 bits.


Relocation against a MIPS literal section.


MIPS ELF relocations.


Fujitsu Frv Relocations.


This is a 24bit GOT-relative reloc for the mn10300.

— : BFD_RELOC_MN10300_GOT32

This is a 32bit GOT-relative reloc for the mn10300, offset by two bytes in the instruction.

— : BFD_RELOC_MN10300_GOT24

This is a 24bit GOT-relative reloc for the mn10300, offset by two bytes in the instruction.

— : BFD_RELOC_MN10300_GOT16

This is a 16bit GOT-relative reloc for the mn10300, offset by two bytes in the instruction.


Copy symbol at runtime.


Create GOT entry.


Create PLT entry.


Adjust by program base.

— : BFD_RELOC_386_GOT32
— : BFD_RELOC_386_PLT32
— : BFD_RELOC_386_TLS_LDO_32
— : BFD_RELOC_386_TLS_IE_32
— : BFD_RELOC_386_TLS_LE_32

i386/elf relocations

— : BFD_RELOC_X86_64_GOT32
— : BFD_RELOC_X86_64_PLT32
— : BFD_RELOC_X86_64_32S
— : BFD_RELOC_X86_64_TPOFF64
— : BFD_RELOC_X86_64_TPOFF32
— : BFD_RELOC_X86_64_GOTPC32

x86-64/elf relocations


ns32k relocations


PDP11 relocations


Picojava relocs. Not all of these appear in object files.


Power(rs6000) and PowerPC relocations.


PowerPC and PowerPC64 thread-local storage relocations.

— : BFD_RELOC_I370_D12

IBM 370/390 relocations


The type of reloc used to build a constructor table - at the moment probably a 32 bit wide absolute relocation, but the target can choose. It generally does map to one of the other relocation types.


ARM 26 bit pc-relative branch. The lowest two bits must be zero and are not stored in the instruction.


ARM 26 bit pc-relative branch. The lowest bit must be zero and is not stored in the instruction. The 2nd lowest bit comes from a 1 bit field in the instruction.


Thumb 22 bit pc-relative branch. The lowest bit must be zero and is not stored in the instruction. The 2nd lowest bit comes from a 1 bit field in the instruction.


Thumb 7-, 9-, 12-, 20-, 23-, and 25-bit pc-relative branches. The lowest bit must be zero and is not stored in the instruction. Note that the corresponding ELF R_ARM_THM_JUMPnn constant has an "nn" one smaller in all cases. Note further that BRANCH23 corresponds to R_ARM_THM_CALL.


12-bit immediate offset, used in ARM-format ldr and str instructions.


5-bit immediate offset, used in Thumb-format ldr and str instructions.


Pc-relative or absolute relocation depending on target. Used for entries in .init_array sections.


Read-only segment base relative address.


Data segment base relative address.


This reloc is used for references to RTTI data from exception handling tables. The actual definition depends on the target. It may be a pc-relative or some form of GOT-indirect relocation.


31-bit PC relative address.


Relocations for setting up GOTs and PLTs for shared libraries.


ARM thread-local storage relocations.


These relocs are only used within the ARM assembler. They are not (at present) written to any object files.


Renesas / SuperH SH relocs. Not all of these appear in object files.


ARC Cores relocs. ARC 22 bit pc-relative branch. The lowest two bits must be zero and are not stored in the instruction. The high 20 bits are installed in bits 26 through 7 of the instruction.


ARC 26 bit absolute branch. The lowest two bits must be zero and are not stored in the instruction. The high 24 bits are installed in bits 23 through 0.


Mitsubishi D10V relocs. This is a 10-bit reloc with the right 2 bits assumed to be 0.


Mitsubishi D10V relocs. This is a 10-bit reloc with the right 2 bits assumed to be 0. This is the same as the previous reloc except it is in the left container, i.e., shifted left 15 bits.

— : BFD_RELOC_D10V_18

This is an 18-bit reloc with the right 2 bits assumed to be 0.


This is an 18-bit reloc with the right 2 bits assumed to be 0.

— : BFD_RELOC_D30V_6

Mitsubishi D30V relocs. This is a 6-bit absolute reloc.


This is a 6-bit pc-relative reloc with the right 3 bits assumed to be 0.


This is a 6-bit pc-relative reloc with the right 3 bits assumed to be 0. Same as the previous reloc but on the right side of the container.

— : BFD_RELOC_D30V_15

This is a 12-bit absolute reloc with the right 3 bitsassumed to be 0.


This is a 12-bit pc-relative reloc with the right 3 bits assumed to be 0.


This is a 12-bit pc-relative reloc with the right 3 bits assumed to be 0. Same as the previous reloc but on the right side of the container.

— : BFD_RELOC_D30V_21

This is an 18-bit absolute reloc with the right 3 bits assumed to be 0.


This is an 18-bit pc-relative reloc with the right 3 bits assumed to be 0.


This is an 18-bit pc-relative reloc with the right 3 bits assumed to be 0. Same as the previous reloc but on the right side of the container.

— : BFD_RELOC_D30V_32

This is a 32-bit absolute reloc.


This is a 32-bit pc-relative reloc.


DLX relocs


DLX relocs


DLX relocs

— : BFD_RELOC_M32R_24

Renesas M32R (formerly Mitsubishi M32R) relocs. This is a 24 bit absolute address.


This is a 10-bit pc-relative reloc with the right 2 bits assumed to be 0.


This is an 18-bit reloc with the right 2 bits assumed to be 0.


This is a 26-bit reloc with the right 2 bits assumed to be 0.


This is a 16-bit reloc containing the high 16 bits of an address used when the lower 16 bits are treated as unsigned.


This is a 16-bit reloc containing the high 16 bits of an address used when the lower 16 bits are treated as signed.


This is a 16-bit reloc containing the lower 16 bits of an address.


This is a 16-bit reloc containing the small data area offset for use in add3, load, and store instructions.


For PIC.


This is a 9-bit reloc


This is a 22-bit reloc


This is a 16 bit offset from the short data area pointer.


This is a 16 bit offset (of which only 15 bits are used) from the short data area pointer.


This is a 16 bit offset from the zero data area pointer.


This is a 16 bit offset (of which only 15 bits are used) from the zero data area pointer.


This is an 8 bit offset (of which only 6 bits are used) from the tiny data area pointer.


This is an 8bit offset (of which only 7 bits are used) from the tiny data area pointer.


This is a 7 bit offset from the tiny data area pointer.


This is a 16 bit offset from the tiny data area pointer.


This is a 5 bit offset (of which only 4 bits are used) from the tiny data area pointer.


This is a 4 bit offset from the tiny data area pointer.


This is a 16 bit offset from the short data area pointer, with the bits placed non-contiguously in the instruction.


This is a 16 bit offset from the zero data area pointer, with the bits placed non-contiguously in the instruction.


This is a 6 bit offset from the call table base pointer.


This is a 16 bit offset from the call table base pointer.


Used for relaxing indirect function calls.


Used for relaxing indirect jumps.


Used to maintain alignment whilst relaxing.


This is a variation of BFD_RELOC_LO16 that can be used in v850e ld.bu instructions.

— : BFD_RELOC_MN10300_32_PCREL

This is a 32bit pcrel reloc for the mn10300, offset by two bytes in the instruction.

— : BFD_RELOC_MN10300_16_PCREL

This is a 16bit pcrel reloc for the mn10300, offset by two bytes in the instruction.


This is a 8bit DP reloc for the tms320c30, where the most significant 8 bits of a 24 bit word are placed into the least significant 8 bits of the opcode.


This is a 7bit reloc for the tms320c54x, where the least significant 7 bits of a 16 bit word are placed into the least significant 7 bits of the opcode.


This is a 9bit DP reloc for the tms320c54x, where the most significant 9 bits of a 16 bit word are placed into the least significant 9 bits of the opcode.


This is an extended address 23-bit reloc for the tms320c54x.

— : BFD_RELOC_TIC54X_16_OF_23

This is a 16-bit reloc for the tms320c54x, where the least significant 16 bits of a 23-bit extended address are placed into the opcode.


This is a reloc for the tms320c54x, where the most significant 7 bits of a 23-bit extended address are placed into the opcode.

— : BFD_RELOC_FR30_48

This is a 48 bit reloc for the FR30 that stores 32 bits.

— : BFD_RELOC_FR30_20

This is a 32 bit reloc for the FR30 that stores 20 bits split up into two sections.

— : BFD_RELOC_FR30_6_IN_4

This is a 16 bit reloc for the FR30 that stores a 6 bit word offset in 4 bits.

— : BFD_RELOC_FR30_8_IN_8

This is a 16 bit reloc for the FR30 that stores an 8 bit byte offset into 8 bits.

— : BFD_RELOC_FR30_9_IN_8

This is a 16 bit reloc for the FR30 that stores a 9 bit short offset into 8 bits.

— : BFD_RELOC_FR30_10_IN_8

This is a 16 bit reloc for the FR30 that stores a 10 bit word offset into 8 bits.


This is a 16 bit reloc for the FR30 that stores a 9 bit pc relative short offset into 8 bits.


This is a 16 bit reloc for the FR30 that stores a 12 bit pc relative short offset into 11 bits.


Motorola Mcore relocations.


These are relocations for the GETA instruction.


These are relocations for a conditional branch instruction.


These are relocations for the PUSHJ instruction.


These are relocations for the JMP instruction.


This is a relocation for a relative address as in a GETA instruction or a branch.


This is a relocation for a relative address as in a JMP instruction.


This is a relocation for an instruction field that may be a general register or a value 0..255.


This is a relocation for an instruction field that may be a general register.


This is a relocation for two instruction fields holding a register and an offset, the equivalent of the relocation.


This relocation is an assertion that the expression is not allocated as a global register. It does not modify contents.


This is a 16 bit reloc for the AVR that stores 8 bit pc relative short offset into 7 bits.


This is a 16 bit reloc for the AVR that stores 13 bit pc relative short offset into 12 bits.


This is a 16 bit reloc for the AVR that stores 17 bit value (usually program memory address) into 16 bits.


This is a 16 bit reloc for the AVR that stores 8 bit value (usually data memory address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores 8 bit value (high 8 bit of data memory address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores 8 bit value (most high 8 bit of program memory address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (usually data memory address) into 8 bit immediate value of SUBI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (high 8 bit of data memory address) into 8 bit immediate value of SUBI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (most high 8 bit of program memory address) into 8 bit immediate value of LDI or SUBI insn.


This is a 16 bit reloc for the AVR that stores 8 bit value (usually command address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores 8 bit value (high 8 bit of command address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores 8 bit value (most high 8 bit of command address) into 8 bit immediate value of LDI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (usually command address) into 8 bit immediate value of SUBI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (high 8 bit of 16 bit command address) into 8 bit immediate value of SUBI insn.


This is a 16 bit reloc for the AVR that stores negated 8 bit value (high 6 bit of 22 bit command address) into 8 bit immediate value of SUBI insn.


This is a 32 bit reloc for the AVR that stores 23 bit value into 22 bits.


This is a 16 bit reloc for the AVR that stores all needed bits for absolute addressing with ldi with overflow check to linktime


This is a 6 bit reloc for the AVR that stores offset for ldd/std instructions


This is a 6 bit reloc for the AVR that stores offset for adiw/sbiw instructions

— : BFD_RELOC_390_12

Direct 12 bit.

— : BFD_RELOC_390_GOT12

12 bit GOT offset.

— : BFD_RELOC_390_PLT32

32 bit PC relative PLT address.


Copy symbol at runtime.


Create GOT entry.


Create PLT entry.


Adjust by program base.


32 bit PC relative offset to GOT.

— : BFD_RELOC_390_GOT16

16 bit GOT offset.


PC relative 16 bit shifted by 1.


16 bit PC rel. PLT shifted by 1.


PC relative 32 bit shifted by 1.


32 bit PC rel. PLT shifted by 1.


32 bit PC rel. GOT shifted by 1.

— : BFD_RELOC_390_GOT64

64 bit GOT offset.

— : BFD_RELOC_390_PLT64

64 bit PC relative PLT address.


32 bit rel. offset to GOT entry.


64 bit offset to GOT.


12-bit offset to symbol-entry within GOT, with PLT handling.


16-bit offset to symbol-entry within GOT, with PLT handling.


32-bit offset to symbol-entry within GOT, with PLT handling.


64-bit offset to symbol-entry within GOT, with PLT handling.


32-bit rel. offset to symbol-entry within GOT, with PLT handling.


16-bit rel. offset from the GOT to a PLT entry.


32-bit rel. offset from the GOT to a PLT entry.


64-bit rel. offset from the GOT to a PLT entry.

— : BFD_RELOC_390_TLS_GD32
— : BFD_RELOC_390_TLS_GD64
— : BFD_RELOC_390_TLS_IE32
— : BFD_RELOC_390_TLS_IE64
— : BFD_RELOC_390_TLS_LE32
— : BFD_RELOC_390_TLS_LE64

s390 tls relocations.

— : BFD_RELOC_390_20
— : BFD_RELOC_390_GOT20

Long displacement extension.


Scenix IP2K - 9-bit register number / data address


Scenix IP2K - 4-bit register/data bank number


Scenix IP2K - low 13 bits of instruction word address


Scenix IP2K - high 3 bits of instruction word address


Scenix IP2K - ext/low/high 8 bits of data address


Scenix IP2K - low/high 8 bits of instruction word address


Scenix IP2K - even/odd PC modifier to modify snb pcl.0


Scenix IP2K - 16 bit word address in text section.


Scenix IP2K - 7-bit sp or dp offset


Scenix VPE4K coprocessor - data/insn-space addressing


These two relocations are used by the linker to determine which of the entries in a C++ virtual function table are actually used. When the –gc-sections option is given, the linker will zero out the entries that are not used, so that the code for those functions need not be included in the output.

VTABLE_INHERIT is a zero-space relocation used to describe to the linker the inheritance tree of a C++ virtual function table. The relocation's symbol should be the parent class' vtable, and the relocation should be located at the child vtable.

VTABLE_ENTRY is a zero-space relocation that describes the use of a virtual function table entry. The reloc's symbol should refer to the table of the class mentioned in the code. Off of that base, an offset describes the entry that is being used. For Rela hosts, this offset is stored in the reloc's addend. For Rel hosts, we are forced to put this offset in the reloc's section offset.


Intel IA64 Relocations.


Motorola 68HC11 reloc. This is the 8 bit high part of an absolute address.


Motorola 68HC11 reloc. This is the 8 bit low part of an absolute address.

— : BFD_RELOC_M68HC11_3B

Motorola 68HC11 reloc. This is the 3 bit of a value.


Motorola 68HC11 reloc. This reloc marks the beginning of a jump/call instruction. It is used for linker relaxation to correctly identify beginning of instruction and change some branches to use PC-relative addressing mode.


Motorola 68HC11 reloc. This reloc marks a group of several instructions that gcc generates and for which the linker relaxation pass can modify and/or remove some of them.

— : BFD_RELOC_M68HC11_LO16

Motorola 68HC11 reloc. This is the 16-bit lower part of an address. It is used for 'call' instruction to specify the symbol address without any special transformation (due to memory bank window).


Motorola 68HC11 reloc. This is a 8-bit reloc that specifies the page number of an address. It is used by 'call' instruction to specify the page number of the symbol.

— : BFD_RELOC_M68HC11_24

Motorola 68HC11 reloc. This is a 24-bit reloc that represents the address with a 16-bit value and a 8-bit page number. The symbol address is transformed to follow the 16K memory bank of 68HC12 (seen as mapped in the window).

— : BFD_RELOC_M68HC12_5B

Motorola 68HC12 reloc. This is the 5 bits of a value.

— : BFD_RELOC_16C_REG04a
— : BFD_RELOC_16C_REG04a_C

NS CR16C Relocations.


NS CRX Relocations.


These relocs are only used within the CRIS assembler. They are not (at present) written to any object files.


Relocs used in ELF shared libraries for CRIS.


32-bit offset to symbol-entry within GOT.


16-bit offset to symbol-entry within GOT.


32-bit offset to symbol-entry within GOT, with PLT handling.


16-bit offset to symbol-entry within GOT, with PLT handling.


32-bit offset to symbol, relative to GOT.


32-bit offset to symbol with PLT entry, relative to GOT.


32-bit offset to symbol with PLT entry, relative to this relocation.

— : BFD_RELOC_860_PC26
— : BFD_RELOC_860_PLT26
— : BFD_RELOC_860_PC16
— : BFD_RELOC_860_LOW0
— : BFD_RELOC_860_LOW1
— : BFD_RELOC_860_LOW2
— : BFD_RELOC_860_LOW3

Intel i860 Relocations.


OpenRISC Relocations.


H8 elf Relocations.


Sony Xstormy16 Relocations.


Relocations used by VAX ELF.

— : BFD_RELOC_MSP430_16

msp430 specific relocation codes

— : BFD_RELOC_IQ2000_UHI16

IQ2000 Relocations.


Special Xtensa relocation used only by PLT entries in ELF shared objects to indicate that the runtime linker should set the value to one of its own internal functions or data structures.


Xtensa relocations for ELF shared objects.


Xtensa relocation used in ELF object files for symbols that may require PLT entries. Otherwise, this is just a generic 32-bit relocation.


Xtensa relocations to mark the difference of two local symbols. These are only needed to support linker relaxation and can be ignored when not relaxing. The field is set to the value of the difference assuming no relaxation. The relocation encodes the position of the first symbol so the linker can determine whether to adjust the field value.


Generic Xtensa relocations for instruction operands. Only the slot number is encoded in the relocation. The relocation applies to the last PC-relative immediate operand, or if there are no PC-relative immediates, to the last immediate operand.


Alternate Xtensa relocations. Only the slot is encoded in the relocation. The meaning of these relocations is opcode-specific.


Xtensa relocations for backward compatibility. These have all been replaced by BFD_RELOC_XTENSA_SLOT0_OP.


Xtensa relocation to mark that the assembler expanded the instructions from an original target. The expansion size is encoded in the reloc size.


Xtensa relocation to mark that the linker should simplify assembler-expanded instructions. This is commonly used internally by the linker after analysis of a BFD_RELOC_XTENSA_ASM_EXPAND.

     typedef enum bfd_reloc_code_real bfd_reloc_code_real_type; bfd_reloc_type_lookup


     reloc_howto_type *bfd_reloc_type_lookup
        (bfd *abfd, bfd_reloc_code_real_type code);

Return a pointer to a howto structure which, when invoked, will perform the relocation code on data from the architecture noted. bfd_default_reloc_type_lookup


     reloc_howto_type *bfd_default_reloc_type_lookup
        (bfd *abfd, bfd_reloc_code_real_type  code);

Provides a default relocation lookup routine for any architecture. bfd_get_reloc_code_name


     const char *bfd_get_reloc_code_name (bfd_reloc_code_real_type code);

Provides a printable name for the supplied relocation code. Useful mainly for printing error messages. bfd_generic_relax_section


     bfd_boolean bfd_generic_relax_section
        (bfd *abfd,
         asection *section,
         struct bfd_link_info *,
         bfd_boolean *);

Provides default handling for relaxing for back ends which don't do relaxing. bfd_generic_gc_sections


     bfd_boolean bfd_generic_gc_sections
        (bfd *, struct bfd_link_info *);

Provides default handling for relaxing for back ends which don't do section gc – i.e., does nothing besides the special case for marking sections having global symbols. bfd_generic_merge_sections


     bfd_boolean bfd_generic_merge_sections
        (bfd *, struct bfd_link_info *);

Provides default handling for SEC_MERGE section merging for back ends which don't have SEC_MERGE support – i.e., does nothing. bfd_generic_get_relocated_section_contents


     bfd_byte *bfd_generic_get_relocated_section_contents
        (bfd *abfd,
         struct bfd_link_info *link_info,
         struct bfd_link_order *link_order,
         bfd_byte *data,
         bfd_boolean relocatable,
         asymbol **symbols);

Provides default handling of relocation effort for back ends which can't be bothered to do it efficiently.

Next: , Previous: Relocations, Up: BFD front end

2.12 Core files

These are functions pertaining to core files. bfd_core_file_failing_command


     const char *bfd_core_file_failing_command (bfd *abfd);

Return a read-only string explaining which program was running when it failed and produced the core file abfd. bfd_core_file_failing_signal


     int bfd_core_file_failing_signal (bfd *abfd);

Returns the signal number which caused the core dump which generated the file the BFD abfd is attached to. core_file_matches_executable_p


     bfd_boolean core_file_matches_executable_p
        (bfd *core_bfd, bfd *exec_bfd);

Return TRUE if the core file attached to core_bfd was generated by a run of the executable file attached to exec_bfd, FALSE otherwise.

Next: , Previous: Core Files, Up: BFD front end

2.13 Targets

Each port of BFD to a different machine requires the creation of a target back end. All the back end provides to the root part of BFD is a structure containing pointers to functions which perform certain low level operations on files. BFD translates the applications's requests through a pointer into calls to the back end routines.

When a file is opened with bfd_openr, its format and target are unknown. BFD uses various mechanisms to determine how to interpret the file. The operations performed are:

Once the BFD has been opened and the target selected, the file format may be determined. This is done by calling bfd_check_format on the BFD with a suggested format. If target_defaulted has been set, each possible target type is tried to see if it recognizes the specified format. bfd_check_format returns TRUE when the caller guesses right.

Previous: Targets, Up: Targets

2.13.1 bfd_target

This structure contains everything that BFD knows about a target. It includes things like its byte order, name, and which routines to call to do various operations.

Every BFD points to a target structure with its xvec member.

The macros below are used to dispatch to functions through the bfd_target vector. They are used in a number of macros further down in bfd.h, and are also used when calling various routines by hand inside the BFD implementation. The arglist argument must be parenthesized; it contains all the arguments to the called function.

They make the documentation (more) unpleasant to read, so if someone wants to fix this and not break the above, please do.

     #define BFD_SEND(bfd, message, arglist) \
       ((*((bfd)->xvec->message)) arglist)
     #ifdef DEBUG_BFD_SEND
     #undef BFD_SEND
     #define BFD_SEND(bfd, message, arglist) \
       (((bfd) && (bfd)->xvec && (bfd)->xvec->message) ? \
         ((*((bfd)->xvec->message)) arglist) : \
         (bfd_assert (__FILE__,__LINE__), NULL))

For operations which index on the BFD format:

     #define BFD_SEND_FMT(bfd, message, arglist) \
       (((bfd)->xvec->message[(int) ((bfd)->format)]) arglist)
     #ifdef DEBUG_BFD_SEND
     #undef BFD_SEND_FMT
     #define BFD_SEND_FMT(bfd, message, arglist) \
       (((bfd) && (bfd)->xvec && (bfd)->xvec->message) ? \
        (((bfd)->xvec->message[(int) ((bfd)->format)]) arglist) : \
        (bfd_assert (__FILE__,__LINE__), NULL))

This is the structure which defines the type of BFD this is. The xvec member of the struct bfd itself points here. Each module that implements access to a different target under BFD, defines one of these.

FIXME, these names should be rationalised with the names of the entry points which call them. Too bad we can't have one macro to define them both!

     enum bfd_flavour
     /* Forward declaration.  */
     typedef struct bfd_link_info _bfd_link_info;
     typedef struct bfd_target
       /* Identifies the kind of target, e.g., SunOS4, Ultrix, etc.  */
       char *name;
      /* The "flavour" of a back end is a general indication about
         the contents of a file.  */
       enum bfd_flavour flavour;
       /* The order of bytes within the data area of a file.  */
       enum bfd_endian byteorder;
      /* The order of bytes within the header parts of a file.  */
       enum bfd_endian header_byteorder;
       /* A mask of all the flags which an executable may have set -
          from the set BFD_NO_FLAGS, HAS_RELOC, ...D_PAGED.  */
       flagword object_flags;
      /* A mask of all the flags which a section may have set - from
         the set SEC_NO_FLAGS, SEC_ALLOC, ...SET_NEVER_LOAD.  */
       flagword section_flags;
      /* The character normally found at the front of a symbol.
         (if any), perhaps `_'.  */
       char symbol_leading_char;
      /* The pad character for file names within an archive header.  */
       char ar_pad_char;
       /* The maximum number of characters in an archive header.  */
       unsigned short ar_max_namelen;
       /* Entries for byte swapping for data. These are different from the
          other entry points, since they don't take a BFD as the first argument.
          Certain other handlers could do the same.  */
       bfd_uint64_t   (*bfd_getx64) (const void *);
       bfd_int64_t    (*bfd_getx_signed_64) (const void *);
       void           (*bfd_putx64) (bfd_uint64_t, void *);
       bfd_vma        (*bfd_getx32) (const void *);
       bfd_signed_vma (*bfd_getx_signed_32) (const void *);
       void           (*bfd_putx32) (bfd_vma, void *);
       bfd_vma        (*bfd_getx16) (const void *);
       bfd_signed_vma (*bfd_getx_signed_16) (const void *);
       void           (*bfd_putx16) (bfd_vma, void *);
       /* Byte swapping for the headers.  */
       bfd_uint64_t   (*bfd_h_getx64) (const void *);
       bfd_int64_t    (*bfd_h_getx_signed_64) (const void *);
       void           (*bfd_h_putx64) (bfd_uint64_t, void *);
       bfd_vma        (*bfd_h_getx32) (const void *);
       bfd_signed_vma (*bfd_h_getx_signed_32) (const void *);
       void           (*bfd_h_putx32) (bfd_vma, void *);
       bfd_vma        (*bfd_h_getx16) (const void *);
       bfd_signed_vma (*bfd_h_getx_signed_16) (const void *);
       void           (*bfd_h_putx16) (bfd_vma, void *);
       /* Format dependent routines: these are vectors of entry points
          within the target vector structure, one for each format to check.  */
       /* Check the format of a file being read.  Return a bfd_target * or zero.  */
       const struct bfd_target *(*_bfd_check_format[bfd_type_end]) (bfd *);
       /* Set the format of a file being written.  */
       bfd_boolean (*_bfd_set_format[bfd_type_end]) (bfd *);
       /* Write cached information into a file being written, at bfd_close.  */
       bfd_boolean (*_bfd_write_contents[bfd_type_end]) (bfd *);

The general target vector. These vectors are initialized using the BFD_JUMP_TABLE macros.

       /* Generic entry points.  */
       NAME##_close_and_cleanup, \
       NAME##_bfd_free_cached_info, \
       NAME##_new_section_hook, \
       NAME##_get_section_contents, \
       /* Called when the BFD is being closed to do any necessary cleanup.  */
       bfd_boolean (*_close_and_cleanup) (bfd *);
       /* Ask the BFD to free all cached information.  */
       bfd_boolean (*_bfd_free_cached_info) (bfd *);
       /* Called when a new section is created.  */
       bfd_boolean (*_new_section_hook) (bfd *, sec_ptr);
       /* Read the contents of a section.  */
       bfd_boolean (*_bfd_get_section_contents)
         (bfd *, sec_ptr, void *, file_ptr, bfd_size_type);
       bfd_boolean (*_bfd_get_section_contents_in_window)
         (bfd *, sec_ptr, bfd_window *, file_ptr, bfd_size_type);
       /* Entry points to copy private data.  */
     #define BFD_JUMP_TABLE_COPY(NAME) \
       NAME##_bfd_copy_private_bfd_data, \
       NAME##_bfd_merge_private_bfd_data, \
       NAME##_bfd_copy_private_section_data, \
       NAME##_bfd_copy_private_symbol_data, \
       NAME##_bfd_copy_private_header_data, \
       NAME##_bfd_set_private_flags, \
       /* Called to copy BFD general private data from one object file
          to another.  */
       bfd_boolean (*_bfd_copy_private_bfd_data) (bfd *, bfd *);
       /* Called to merge BFD general private data from one object file
          to a common output file when linking.  */
       bfd_boolean (*_bfd_merge_private_bfd_data) (bfd *, bfd *);
       /* Called to copy BFD private section data from one object file
          to another.  */
       bfd_boolean (*_bfd_copy_private_section_data)
         (bfd *, sec_ptr, bfd *, sec_ptr);
       /* Called to copy BFD private symbol data from one symbol
          to another.  */
       bfd_boolean (*_bfd_copy_private_symbol_data)
         (bfd *, asymbol *, bfd *, asymbol *);
       /* Called to copy BFD private header data from one object file
          to another.  */
       bfd_boolean (*_bfd_copy_private_header_data)
         (bfd *, bfd *);
       /* Called to set private backend flags.  */
       bfd_boolean (*_bfd_set_private_flags) (bfd *, flagword);
       /* Called to print private BFD data.  */
       bfd_boolean (*_bfd_print_private_bfd_data) (bfd *, void *);
       /* Core file entry points.  */
     #define BFD_JUMP_TABLE_CORE(NAME) \
       NAME##_core_file_failing_command, \
       NAME##_core_file_failing_signal, \
       char *      (*_core_file_failing_command) (bfd *);
       int         (*_core_file_failing_signal) (bfd *);
       bfd_boolean (*_core_file_matches_executable_p) (bfd *, bfd *);
       /* Archive entry points.  */
       NAME##_slurp_armap, \
       NAME##_slurp_extended_name_table, \
       NAME##_construct_extended_name_table, \
       NAME##_truncate_arname, \
       NAME##_write_armap, \
       NAME##_read_ar_hdr, \
       NAME##_openr_next_archived_file, \
       NAME##_get_elt_at_index, \
       NAME##_generic_stat_arch_elt, \
       bfd_boolean (*_bfd_slurp_armap) (bfd *);
       bfd_boolean (*_bfd_slurp_extended_name_table) (bfd *);
       bfd_boolean (*_bfd_construct_extended_name_table)
         (bfd *, char **, bfd_size_type *, const char **);
       void        (*_bfd_truncate_arname) (bfd *, const char *, char *);
       bfd_boolean (*write_armap)
         (bfd *, unsigned int, struct orl *, unsigned int, int);
       void *      (*_bfd_read_ar_hdr_fn) (bfd *);
       bfd *       (*openr_next_archived_file) (bfd *, bfd *);
     #define bfd_get_elt_at_index(b,i) BFD_SEND (b, _bfd_get_elt_at_index, (b,i))
       bfd *       (*_bfd_get_elt_at_index) (bfd *, symindex);
       int         (*_bfd_stat_arch_elt) (bfd *, struct stat *);
       bfd_boolean (*_bfd_update_armap_timestamp) (bfd *);
       /* Entry points used for symbols.  */
       NAME##_get_symtab_upper_bound, \
       NAME##_canonicalize_symtab, \
       NAME##_make_empty_symbol, \
       NAME##_print_symbol, \
       NAME##_get_symbol_info, \
       NAME##_bfd_is_local_label_name, \
       NAME##_bfd_is_target_special_symbol, \
       NAME##_get_lineno, \
       NAME##_find_nearest_line, \
       _bfd_generic_find_line, \
       NAME##_find_inliner_info, \
       NAME##_bfd_make_debug_symbol, \
       NAME##_read_minisymbols, \
       long        (*_bfd_get_symtab_upper_bound) (bfd *);
       long        (*_bfd_canonicalize_symtab)
         (bfd *, struct bfd_symbol **);
       struct bfd_symbol *
                   (*_bfd_make_empty_symbol) (bfd *);
       void        (*_bfd_print_symbol)
         (bfd *, void *, struct bfd_symbol *, bfd_print_symbol_type);
     #define bfd_print_symbol(b,p,s,e) BFD_SEND (b, _bfd_print_symbol, (b,p,s,e))
       void        (*_bfd_get_symbol_info)
         (bfd *, struct bfd_symbol *, symbol_info *);
     #define bfd_get_symbol_info(b,p,e) BFD_SEND (b, _bfd_get_symbol_info, (b,p,e))
       bfd_boolean (*_bfd_is_local_label_name) (bfd *, const char *);
       bfd_boolean (*_bfd_is_target_special_symbol) (bfd *, asymbol *);
       alent *     (*_get_lineno) (bfd *, struct bfd_symbol *);
       bfd_boolean (*_bfd_find_nearest_line)
         (bfd *, struct bfd_section *, struct bfd_symbol **, bfd_vma,
          const char **, const char **, unsigned int *);
       bfd_boolean (*_bfd_find_line)
         (bfd *, struct bfd_symbol **, struct bfd_symbol *,
          const char **, unsigned int *);
       bfd_boolean (*_bfd_find_inliner_info)
         (bfd *, const char **, const char **, unsigned int *);
      /* Back-door to allow format-aware applications to create debug symbols
         while using BFD for everything else.  Currently used by the assembler
         when creating COFF files.  */
       asymbol *   (*_bfd_make_debug_symbol)
         (bfd *, void *, unsigned long size);
     #define bfd_read_minisymbols(b, d, m, s) \
       BFD_SEND (b, _read_minisymbols, (b, d, m, s))
       long        (*_read_minisymbols)
         (bfd *, bfd_boolean, void **, unsigned int *);
     #define bfd_minisymbol_to_symbol(b, d, m, f) \
       BFD_SEND (b, _minisymbol_to_symbol, (b, d, m, f))
       asymbol *   (*_minisymbol_to_symbol)
         (bfd *, bfd_boolean, const void *, asymbol *);
       /* Routines for relocs.  */
       NAME##_get_reloc_upper_bound, \
       NAME##_canonicalize_reloc, \
       long        (*_get_reloc_upper_bound) (bfd *, sec_ptr);
       long        (*_bfd_canonicalize_reloc)
         (bfd *, sec_ptr, arelent **, struct bfd_symbol **);
       /* See documentation on reloc types.  */
       reloc_howto_type *
                   (*reloc_type_lookup) (bfd *, bfd_reloc_code_real_type);
       /* Routines used when writing an object file.  */
       NAME##_set_arch_mach, \
       bfd_boolean (*_bfd_set_arch_mach)
         (bfd *, enum bfd_architecture, unsigned long);
       bfd_boolean (*_bfd_set_section_contents)
         (bfd *, sec_ptr, const void *, file_ptr, bfd_size_type);
       /* Routines used by the linker.  */
     #define BFD_JUMP_TABLE_LINK(NAME) \
       NAME##_sizeof_headers, \
       NAME##_bfd_get_relocated_section_contents, \
       NAME##_bfd_relax_section, \
       NAME##_bfd_link_hash_table_create, \
       NAME##_bfd_link_hash_table_free, \
       NAME##_bfd_link_add_symbols, \
       NAME##_bfd_link_just_syms, \
       NAME##_bfd_final_link, \
       NAME##_bfd_link_split_section, \
       NAME##_bfd_gc_sections, \
       NAME##_bfd_merge_sections, \
       NAME##_bfd_is_group_section, \
       NAME##_bfd_discard_group, \
       NAME##_section_already_linked \
       int         (*_bfd_sizeof_headers) (bfd *, bfd_boolean);
       bfd_byte *  (*_bfd_get_relocated_section_contents)
         (bfd *, struct bfd_link_info *, struct bfd_link_order *,
          bfd_byte *, bfd_boolean, struct bfd_symbol **);
       bfd_boolean (*_bfd_relax_section)
         (bfd *, struct bfd_section *, struct bfd_link_info *, bfd_boolean *);
       /* Create a hash table for the linker.  Different backends store
          different information in this table.  */
       struct bfd_link_hash_table *
                   (*_bfd_link_hash_table_create) (bfd *);
       /* Release the memory associated with the linker hash table.  */
       void        (*_bfd_link_hash_table_free) (struct bfd_link_hash_table *);
       /* Add symbols from this object file into the hash table.  */
       bfd_boolean (*_bfd_link_add_symbols) (bfd *, struct bfd_link_info *);
       /* Indicate that we are only retrieving symbol values from this section.  */
       void        (*_bfd_link_just_syms) (asection *, struct bfd_link_info *);
       /* Do a link based on the link_order structures attached to each
          section of the BFD.  */
       bfd_boolean (*_bfd_final_link) (bfd *, struct bfd_link_info *);
       /* Should this section be split up into smaller pieces during linking.  */
       bfd_boolean (*_bfd_link_split_section) (bfd *, struct bfd_section *);
       /* Remove sections that are not referenced from the output.  */
       bfd_boolean (*_bfd_gc_sections) (bfd *, struct bfd_link_info *);
       /* Attempt to merge SEC_MERGE sections.  */
       bfd_boolean (*_bfd_merge_sections) (bfd *, struct bfd_link_info *);
       /* Is this section a member of a group?  */
       bfd_boolean (*_bfd_is_group_section) (bfd *, const struct bfd_section *);
       /* Discard members of a group.  */
       bfd_boolean (*_bfd_discard_group) (bfd *, struct bfd_section *);
       /* Check if SEC has been already linked during a reloceatable or
          final link.  */
       void (*_section_already_linked) (bfd *, struct bfd_section *);
       /* Routines to handle dynamic symbols and relocs.  */
       NAME##_get_dynamic_symtab_upper_bound, \
       NAME##_canonicalize_dynamic_symtab, \
       NAME##_get_synthetic_symtab, \
       NAME##_get_dynamic_reloc_upper_bound, \
       /* Get the amount of memory required to hold the dynamic symbols.  */
       long        (*_bfd_get_dynamic_symtab_upper_bound) (bfd *);
       /* Read in the dynamic symbols.  */
       long        (*_bfd_canonicalize_dynamic_symtab)
         (bfd *, struct bfd_symbol **);
       /* Create synthetized symbols.  */
       long        (*_bfd_get_synthetic_symtab)
         (bfd *, long, struct bfd_symbol **, long, struct bfd_symbol **,
          struct bfd_symbol **);
       /* Get the amount of memory required to hold the dynamic relocs.  */
       long        (*_bfd_get_dynamic_reloc_upper_bound) (bfd *);
       /* Read in the dynamic relocs.  */
       long        (*_bfd_canonicalize_dynamic_reloc)
         (bfd *, arelent **, struct bfd_symbol **);

A pointer to an alternative bfd_target in case the current one is not satisfactory. This can happen when the target cpu supports both big and little endian code, and target chosen by the linker has the wrong endianness. The function open_output() in ld/ldlang.c uses this field to find an alternative output format that is suitable.

       /* Opposite endian version of this target.  */
       const struct bfd_target * alternative_target;
       /* Data for use by back-end routines, which isn't
          generic enough to belong in this structure.  */
       const void *backend_data;
     } bfd_target; bfd_set_default_target


     bfd_boolean bfd_set_default_target (const char *name);

Set the default target vector to use when recognizing a BFD. This takes the name of the target, which may be a BFD target name or a configuration triplet. bfd_find_target


     const bfd_target *bfd_find_target (const char *target_name, bfd *abfd);

Return a pointer to the transfer vector for the object target named target_name. If target_name is NULL, choose the one in the environment variable GNUTARGET; if that is null or not defined, then choose the first entry in the target list. Passing in the string "default" or setting the environment variable to "default" will cause the first entry in the target list to be returned, and "target_defaulted" will be set in the BFD. This causes bfd_check_format to loop over all the targets to find the one that matches the file being read. bfd_target_list


     const char ** bfd_target_list (void);

Return a freshly malloced NULL-terminated vector of the names of all the valid BFD targets. Do not modify the names. bfd_seach_for_target


     const bfd_target *bfd_search_for_target
        (int (*search_func) (const bfd_target *, void *),
         void *);

Return a pointer to the first transfer vector in the list of transfer vectors maintained by BFD that produces a non-zero result when passed to the function search_func. The parameter data is passed, unexamined, to the search function.

Next: , Previous: Targets, Up: BFD front end

2.14 Architectures

BFD keeps one atom in a BFD describing the architecture of the data attached to the BFD: a pointer to a bfd_arch_info_type.

Pointers to structures can be requested independently of a BFD so that an architecture's information can be interrogated without access to an open BFD.

The architecture information is provided by each architecture package. The set of default architectures is selected by the macro SELECT_ARCHITECTURES. This is normally set up in the config/ file of your choice. If the name is not defined, then all the architectures supported are included.

When BFD starts up, all the architectures are called with an initialize method. It is up to the architecture back end to insert as many items into the list of architectures as it wants to; generally this would be one for each machine and one for the default case (an item with a machine field of 0).

BFD's idea of an architecture is implemented in archures.c.

2.14.1 bfd_architecture

This enum gives the object file's CPU architecture, in a global sense—i.e., what processor family does it belong to? Another field indicates which processor within the family is in use. The machine gives a number which distinguishes different versions of the architecture, containing, for example, 2 and 3 for Intel i960 KA and i960 KB, and 68020 and 68030 for Motorola 68020 and 68030.

     enum bfd_architecture
       bfd_arch_unknown,   /* File arch not known.  */
       bfd_arch_obscure,   /* Arch known, not one of these.  */
       bfd_arch_m68k,      /* Motorola 68xxx */
     #define bfd_mach_m68000 1
     #define bfd_mach_m68008 2
     #define bfd_mach_m68010 3
     #define bfd_mach_m68020 4
     #define bfd_mach_m68030 5
     #define bfd_mach_m68040 6
     #define bfd_mach_m68060 7
     #define bfd_mach_cpu32  8
     #define bfd_mach_mcf5200  9
     #define bfd_mach_mcf5206e 10
     #define bfd_mach_mcf5307  11
     #define bfd_mach_mcf5407  12
     #define bfd_mach_mcf528x  13
     #define bfd_mach_mcfv4e   14
     #define bfd_mach_mcf521x   15
     #define bfd_mach_mcf5249   16
     #define bfd_mach_mcf547x   17
     #define bfd_mach_mcf548x   18
       bfd_arch_vax,       /* DEC Vax */
       bfd_arch_i960,      /* Intel 960 */
         /* The order of the following is important.
            lower number indicates a machine type that
            only accepts a subset of the instructions
            available to machines with higher numbers.
            The exception is the "ca", which is
            incompatible with all other machines except
            "core".  */
     #define bfd_mach_i960_core      1
     #define bfd_mach_i960_ka_sa     2
     #define bfd_mach_i960_kb_sb     3
     #define bfd_mach_i960_mc        4
     #define bfd_mach_i960_xa        5
     #define bfd_mach_i960_ca        6
     #define bfd_mach_i960_jx        7
     #define bfd_mach_i960_hx        8
       bfd_arch_or32,      /* OpenRISC 32 */
       bfd_arch_a29k,      /* AMD 29000 */
       bfd_arch_sparc,     /* SPARC */
     #define bfd_mach_sparc                 1
     /* The difference between v8plus and v9 is that v9 is a true 64 bit env.  */
     #define bfd_mach_sparc_sparclet        2
     #define bfd_mach_sparc_sparclite       3
     #define bfd_mach_sparc_v8plus          4
     #define bfd_mach_sparc_v8plusa         5 /* with ultrasparc add'ns.  */
     #define bfd_mach_sparc_sparclite_le    6
     #define bfd_mach_sparc_v9              7
     #define bfd_mach_sparc_v9a             8 /* with ultrasparc add'ns.  */
     #define bfd_mach_sparc_v8plusb         9 /* with cheetah add'ns.  */
     #define bfd_mach_sparc_v9b             10 /* with cheetah add'ns.  */
     /* Nonzero if MACH has the v9 instruction set.  */
     #define bfd_mach_sparc_v9_p(mach) \
       ((mach) >= bfd_mach_sparc_v8plus && (mach) <= bfd_mach_sparc_v9b \
        && (mach) != bfd_mach_sparc_sparclite_le)
     /* Nonzero if MACH is a 64 bit sparc architecture.  */
     #define bfd_mach_sparc_64bit_p(mach) \
       ((mach) >= bfd_mach_sparc_v9 && (mach) != bfd_mach_sparc_v8plusb)
       bfd_arch_mips,      /* MIPS Rxxxx */
     #define bfd_mach_mips3000              3000
     #define bfd_mach_mips3900              3900
     #define bfd_mach_mips4000              4000
     #define bfd_mach_mips4010              4010
     #define bfd_mach_mips4100              4100
     #define bfd_mach_mips4111              4111
     #define bfd_mach_mips4120              4120
     #define bfd_mach_mips4300              4300
     #define bfd_mach_mips4400              4400
     #define bfd_mach_mips4600              4600
     #define bfd_mach_mips4650              4650
     #define bfd_mach_mips5000              5000
     #define bfd_mach_mips5400              5400
     #define bfd_mach_mips5500              5500
     #define bfd_mach_mips6000              6000
     #define bfd_mach_mips7000              7000
     #define bfd_mach_mips8000              8000
     #define bfd_mach_mips9000              9000
     #define bfd_mach_mips10000             10000
     #define bfd_mach_mips12000             12000
     #define bfd_mach_mips16                16
     #define bfd_mach_mips5                 5
     #define bfd_mach_mips_sb1              12310201 /* octal 'SB', 01 */
     #define bfd_mach_mipsisa32             32
     #define bfd_mach_mipsisa32r2           33
     #define bfd_mach_mipsisa64             64
     #define bfd_mach_mipsisa64r2           65
       bfd_arch_i386,      /* Intel 386 */
     #define bfd_mach_i386_i386 1
     #define bfd_mach_i386_i8086 2
     #define bfd_mach_i386_i386_intel_syntax 3
     #define bfd_mach_x86_64 64
     #define bfd_mach_x86_64_intel_syntax 65
       bfd_arch_we32k,     /* AT&T WE32xxx */
       bfd_arch_tahoe,     /* CCI/Harris Tahoe */
       bfd_arch_i860,      /* Intel 860 */
       bfd_arch_i370,      /* IBM 360/370 Mainframes */
       bfd_arch_romp,      /* IBM ROMP PC/RT */
       bfd_arch_alliant,   /* Alliant */
       bfd_arch_convex,    /* Convex */
       bfd_arch_m88k,      /* Motorola 88xxx */
       bfd_arch_m98k,      /* Motorola 98xxx */
       bfd_arch_pyramid,   /* Pyramid Technology */
       bfd_arch_h8300,     /* Renesas H8/300 (formerly Hitachi H8/300) */
     #define bfd_mach_h8300    1
     #define bfd_mach_h8300h   2
     #define bfd_mach_h8300s   3
     #define bfd_mach_h8300hn  4
     #define bfd_mach_h8300sn  5
     #define bfd_mach_h8300sx  6
     #define bfd_mach_h8300sxn 7
       bfd_arch_pdp11,     /* DEC PDP-11 */
       bfd_arch_powerpc,   /* PowerPC */
     #define bfd_mach_ppc           32
     #define bfd_mach_ppc64         64
     #define bfd_mach_ppc_403       403
     #define bfd_mach_ppc_403gc     4030
     #define bfd_mach_ppc_505       505
     #define bfd_mach_ppc_601       601
     #define bfd_mach_ppc_602       602
     #define bfd_mach_ppc_603       603
     #define bfd_mach_ppc_ec603e    6031
     #define bfd_mach_ppc_604       604
     #define bfd_mach_ppc_620       620
     #define bfd_mach_ppc_630       630
     #define bfd_mach_ppc_750       750
     #define bfd_mach_ppc_860       860
     #define bfd_mach_ppc_a35       35
     #define bfd_mach_ppc_rs64ii    642
     #define bfd_mach_ppc_rs64iii   643
     #define bfd_mach_ppc_7400      7400
     #define bfd_mach_ppc_e500      500
       bfd_arch_rs6000,    /* IBM RS/6000 */
     #define bfd_mach_rs6k          6000
     #define bfd_mach_rs6k_rs1      6001
     #define bfd_mach_rs6k_rsc      6003
     #define bfd_mach_rs6k_rs2      6002
       bfd_arch_hppa,      /* HP PA RISC */
     #define bfd_mach_hppa10        10
     #define bfd_mach_hppa11        11
     #define bfd_mach_hppa20        20
     #define bfd_mach_hppa20w       25
       bfd_arch_d10v,      /* Mitsubishi D10V */
     #define bfd_mach_d10v          1
     #define bfd_mach_d10v_ts2      2
     #define bfd_mach_d10v_ts3      3
       bfd_arch_d30v,      /* Mitsubishi D30V */
       bfd_arch_dlx,       /* DLX */
       bfd_arch_m68hc11,   /* Motorola 68HC11 */
       bfd_arch_m68hc12,   /* Motorola 68HC12 */
     #define bfd_mach_m6812_default 0
     #define bfd_mach_m6812         1
     #define bfd_mach_m6812s        2
       bfd_arch_z8k,       /* Zilog Z8000 */
     #define bfd_mach_z8001         1
     #define bfd_mach_z8002         2
       bfd_arch_h8500,     /* Renesas H8/500 (formerly Hitachi H8/500) */
       bfd_arch_sh,        /* Renesas / SuperH SH (formerly Hitachi SH) */
     #define bfd_mach_sh            1
     #define bfd_mach_sh2        0x20
     #define bfd_mach_sh_dsp     0x2d
     #define bfd_mach_sh2a       0x2a
     #define bfd_mach_sh2a_nofpu 0x2b
     #define bfd_mach_sh2a_nofpu_or_sh4_nommu_nofpu 0x2a1
     #define bfd_mach_sh2a_nofpu_or_sh3_nommu 0x2a2
     #define bfd_mach_sh2a_or_sh4  0x2a3
     #define bfd_mach_sh2a_or_sh3e 0x2a4
     #define bfd_mach_sh2e       0x2e
     #define bfd_mach_sh3        0x30
     #define bfd_mach_sh3_nommu  0x31
     #define bfd_mach_sh3_dsp    0x3d
     #define bfd_mach_sh3e       0x3e
     #define bfd_mach_sh4        0x40
     #define bfd_mach_sh4_nofpu  0x41
     #define bfd_mach_sh4_nommu_nofpu  0x42
     #define bfd_mach_sh4a       0x4a
     #define bfd_mach_sh4a_nofpu 0x4b
     #define bfd_mach_sh4al_dsp  0x4d
     #define bfd_mach_sh5        0x50
       bfd_arch_alpha,     /* Dec Alpha */
     #define bfd_mach_alpha_ev4  0x10
     #define bfd_mach_alpha_ev5  0x20
     #define bfd_mach_alpha_ev6  0x30
       bfd_arch_arm,       /* Advanced Risc Machines ARM.  */
     #define bfd_mach_arm_unknown   0
     #define bfd_mach_arm_2         1
     #define bfd_mach_arm_2a        2
     #define bfd_mach_arm_3         3
     #define bfd_mach_arm_3M        4
     #define bfd_mach_arm_4         5
     #define bfd_mach_arm_4T        6
     #define bfd_mach_arm_5         7
     #define bfd_mach_arm_5T        8
     #define bfd_mach_arm_5TE       9
     #define bfd_mach_arm_XScale    10
     #define bfd_mach_arm_ep9312    11
     #define bfd_mach_arm_iWMMXt    12
       bfd_arch_ns32k,     /* National Semiconductors ns32000 */
       bfd_arch_w65,       /* WDC 65816 */
       bfd_arch_tic30,     /* Texas Instruments TMS320C30 */
       bfd_arch_tic4x,     /* Texas Instruments TMS320C3X/4X */
     #define bfd_mach_tic3x         30
     #define bfd_mach_tic4x         40
       bfd_arch_tic54x,    /* Texas Instruments TMS320C54X */
       bfd_arch_tic80,     /* TI TMS320c80 (MVP) */
       bfd_arch_v850,      /* NEC V850 */
     #define bfd_mach_v850          1
     #define bfd_mach_v850e         'E'
     #define bfd_mach_v850e1        '1'
       bfd_arch_arc,       /* ARC Cores */
     #define bfd_mach_arc_5         5
     #define bfd_mach_arc_6         6
     #define bfd_mach_arc_7         7
     #define bfd_mach_arc_8         8
       bfd_arch_m32r,      /* Renesas M32R (formerly Mitsubishi M32R/D) */
     #define bfd_mach_m32r          1 /* For backwards compatibility.  */
     #define bfd_mach_m32rx         'x'
     #define bfd_mach_m32r2         '2'
       bfd_arch_mn10200,   /* Matsushita MN10200 */
       bfd_arch_mn10300,   /* Matsushita MN10300 */
     #define bfd_mach_mn10300               300
     #define bfd_mach_am33          330
     #define bfd_mach_am33_2        332
     #define bfd_mach_fr30          0x46523330
     #define bfd_mach_frv           1
     #define bfd_mach_frvsimple     2
     #define bfd_mach_fr300         300
     #define bfd_mach_fr400         400
     #define bfd_mach_fr450         450
     #define bfd_mach_frvtomcat     499     /* fr500 prototype */
     #define bfd_mach_fr500         500
     #define bfd_mach_fr550         550
       bfd_arch_ia64,      /* HP/Intel ia64 */
     #define bfd_mach_ia64_elf64    64
     #define bfd_mach_ia64_elf32    32
       bfd_arch_ip2k,      /* Ubicom IP2K microcontrollers. */
     #define bfd_mach_ip2022        1
     #define bfd_mach_ip2022ext     2
      bfd_arch_iq2000,     /* Vitesse IQ2000.  */
     #define bfd_mach_iq2000        1
     #define bfd_mach_iq10          2
     #define bfd_mach_ms1           1
     #define bfd_mach_mrisc2        2
       bfd_arch_avr,       /* Atmel AVR microcontrollers.  */
     #define bfd_mach_avr1          1
     #define bfd_mach_avr2          2
     #define bfd_mach_avr3          3
     #define bfd_mach_avr4          4
     #define bfd_mach_avr5          5
       bfd_arch_cr16c,       /* National Semiconductor CompactRISC. */
     #define bfd_mach_cr16c         1
       bfd_arch_crx,       /*  National Semiconductor CRX.  */
     #define bfd_mach_crx           1
       bfd_arch_cris,      /* Axis CRIS */
     #define bfd_mach_cris_v0_v10   255
     #define bfd_mach_cris_v32      32
     #define bfd_mach_cris_v10_v32  1032
       bfd_arch_s390,      /* IBM s390 */
     #define bfd_mach_s390_31       31
     #define bfd_mach_s390_64       64
       bfd_arch_openrisc,  /* OpenRISC */
       bfd_arch_mmix,      /* Donald Knuth's educational processor.  */
     #define bfd_mach_xstormy16     1
       bfd_arch_msp430,    /* Texas Instruments MSP430 architecture.  */
     #define bfd_mach_msp11          11
     #define bfd_mach_msp110         110
     #define bfd_mach_msp12          12
     #define bfd_mach_msp13          13
     #define bfd_mach_msp14          14
     #define bfd_mach_msp15          15
     #define bfd_mach_msp16          16
     #define bfd_mach_msp31          31
     #define bfd_mach_msp32          32
     #define bfd_mach_msp33          33
     #define bfd_mach_msp41          41
     #define bfd_mach_msp42          42
     #define bfd_mach_msp43          43
     #define bfd_mach_msp44          44
       bfd_arch_xtensa,    /* Tensilica's Xtensa cores.  */
     #define bfd_mach_xtensa        1
        bfd_arch_maxq,     /* Dallas MAXQ 10/20 */
     #define bfd_mach_maxq10    10
     #define bfd_mach_maxq20    20

2.14.2 bfd_arch_info

This structure contains information on architectures for use within BFD.

     typedef struct bfd_arch_info
       int bits_per_word;
       int bits_per_address;
       int bits_per_byte;
       enum bfd_architecture arch;
       unsigned long mach;
       const char *arch_name;
       const char *printable_name;
       unsigned int section_align_power;
       /* TRUE if this is the default machine for the architecture.
          The default arch should be the first entry for an arch so that
          all the entries for that arch can be accessed via next.  */
       bfd_boolean the_default;
       const struct bfd_arch_info * (*compatible)
         (const struct bfd_arch_info *a, const struct bfd_arch_info *b);
       bfd_boolean (*scan) (const struct bfd_arch_info *, const char *);
       const struct bfd_arch_info *next;
     bfd_arch_info_type; bfd_printable_name


     const char *bfd_printable_name (bfd *abfd);

Return a printable string representing the architecture and machine from the pointer to the architecture info structure. bfd_scan_arch


     const bfd_arch_info_type *bfd_scan_arch (const char *string);

Figure out if BFD supports any cpu which could be described with the name string. Return a pointer to an arch_info structure if a machine is found, otherwise NULL. bfd_arch_list


     const char **bfd_arch_list (void);

Return a freshly malloced NULL-terminated vector of the names of all the valid BFD architectures. Do not modify the names. bfd_arch_get_compatible


     const bfd_arch_info_type *bfd_arch_get_compatible
        (const bfd *abfd, const bfd *bbfd, bfd_boolean accept_unknowns);

Determine whether two BFDs' architectures and machine types are compatible. Calculates the lowest common denominator between the two architectures and machine types implied by the BFDs and returns a pointer to an arch_info structure describing the compatible machine. bfd_default_arch_struct

The bfd_default_arch_struct is an item of bfd_arch_info_type which has been initialized to a fairly generic state. A BFD starts life by pointing to this structure, until the correct back end has determined the real architecture of the file.

     extern const bfd_arch_info_type bfd_default_arch_struct; bfd_set_arch_info


     void bfd_set_arch_info (bfd *abfd, const bfd_arch_info_type *arg);

Set the architecture info of abfd to arg. bfd_default_set_arch_mach


     bfd_boolean bfd_default_set_arch_mach
        (bfd *abfd, enum bfd_architecture arch, unsigned long mach);

Set the architecture and machine type in BFD abfd to arch and mach. Find the correct pointer to a structure and insert it into the arch_info pointer. bfd_get_arch


     enum bfd_architecture bfd_get_arch (bfd *abfd);

Return the enumerated type which describes the BFD abfd's architecture. bfd_get_mach


     unsigned long bfd_get_mach (bfd *abfd);

Return the long type which describes the BFD abfd's machine. bfd_arch_bits_per_byte


     unsigned int bfd_arch_bits_per_byte (bfd *abfd);

Return the number of bits in one of the BFD abfd's architecture's bytes. bfd_arch_bits_per_address


     unsigned int bfd_arch_bits_per_address (bfd *abfd);

Return the number of bits in one of the BFD abfd's architecture's addresses. bfd_default_compatible


     const bfd_arch_info_type *bfd_default_compatible
        (const bfd_arch_info_type *a, const bfd_arch_info_type *b);

The default function for testing for compatibility. bfd_default_scan


     bfd_boolean bfd_default_scan
        (const struct bfd_arch_info *info, const char *string);

The default function for working out whether this is an architecture hit and a machine hit. bfd_get_arch_info


     const bfd_arch_info_type *bfd_get_arch_info (bfd *abfd);

Return the architecture info struct in abfd. bfd_lookup_arch


     const bfd_arch_info_type *bfd_lookup_arch
        (enum bfd_architecture arch, unsigned long machine);

Look for the architecture info structure which matches the arguments arch and machine. A machine of 0 matches the machine/architecture structure which marks itself as the default. bfd_printable_arch_mach


     const char *bfd_printable_arch_mach
        (enum bfd_architecture arch, unsigned long machine);

Return a printable string representing the architecture and machine type.

This routine is depreciated. bfd_octets_per_byte


     unsigned int bfd_octets_per_byte (bfd *abfd);

Return the number of octets (8-bit quantities) per target byte (minimum addressable unit). In most cases, this will be one, but some DSP targets have 16, 32, or even 48 bits per byte. bfd_arch_mach_octets_per_byte


     unsigned int bfd_arch_mach_octets_per_byte
        (enum bfd_architecture arch, unsigned long machine);

See bfd_octets_per_byte.

This routine is provided for those cases where a bfd * is not available

Next: , Previous: Architectures, Up: BFD front end

2.15 Opening and closing BFDs bfd_fopen


     bfd *bfd_fopen (const char *filename, const char *target,
         const char *mode, int fd);

Open the file filename with the target target. Return a pointer to the created BFD. If fd is not -1, then fdopen is used to open the file; otherwise, fopen is used. mode is passed directly to fopen or fdopen.

Calls bfd_find_target, so target is interpreted as by that function.

The new BFD is marked as cacheable iff fd is -1.

If NULL is returned then an error has occured. Possible errors are bfd_error_no_memory, bfd_error_invalid_target or system_call error. bfd_openr


     bfd *bfd_openr (const char *filename, const char *target);

Open the file filename (using fopen) with the target target. Return a pointer to the created BFD.

Calls bfd_find_target, so target is interpreted as by that function.

If NULL is returned then an error has occured. Possible errors are bfd_error_no_memory, bfd_error_invalid_target or system_call error. bfd_fdopenr


     bfd *bfd_fdopenr (const char *filename, const char *target, int fd);

bfd_fdopenr is to bfd_fopenr much like fdopen is to fopen. It opens a BFD on a file already described by the fd supplied.

When the file is later bfd_closed, the file descriptor will be closed. If the caller desires that this file descriptor be cached by BFD (opened as needed, closed as needed to free descriptors for other opens), with the supplied fd used as an initial file descriptor (but subject to closure at any time), call bfd_set_cacheable(bfd, 1) on the returned BFD. The default is to assume no caching; the file descriptor will remain open until bfd_close, and will not be affected by BFD operations on other files.

Possible errors are bfd_error_no_memory, bfd_error_invalid_target and bfd_error_system_call. bfd_openstreamr


     bfd *bfd_openstreamr (const char *, const char *, void *);

Open a BFD for read access on an existing stdio stream. When the BFD is passed to bfd_close, the stream will be closed. bfd_openr_iovec


     bfd *bfd_openr_iovec (const char *filename, const char *target,
         void *(*open) (struct bfd *nbfd,
         void *open_closure),
         void *open_closure,
         file_ptr (*pread) (struct bfd *nbfd,
         void *stream,
         void *buf,
         file_ptr nbytes,
         file_ptr offset),
         int (*close) (struct bfd *nbfd,
         void *stream));

Create and return a BFD backed by a read-only stream. The stream is created using open, accessed using pread and destroyed using close.

Calls bfd_find_target, so target is interpreted as by that function.

Calls open (which can call bfd_zalloc and bfd_get_filename) to obtain the read-only stream backing the BFD. open either succeeds returning the non-NULL stream, or fails returning NULL (setting bfd_error).

Calls pread to request nbytes of data from stream starting at offset (e.g., via a call to bfd_read). pread either succeeds returning the number of bytes read (which can be less than nbytes when end-of-file), or fails returning -1 (setting bfd_error).

Calls close when the BFD is later closed using bfd_close. close either succeeds returning 0, or fails returning -1 (setting bfd_error).

If bfd_openr_iovec returns NULL then an error has occurred. Possible errors are bfd_error_no_memory, bfd_error_invalid_target and bfd_error_system_call. bfd_openw


     bfd *bfd_openw (const char *filename, const char *target);

Create a BFD, associated with file filename, using the file format target, and return a pointer to it.

Possible errors are bfd_error_system_call, bfd_error_no_memory, bfd_error_invalid_target. bfd_close


     bfd_boolean bfd_close (bfd *abfd);

Close a BFD. If the BFD was open for writing, then pending operations are completed and the file written out and closed. If the created file is executable, then chmod is called to mark it as such.

All memory attached to the BFD is released.

The file descriptor associated with the BFD is closed (even if it was passed in to BFD by bfd_fdopenr).

TRUE is returned if all is ok, otherwise FALSE. bfd_close_all_done


     bfd_boolean bfd_close_all_done (bfd *);

Close a BFD. Differs from bfd_close since it does not complete any pending operations. This routine would be used if the application had just used BFD for swapping and didn't want to use any of the writing code.

If the created file is executable, then chmod is called to mark it as such.

All memory attached to the BFD is released.

TRUE is returned if all is ok, otherwise FALSE. bfd_create


     bfd *bfd_create (const char *filename, bfd *templ);

Create a new BFD in the manner of bfd_openw, but without opening a file. The new BFD takes the target from the target used by template. The format is always set to bfd_object. bfd_make_writable


     bfd_boolean bfd_make_writable (bfd *abfd);

Takes a BFD as created by bfd_create and converts it into one like as returned by bfd_openw. It does this by converting the BFD to BFD_IN_MEMORY. It's assumed that you will call bfd_make_readable on this bfd later.

TRUE is returned if all is ok, otherwise FALSE. bfd_make_readable


     bfd_boolean bfd_make_readable (bfd *abfd);

Takes a BFD as created by bfd_create and bfd_make_writable and converts it into one like as returned by bfd_openr. It does this by writing the contents out to the memory buffer, then reversing the direction.

TRUE is returned if all is ok, otherwise FALSE. bfd_alloc


     void *bfd_alloc (bfd *abfd, bfd_size_type wanted);

Allocate a block of wanted bytes of memory attached to abfd and return a pointer to it. bfd_alloc2


     void *bfd_alloc2 (bfd *abfd, bfd_size_type nmemb, bfd_size_type size);

Allocate a block of nmemb elements of size bytes each of memory attached to abfd and return a pointer to it. bfd_zalloc


     void *bfd_zalloc (bfd *abfd, bfd_size_type wanted);

Allocate a block of wanted bytes of zeroed memory attached to abfd and return a pointer to it. bfd_zalloc2


     void *bfd_zalloc2 (bfd *abfd, bfd_size_type nmemb, bfd_size_type size);

Allocate a block of nmemb elements of size bytes each of zeroed memory attached to abfd and return a pointer to it. bfd_calc_gnu_debuglink_crc32


     unsigned long bfd_calc_gnu_debuglink_crc32
        (unsigned long crc, const unsigned char *buf, bfd_size_type len);

Computes a CRC value as used in the .gnu_debuglink section. Advances the previously computed crc value by computing and adding in the crc32 for len bytes of buf.

Return the updated CRC32 value. get_debug_link_info


     char *get_debug_link_info (bfd *abfd, unsigned long *crc32_out);

fetch the filename and CRC32 value for any separate debuginfo associated with abfd. Return NULL if no such info found, otherwise return filename and update crc32_out. separate_debug_file_exists


     bfd_boolean separate_debug_file_exists
        (char *name, unsigned long crc32);

Checks to see if name is a file and if its contents match crc32. find_separate_debug_file


     char *find_separate_debug_file (bfd *abfd);

Searches abfd for a reference to separate debugging information, scans various locations in the filesystem, including the file tree rooted at debug_file_directory, and returns a filename of such debugging information if the file is found and has matching CRC32. Returns NULL if no reference to debugging file exists, or file cannot be found. bfd_follow_gnu_debuglink


     char *bfd_follow_gnu_debuglink (bfd *abfd, const char *dir);

Takes a BFD and searches it for a .gnu_debuglink section. If this section is found, it examines the section for the name and checksum of a '.debug' file containing auxiliary debugging information. It then searches the filesystem for this .debug file in some standard locations, including the directory tree rooted at dir, and if found returns the full filename.

If dir is NULL, it will search a default path configured into libbfd at build time. [XXX this feature is not currently implemented].

NULL on any errors or failure to locate the .debug file, otherwise a pointer to a heap-allocated string containing the filename. The caller is responsible for freeing this string. bfd_create_gnu_debuglink_section


     struct bfd_section *bfd_create_gnu_debuglink_section
        (bfd *abfd, const char *filename);

Takes a BFD and adds a .gnu_debuglink section to it. The section is sized to be big enough to contain a link to the specified filename.

A pointer to the new section is returned if all is ok. Otherwise NULL is returned and bfd_error is set. bfd_fill_in_gnu_debuglink_section


     bfd_boolean bfd_fill_in_gnu_debuglink_section
        (bfd *abfd, struct bfd_section *sect, const char *filename);

Takes a BFD and containing a .gnu_debuglink section SECT and fills in the contents of the section to contain a link to the specified filename. The filename should be relative to the current directory.

TRUE is returned if all is ok. Otherwise FALSE is returned and bfd_error is set.

Next: , Previous: Opening and Closing, Up: BFD front end

2.16 Internal functions

These routines are used within BFD. They are not intended for export, but are documented here for completeness. bfd_write_bigendian_4byte_int


     bfd_boolean bfd_write_bigendian_4byte_int (bfd *, unsigned int);

Write a 4 byte integer i to the output BFD abfd, in big endian order regardless of what else is going on. This is useful in archives. bfd_put_size bfd_get_size

These macros as used for reading and writing raw data in sections; each access (except for bytes) is vectored through the target format of the BFD and mangled accordingly. The mangling performs any necessary endian translations and removes alignment restrictions. Note that types accepted and returned by these macros are identical so they can be swapped around in macros—for example, libaout.h defines GET_WORD to either bfd_get_32 or bfd_get_64.

In the put routines, val must be a bfd_vma. If we are on a system without prototypes, the caller is responsible for making sure that is true, with a cast if necessary. We don't cast them in the macro definitions because that would prevent lint or gcc -Wall from detecting sins such as passing a pointer. To detect calling these with less than a bfd_vma, use gcc -Wconversion on a host with 64 bit bfd_vma's.

     /* Byte swapping macros for user section data.  */
     #define bfd_put_8(abfd, val, ptr) \
       ((void) (*((unsigned char *) (ptr)) = (val) & 0xff))
     #define bfd_put_signed_8 \
     #define bfd_get_8(abfd, ptr) \
       (*(unsigned char *) (ptr) & 0xff)
     #define bfd_get_signed_8(abfd, ptr) \
       (((*(unsigned char *) (ptr) & 0xff) ^ 0x80) - 0x80)
     #define bfd_put_16(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_putx16, ((val),(ptr)))
     #define bfd_put_signed_16 \
     #define bfd_get_16(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx16, (ptr))
     #define bfd_get_signed_16(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx_signed_16, (ptr))
     #define bfd_put_32(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_putx32, ((val),(ptr)))
     #define bfd_put_signed_32 \
     #define bfd_get_32(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx32, (ptr))
     #define bfd_get_signed_32(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx_signed_32, (ptr))
     #define bfd_put_64(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_putx64, ((val), (ptr)))
     #define bfd_put_signed_64 \
     #define bfd_get_64(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx64, (ptr))
     #define bfd_get_signed_64(abfd, ptr) \
       BFD_SEND (abfd, bfd_getx_signed_64, (ptr))
     #define bfd_get(bits, abfd, ptr)                       \
       ((bits) == 8 ? (bfd_vma) bfd_get_8 (abfd, ptr)       \
        : (bits) == 16 ? bfd_get_16 (abfd, ptr)             \
        : (bits) == 32 ? bfd_get_32 (abfd, ptr)             \
        : (bits) == 64 ? bfd_get_64 (abfd, ptr)             \
        : (abort (), (bfd_vma) - 1))
     #define bfd_put(bits, abfd, val, ptr)                  \
       ((bits) == 8 ? bfd_put_8  (abfd, val, ptr)           \
        : (bits) == 16 ? bfd_put_16 (abfd, val, ptr)                \
        : (bits) == 32 ? bfd_put_32 (abfd, val, ptr)                \
        : (bits) == 64 ? bfd_put_64 (abfd, val, ptr)                \
        : (abort (), (void) 0)) bfd_h_put_size

These macros have the same function as their bfd_get_x brethren, except that they are used for removing information for the header records of object files. Believe it or not, some object files keep their header records in big endian order and their data in little endian order.

     /* Byte swapping macros for file header data.  */
     #define bfd_h_put_8(abfd, val, ptr) \
       bfd_put_8 (abfd, val, ptr)
     #define bfd_h_put_signed_8(abfd, val, ptr) \
       bfd_put_8 (abfd, val, ptr)
     #define bfd_h_get_8(abfd, ptr) \
       bfd_get_8 (abfd, ptr)
     #define bfd_h_get_signed_8(abfd, ptr) \
       bfd_get_signed_8 (abfd, ptr)
     #define bfd_h_put_16(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_h_putx16, (val, ptr))
     #define bfd_h_put_signed_16 \
     #define bfd_h_get_16(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx16, (ptr))
     #define bfd_h_get_signed_16(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx_signed_16, (ptr))
     #define bfd_h_put_32(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_h_putx32, (val, ptr))
     #define bfd_h_put_signed_32 \
     #define bfd_h_get_32(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx32, (ptr))
     #define bfd_h_get_signed_32(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx_signed_32, (ptr))
     #define bfd_h_put_64(abfd, val, ptr) \
       BFD_SEND (abfd, bfd_h_putx64, (val, ptr))
     #define bfd_h_put_signed_64 \
     #define bfd_h_get_64(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx64, (ptr))
     #define bfd_h_get_signed_64(abfd, ptr) \
       BFD_SEND (abfd, bfd_h_getx_signed_64, (ptr))
     /* Aliases for the above, which should eventually go away.  */
     #define H_PUT_64  bfd_h_put_64
     #define H_PUT_32  bfd_h_put_32
     #define H_PUT_16  bfd_h_put_16
     #define H_PUT_8   bfd_h_put_8
     #define H_PUT_S64 bfd_h_put_signed_64
     #define H_PUT_S32 bfd_h_put_signed_32
     #define H_PUT_S16 bfd_h_put_signed_16
     #define H_PUT_S8  bfd_h_put_signed_8
     #define H_GET_64  bfd_h_get_64
     #define H_GET_32  bfd_h_get_32
     #define H_GET_16  bfd_h_get_16
     #define H_GET_8   bfd_h_get_8
     #define H_GET_S64 bfd_h_get_signed_64
     #define H_GET_S32 bfd_h_get_signed_32
     #define H_GET_S16 bfd_h_get_signed_16
     #define H_GET_S8  bfd_h_get_signed_8


     unsigned int bfd_log2 (bfd_vma x);

Return the log base 2 of the value supplied, rounded up. E.g., an x of 1025 returns 11. A x of 0 returns 0.

Next: , Previous: Internal, Up: BFD front end

2.17 File caching

The file caching mechanism is embedded within BFD and allows the application to open as many BFDs as it wants without regard to the underlying operating system's file descriptor limit (often as low as 20 open files). The module in cache.c maintains a least recently used list of BFD_CACHE_MAX_OPEN files, and exports the name bfd_cache_lookup, which runs around and makes sure that the required BFD is open. If not, then it chooses a file to close, closes it and opens the one wanted, returning its file handle. BFD_CACHE_MAX_OPEN macro

The maximum number of files which the cache will keep open at one time.

     #define BFD_CACHE_MAX_OPEN 10 bfd_last_cache


     extern bfd *bfd_last_cache;

Zero, or a pointer to the topmost BFD on the chain. This is used by the bfd_cache_lookup macro in libbfd.h to determine when it can avoid a function call. bfd_cache_lookup

Check to see if the required BFD is the same as the last one looked up. If so, then it can use the stream in the BFD with impunity, since it can't have changed since the last lookup; otherwise, it has to perform the complicated lookup function.

     #define bfd_cache_lookup(x) \
         ((x) == bfd_last_cache ? \
           (FILE *) (bfd_last_cache->iostream): \
            bfd_cache_lookup_worker (x)) bfd_cache_init


     bfd_boolean bfd_cache_init (bfd *abfd);

Add a newly opened BFD to the cache. bfd_cache_close


     bfd_boolean bfd_cache_close (bfd *abfd);

Remove the BFD abfd from the cache. If the attached file is open, then close it too.

FALSE is returned if closing the file fails, TRUE is returned if all is well. bfd_cache_close_all


     bfd_boolean bfd_cache_close_all (void);

Remove all BFDs from the cache. If the attached file is open, then close it too.

FALSE is returned if closing one of the file fails, TRUE is returned if all is well. bfd_open_file


     FILE* bfd_open_file (bfd *abfd);

Call the OS to open a file for abfd. Return the FILE * (possibly NULL) that results from this operation. Set up the BFD so that future accesses know the file is open. If the FILE * returned is NULL, then it won't have been put in the cache, so it won't have to be removed from it. bfd_cache_lookup_worker


     FILE *bfd_cache_lookup_worker (bfd *abfd);

Called when the macro bfd_cache_lookup fails to find a quick answer. Find a file descriptor for abfd. If necessary, it open it. If there are already more than BFD_CACHE_MAX_OPEN files open, it tries to close one first, to avoid running out of file descriptors. It will abort rather than returning NULL if it is unable to (re)open the abfd.

Next: , Previous: File Caching, Up: BFD front end

2.18 Linker Functions

The linker uses three special entry points in the BFD target vector. It is not necessary to write special routines for these entry points when creating a new BFD back end, since generic versions are provided. However, writing them can speed up linking and make it use significantly less runtime memory.

The first routine creates a hash table used by the other routines. The second routine adds the symbols from an object file to the hash table. The third routine takes all the object files and links them together to create the output file. These routines are designed so that the linker proper does not need to know anything about the symbols in the object files that it is linking. The linker merely arranges the sections as directed by the linker script and lets BFD handle the details of symbols and relocs.

The second routine and third routines are passed a pointer to a struct bfd_link_info structure (defined in bfdlink.h) which holds information relevant to the link, including the linker hash table (which was created by the first routine) and a set of callback functions to the linker proper.

The generic linker routines are in linker.c, and use the header file genlink.h. As of this writing, the only back ends which have implemented versions of these routines are a.out (in aoutx.h) and ECOFF (in ecoff.c). The a.out routines are used as examples throughout this section.

Next: , Previous: Linker Functions, Up: Linker Functions

2.18.1 Creating a linker hash table

The linker routines must create a hash table, which must be derived from struct bfd_link_hash_table described in bfdlink.c. See Hash Tables, for information on how to create a derived hash table. This entry point is called using the target vector of the linker output file.

The _bfd_link_hash_table_create entry point must allocate and initialize an instance of the desired hash table. If the back end does not require any additional information to be stored with the entries in the hash table, the entry point may simply create a struct bfd_link_hash_table. Most likely, however, some additional information will be needed.

For example, with each entry in the hash table the a.out linker keeps the index the symbol has in the final output file (this index number is used so that when doing a relocatable link the symbol index used in the output file can be quickly filled in when copying over a reloc). The a.out linker code defines the required structures and functions for a hash table derived from struct bfd_link_hash_table. The a.out linker hash table is created by the function NAME(aout,link_hash_table_create); it simply allocates space for the hash table, initializes it, and returns a pointer to it.

When writing the linker routines for a new back end, you will generally not know exactly which fields will be required until you have finished. You should simply create a new hash table which defines no additional fields, and then simply add fields as they become necessary.

Next: , Previous: Creating a Linker Hash Table, Up: Linker Functions

2.18.2 Adding symbols to the hash table

The linker proper will call the _bfd_link_add_symbols entry point for each object file or archive which is to be linked (typically these are the files named on the command line, but some may also come from the linker script). The entry point is responsible for examining the file. For an object file, BFD must add any relevant symbol information to the hash table. For an archive, BFD must determine which elements of the archive should be used and adding them to the link.

The a.out version of this entry point is NAME(aout,link_add_symbols).

Next: , Previous: Adding Symbols to the Hash Table, Up: Adding Symbols to the Hash Table Differing file formats

Normally all the files involved in a link will be of the same format, but it is also possible to link together different format object files, and the back end must support that. The _bfd_link_add_symbols entry point is called via the target vector of the file to be added. This has an important consequence: the function may not assume that the hash table is the type created by the corresponding _bfd_link_hash_table_create vector. All the _bfd_link_add_symbols function can assume about the hash table is that it is derived from struct bfd_link_hash_table.

Sometimes the _bfd_link_add_symbols function must store some information in the hash table entry to be used by the _bfd_final_link function. In such a case the creator field of the hash table must be checked to make sure that the hash table was created by an object file of the same format.

The _bfd_final_link routine must be prepared to handle a hash entry without any extra information added by the _bfd_link_add_symbols function. A hash entry without extra information will also occur when the linker script directs the linker to create a symbol. Note that, regardless of how a hash table entry is added, all the fields will be initialized to some sort of null value by the hash table entry initialization function.

See ecoff_link_add_externals for an example of how to check the creator field before saving information (in this case, the ECOFF external symbol debugging information) in a hash table entry.

Next: , Previous: Differing file formats, Up: Adding Symbols to the Hash Table Adding symbols from an object file

When the _bfd_link_add_symbols routine is passed an object file, it must add all externally visible symbols in that object file to the hash table. The actual work of adding the symbol to the hash table is normally handled by the function _bfd_generic_link_add_one_symbol. The _bfd_link_add_symbols routine is responsible for reading all the symbols from the object file and passing the correct information to _bfd_generic_link_add_one_symbol.

The _bfd_link_add_symbols routine should not use bfd_canonicalize_symtab to read the symbols. The point of providing this routine is to avoid the overhead of converting the symbols into generic asymbol structures.

_bfd_generic_link_add_one_symbol handles the details of combining common symbols, warning about multiple definitions, and so forth. It takes arguments which describe the symbol to add, notably symbol flags, a section, and an offset. The symbol flags include such things as BSF_WEAK or BSF_INDIRECT. The section is a section in the object file, or something like bfd_und_section_ptr for an undefined symbol or bfd_com_section_ptr for a common symbol.

If the _bfd_final_link routine is also going to need to read the symbol information, the _bfd_link_add_symbols routine should save it somewhere attached to the object file BFD. However, the information should only be saved if the keep_memory field of the info argument is TRUE, so that the -no-keep-memory linker switch is effective.

The a.out function which adds symbols from an object file is aout_link_add_object_symbols, and most of the interesting work is in aout_link_add_symbols. The latter saves pointers to the hash tables entries created by _bfd_generic_link_add_one_symbol indexed by symbol number, so that the _bfd_final_link routine does not have to call the hash table lookup routine to locate the entry.

Previous: Adding symbols from an object file, Up: Adding Symbols to the Hash Table Adding symbols from an archive

When the _bfd_link_add_symbols routine is passed an archive, it must look through the symbols defined by the archive and decide which elements of the archive should be included in the link. For each such element it must call the add_archive_element linker callback, and it must add the symbols from the object file to the linker hash table.

In most cases the work of looking through the symbols in the archive should be done by the _bfd_generic_link_add_archive_symbols function. This function builds a hash table from the archive symbol table and looks through the list of undefined symbols to see which elements should be included. _bfd_generic_link_add_archive_symbols is passed a function to call to make the final decision about adding an archive element to the link and to do the actual work of adding the symbols to the linker hash table.

The function passed to _bfd_generic_link_add_archive_symbols must read the symbols of the archive element and decide whether the archive element should be included in the link. If the element is to be included, the add_archive_element linker callback routine must be called with the element as an argument, and the elements symbols must be added to the linker hash table just as though the element had itself been passed to the _bfd_link_add_symbols function.

When the a.out _bfd_link_add_symbols function receives an archive, it calls _bfd_generic_link_add_archive_symbols passing aout_link_check_archive_element as the function argument. aout_link_check_archive_element calls aout_link_check_ar_symbols. If the latter decides to add the element (an element is only added if it provides a real, non-common, definition for a previously undefined or common symbol) it calls the add_archive_element callback and then aout_link_check_archive_element calls aout_link_add_symbols to actually add the symbols to the linker hash table.

The ECOFF back end is unusual in that it does not normally call _bfd_generic_link_add_archive_symbols, because ECOFF archives already contain a hash table of symbols. The ECOFF back end searches the archive itself to avoid the overhead of creating a new hash table.

Previous: Adding Symbols to the Hash Table, Up: Linker Functions

2.18.3 Performing the final link

When all the input files have been processed, the linker calls the _bfd_final_link entry point of the output BFD. This routine is responsible for producing the final output file, which has several aspects. It must relocate the contents of the input sections and copy the data into the output sections. It must build an output symbol table including any local symbols from the input files and the global symbols from the hash table. When producing relocatable output, it must modify the input relocs and write them into the output file. There may also be object format dependent work to be done.

The linker will also call the write_object_contents entry point when the BFD is closed. The two entry points must work together in order to produce the correct output file.

The details of how this works are inevitably dependent upon the specific object file format. The a.out _bfd_final_link routine is NAME(aout,final_link).

Next: , Previous: Performing the Final Link, Up: Performing the Final Link Information provided by the linker

Before the linker calls the _bfd_final_link entry point, it sets up some data structures for the function to use.

The input_bfds field of the bfd_link_info structure will point to a list of all the input files included in the link. These files are linked through the link_next field of the bfd structure.

Each section in the output file will have a list of link_order structures attached to the map_head.link_order field (the link_order structure is defined in bfdlink.h). These structures describe how to create the contents of the output section in terms of the contents of various input sections, fill constants, and, eventually, other types of information. They also describe relocs that must be created by the BFD backend, but do not correspond to any input file; this is used to support -Ur, which builds constructors while generating a relocatable object file.

Next: , Previous: Information provided by the linker, Up: Performing the Final Link Relocating the section contents

The _bfd_final_link function should look through the link_order structures attached to each section of the output file. Each link_order structure should either be handled specially, or it should be passed to the function _bfd_default_link_order which will do the right thing (_bfd_default_link_order is defined in linker.c).

For efficiency, a link_order of type bfd_indirect_link_order whose associated section belongs to a BFD of the same format as the output BFD must be handled specially. This type of link_order describes part of an output section in terms of a section belonging to one of the input files. The _bfd_final_link function should read the contents of the section and any associated relocs, apply the relocs to the section contents, and write out the modified section contents. If performing a relocatable link, the relocs themselves must also be modified and written out.

The functions _bfd_relocate_contents and _bfd_final_link_relocate provide some general support for performing the actual relocations, notably overflow checking. Their arguments include information about the symbol the relocation is against and a reloc_howto_type argument which describes the relocation to perform. These functions are defined in reloc.c.

The a.out function which handles reading, relocating, and writing section contents is aout_link_input_section. The actual relocation is done in aout_link_input_section_std and aout_link_input_section_ext.

Previous: Relocating the section contents, Up: Performing the Final Link Writing the symbol table

The _bfd_final_link function must gather all the symbols in the input files and write them out. It must also write out all the symbols in the global hash table. This must be controlled by the strip and discard fields of the bfd_link_info structure.

The local symbols of the input files will not have been entered into the linker hash table. The _bfd_final_link routine must consider each input file and include the symbols in the output file. It may be convenient to do this when looking through the link_order structures, or it may be done by stepping through the input_bfds list.

The _bfd_final_link routine must also traverse the global hash table to gather all the externally visible symbols. It is possible that most of the externally visible symbols may be written out when considering the symbols of each input file, but it is still necessary to traverse the hash table since the linker script may have defined some symbols that are not in any of the input files.

The strip field of the bfd_link_info structure controls which symbols are written out. The possible values are listed in bfdlink.h. If the value is strip_some, then the keep_hash field of the bfd_link_info structure is a hash table of symbols to keep; each symbol should be looked up in this hash table, and only symbols which are present should be included in the output file.

If the strip field of the bfd_link_info structure permits local symbols to be written out, the discard field is used to further controls which local symbols are included in the output file. If the value is discard_l, then all local symbols which begin with a certain prefix are discarded; this is controlled by the bfd_is_local_label_name entry point.

The a.out backend handles symbols by calling aout_link_write_symbols on each input BFD and then traversing the global hash table with the function aout_link_write_other_symbol. It builds a string table while writing out the symbols, which is written to the output file at the end of NAME(aout,final_link). bfd_link_split_section


     bfd_boolean bfd_link_split_section (bfd *abfd, asection *sec);

Return nonzero if sec should be split during a reloceatable or final link.

     #define bfd_link_split_section(abfd, sec) \
            BFD_SEND (abfd, _bfd_link_split_section, (abfd, sec)) bfd_section_already_linked


     void bfd_section_already_linked (bfd *abfd, asection *sec);

Check if sec has been already linked during a reloceatable or final link.

     #define bfd_section_already_linked(abfd, sec) \
            BFD_SEND (abfd, _section_already_linked, (abfd, sec))

Previous: Linker Functions, Up: BFD front end

2.19 Hash Tables

BFD provides a simple set of hash table functions. Routines are provided to initialize a hash table, to free a hash table, to look up a string in a hash table and optionally create an entry for it, and to traverse a hash table. There is currently no routine to delete an string from a hash table.

The basic hash table does not permit any data to be stored with a string. However, a hash table is designed to present a base class from which other types of hash tables may be derived. These derived types may store additional information with the string. Hash tables were implemented in this way, rather than simply providing a data pointer in a hash table entry, because they were designed for use by the linker back ends. The linker may create thousands of hash table entries, and the overhead of allocating private data and storing and following pointers becomes noticeable.

The basic hash table code is in hash.c.

Next: , Previous: Hash Tables, Up: Hash Tables

2.19.1 Creating and freeing a hash table

To create a hash table, create an instance of a struct bfd_hash_table (defined in bfd.h) and call bfd_hash_table_init (if you know approximately how many entries you will need, the function bfd_hash_table_init_n, which takes a size argument, may be used). bfd_hash_table_init returns FALSE if some sort of error occurs.

The function bfd_hash_table_init take as an argument a function to use to create new entries. For a basic hash table, use the function bfd_hash_newfunc. See Deriving a New Hash Table Type, for why you would want to use a different value for this argument.

bfd_hash_table_init will create an objalloc which will be used to allocate new entries. You may allocate memory on this objalloc using bfd_hash_allocate.

Use bfd_hash_table_free to free up all the memory that has been allocated for a hash table. This will not free up the struct bfd_hash_table itself, which you must provide.

Use bfd_hash_set_default_size to set the default size of hash table to use.

Next: , Previous: Creating and Freeing a Hash Table, Up: Hash Tables

2.19.2 Looking up or entering a string

The function bfd_hash_lookup is used both to look up a string in the hash table and to create a new entry.

If the create argument is FALSE, bfd_hash_lookup will look up a string. If the string is found, it will returns a pointer to a struct bfd_hash_entry. If the string is not found in the table bfd_hash_lookup will return NULL. You should not modify any of the fields in the returns struct bfd_hash_entry.

If the create argument is TRUE, the string will be entered into the hash table if it is not already there. Either way a pointer to a struct bfd_hash_entry will be returned, either to the existing structure or to a newly created one. In this case, a NULL return means that an error occurred.

If the create argument is TRUE, and a new entry is created, the copy argument is used to decide whether to copy the string onto the hash table objalloc or not. If copy is passed as FALSE, you must be careful not to deallocate or modify the string as long as the hash table exists.

Next: , Previous: Looking Up or Entering a String, Up: Hash Tables

2.19.3 Traversing a hash table

The function bfd_hash_traverse may be used to traverse a hash table, calling a function on each element. The traversal is done in a random order.

bfd_hash_traverse takes as arguments a function and a generic void * pointer. The function is called with a hash table entry (a struct bfd_hash_entry *) and the generic pointer passed to bfd_hash_traverse. The function must return a boolean value, which indicates whether to continue traversing the hash table. If the function returns FALSE, bfd_hash_traverse will stop the traversal and return immediately.

Previous: Traversing a Hash Table, Up: Hash Tables

2.19.4 Deriving a new hash table type

Many uses of hash tables want to store additional information which each entry in the hash table. Some also find it convenient to store additional information with the hash table itself. This may be done using a derived hash table.

Since C is not an object oriented language, creating a derived hash table requires sticking together some boilerplate routines with a few differences specific to the type of hash table you want to create.

An example of a derived hash table is the linker hash table. The structures for this are defined in bfdlink.h. The functions are in linker.c.

You may also derive a hash table from an already derived hash table. For example, the a.out linker backend code uses a hash table derived from the linker hash table.

Next: , Previous: Deriving a New Hash Table Type, Up: Deriving a New Hash Table Type Define the derived structures

You must define a structure for an entry in the hash table, and a structure for the hash table itself.

The first field in the structure for an entry in the hash table must be of the type used for an entry in the hash table you are deriving from. If you are deriving from a basic hash table this is struct bfd_hash_entry, which is defined in bfd.h. The first field in the structure for the hash table itself must be of the type of the hash table you are deriving from itself. If you are deriving from a basic hash table, this is struct bfd_hash_table.

For example, the linker hash table defines struct bfd_link_hash_entry (in bfdlink.h). The first field, root, is of type struct bfd_hash_entry. Similarly, the first field in struct bfd_link_hash_table, table, is of type struct bfd_hash_table.

Next: , Previous: Define the Derived Structures, Up: Deriving a New Hash Table Type Write the derived creation routine

You must write a routine which will create and initialize an entry in the hash table. This routine is passed as the function argument to bfd_hash_table_init.

In order to permit other hash tables to be derived from the hash table you are creating, this routine must be written in a standard way.

The first argument to the creation routine is a pointer to a hash table entry. This may be NULL, in which case the routine should allocate the right amount of space. Otherwise the space has already been allocated by a hash table type derived from this one.

After allocating space, the creation routine must call the creation routine of the hash table type it is derived from, passing in a pointer to the space it just allocated. This will initialize any fields used by the base hash table.

Finally the creation routine must initialize any local fields for the new hash table type.

Here is a boilerplate example of a creation routine. function_name is the name of the routine. entry_type is the type of an entry in the hash table you are creating. base_newfunc is the name of the creation routine of the hash table type your hash table is derived from.

     struct bfd_hash_entry *
     function_name (struct bfd_hash_entry *entry,
                          struct bfd_hash_table *table,
                          const char *string)
       struct entry_type *ret = (entry_type *) entry;
      /* Allocate the structure if it has not already been allocated by a
         derived class.  */
       if (ret == NULL)
           ret = bfd_hash_allocate (table, sizeof (* ret));
           if (ret == NULL)
             return NULL;
      /* Call the allocation method of the base class.  */
       ret = ((entry_type *)
             base_newfunc ((struct bfd_hash_entry *) ret, table, string));
      /* Initialize the local fields here.  */
       return (struct bfd_hash_entry *) ret;

The creation routine for the linker hash table, which is in linker.c, looks just like this example. function_name is _bfd_link_hash_newfunc. entry_type is struct bfd_link_hash_entry. base_newfunc is bfd_hash_newfunc, the creation routine for a basic hash table.

_bfd_link_hash_newfunc also initializes the local fields in a linker hash table entry: type, written and next.

Previous: Write the Derived Creation Routine, Up: Deriving a New Hash Table Type Write other derived routines

You will want to write other routines for your new hash table, as well.

You will want an initialization routine which calls the initialization routine of the hash table you are deriving from and initializes any other local fields. For the linker hash table, this is _bfd_link_hash_table_init in linker.c.

You will want a lookup routine which calls the lookup routine of the hash table you are deriving from and casts the result. The linker hash table uses bfd_link_hash_lookup in linker.c (this actually takes an additional argument which it uses to decide how to return the looked up value).

You may want a traversal routine. This should just call the traversal routine of the hash table you are deriving from with appropriate casts. The linker hash table uses bfd_link_hash_traverse in linker.c.

These routines may simply be defined as macros. For example, the a.out backend linker hash table, which is derived from the linker hash table, uses macros for the lookup and traversal routines. These are aout_link_hash_lookup and aout_link_hash_traverse in aoutx.h.

Next: , Previous: BFD front end, Up: Top

3 BFD back ends

Next: , Previous: BFD back ends, Up: BFD back ends

All of BFD lives in one directory.

Next: , Previous: What to Put Where, Up: BFD back ends

3.1 a.out backends

BFD supports a number of different flavours of a.out format, though the major differences are only the sizes of the structures on disk, and the shape of the relocation information.

The support is split into a basic support file aoutx.h and other files which derive functions from the base. One derivation file is aoutf1.h (for a.out flavour 1), and adds to the basic a.out functions support for sun3, sun4, 386 and 29k a.out files, to create a target jump vector for a specific target.

This information is further split out into more specific files for each machine, including sunos.c for sun3 and sun4, newsos3.c for the Sony NEWS, and demo64.c for a demonstration of a 64 bit a.out format.

The base file aoutx.h defines general mechanisms for reading and writing records to and from disk and various other methods which BFD requires. It is included by aout32.c and aout64.c to form the names aout_32_swap_exec_header_in, aout_64_swap_exec_header_in, etc.

As an example, this is what goes on to make the back end for a sun4, from aout32.c:

            #define ARCH_SIZE 32
            #include "aoutx.h"

Which exports names:


from sunos.c:

            #define TARGET_NAME "a.out-sunos-big"
            #define VECNAME    sunos_big_vec
            #include "aoutf1.h"

requires all the names from aout32.c, and produces the jump vector


The file host-aout.c is a special case. It is for a large set of hosts that use “more or less standard” a.out files, and for which cross-debugging is not interesting. It uses the standard 32-bit a.out support routines, but determines the file offsets and addresses of the text, data, and BSS sections, the machine architecture and machine type, and the entry point address, in a host-dependent manner. Once these values have been determined, generic code is used to handle the object file.

When porting it to run on a new system, you must supply:

             HOST_MACHINE_ARCH       (optional)
             HOST_MACHINE_MACHINE    (optional)

in the file ../include/sys/h-XXX.h (for your host). These values, plus the structures and macros defined in a.out.h on your host system, will produce a BFD target that will access ordinary a.out files on your host. To configure a new machine to use host-aout.c, specify:

            TDEFAULTS = -DDEFAULT_VECTOR=host_aout_big_vec
            TDEPFILES= host-aout.o trad-core.o

in the config/ file, and modify to use the file (by setting "bfd_target=XXX") when your configuration is selected.

3.1.1 Relocations

The file aoutx.h provides for both the standard and extended forms of a.out relocation records.

The standard records contain only an address, a symbol index, and a type field. The extended records (used on 29ks and sparcs) also have a full integer for an addend.

3.1.2 Internal entry points

aoutx.h exports several routines for accessing the contents of an a.out file, which are gathered and exported in turn by various format specific files (eg sunos.c). aout_size_swap_exec_header_in


     void aout_size_swap_exec_header_in,
        (bfd *abfd,
         struct external_exec *bytes,
         struct internal_exec *execp);

Swap the information in an executable header raw_bytes taken from a raw byte stream memory image into the internal exec header structure execp. aout_size_swap_exec_header_out


     void aout_size_swap_exec_header_out
        (bfd *abfd,
         struct internal_exec *execp,
         struct external_exec *raw_bytes);

Swap the information in an internal exec header structure execp into the buffer raw_bytes ready for writing to disk. aout_size_some_aout_object_p


     const bfd_target *aout_size_some_aout_object_p
        (bfd *abfd,
         struct internal_exec *execp,
         const bfd_target *(*callback_to_real_object_p) (bfd *));

Some a.out variant thinks that the file open in abfd checking is an a.out file. Do some more checking, and set up for access if it really is. Call back to the calling environment's "finish up" function just before returning, to handle any last-minute setup. aout_size_mkobject


     bfd_boolean aout_size_mkobject, (bfd *abfd);

Initialize BFD abfd for use with a.out files. aout_size_machine_type


     enum machine_type  aout_size_machine_type
        (enum bfd_architecture arch,
         unsigned long machine,
         bfd_boolean *unknown);

Keep track of machine architecture and machine type for a.out's. Return the machine_type for a particular architecture and machine, or M_UNKNOWN if that exact architecture and machine can't be represented in a.out format.

If the architecture is understood, machine type 0 (default) is always understood. aout_size_set_arch_mach


     bfd_boolean aout_size_set_arch_mach,
        (bfd *,
         enum bfd_architecture arch,
         unsigned long machine);

Set the architecture and the machine of the BFD abfd to the values arch and machine. Verify that abfd's format can support the architecture required. aout_size_new_section_hook


     bfd_boolean aout_size_new_section_hook,
        (bfd *abfd,
         asection *newsect);

Called by the BFD in response to a bfd_make_section request.

Next: , Previous: aout, Up: BFD back ends

3.2 coff backends

BFD supports a number of different flavours of coff format. The major differences between formats are the sizes and alignments of fields in structures on disk, and the occasional extra field.

Coff in all its varieties is implemented with a few common files and a number of implementation specific files. For example, The 88k bcs coff format is implemented in the file coff-m88k.c. This file #includes coff/m88k.h which defines the external structure of the coff format for the 88k, and coff/internal.h which defines the internal structure. coff-m88k.c also defines the relocations used by the 88k format See Relocations.

The Intel i960 processor version of coff is implemented in coff-i960.c. This file has the same structure as coff-m88k.c, except that it includes coff/i960.h rather than coff-m88k.h.

3.2.1 Porting to a new version of coff

The recommended method is to select from the existing implementations the version of coff which is most like the one you want to use. For example, we'll say that i386 coff is the one you select, and that your coff flavour is called foo. Copy i386coff.c to foocoff.c, copy ../include/coff/i386.h to ../include/coff/foo.h, and add the lines to targets.c and so that your new back end is used. Alter the shapes of the structures in ../include/coff/foo.h so that they match what you need. You will probably also have to add #ifdefs to the code in coff/internal.h and coffcode.h if your version of coff is too wild.

You can verify that your new BFD backend works quite simply by building objdump from the binutils directory, and making sure that its version of what's going on and your host system's idea (assuming it has the pretty standard coff dump utility, usually called att-dump or just dump) are the same. Then clean up your code, and send what you've done to Cygnus. Then your stuff will be in the next release, and you won't have to keep integrating it.

3.2.2 How the coff backend works File layout

The Coff backend is split into generic routines that are applicable to any Coff target and routines that are specific to a particular target. The target-specific routines are further split into ones which are basically the same for all Coff targets except that they use the external symbol format or use different values for certain constants.

The generic routines are in coffgen.c. These routines work for any Coff target. They use some hooks into the target specific code; the hooks are in a bfd_coff_backend_data structure, one of which exists for each target.

The essentially similar target-specific routines are in coffcode.h. This header file includes executable C code. The various Coff targets first include the appropriate Coff header file, make any special defines that are needed, and then include coffcode.h.

Some of the Coff targets then also have additional routines in the target source file itself.

For example, coff-i960.c includes coff/internal.h and coff/i960.h. It then defines a few constants, such as I960, and includes coffcode.h. Since the i960 has complex relocation types, coff-i960.c also includes some code to manipulate the i960 relocs. This code is not in coffcode.h because it would not be used by any other target. Bit twiddling

Each flavour of coff supported in BFD has its own header file describing the external layout of the structures. There is also an internal description of the coff layout, in coff/internal.h. A major function of the coff backend is swapping the bytes and twiddling the bits to translate the external form of the structures into the normal internal form. This is all performed in the bfd_swap_thing_direction routines. Some elements are different sizes between different versions of coff; it is the duty of the coff version specific include file to override the definitions of various packing routines in coffcode.h. E.g., the size of line number entry in coff is sometimes 16 bits, and sometimes 32 bits. #defineing PUT_LNSZ_LNNO and GET_LNSZ_LNNO will select the correct one. No doubt, some day someone will find a version of coff which has a varying field size not catered to at the moment. To port BFD, that person will have to add more #defines. Three of the bit twiddling routines are exported to gdb; coff_swap_aux_in, coff_swap_sym_in and coff_swap_lineno_in. GDB reads the symbol table on its own, but uses BFD to fix things up. More of the bit twiddlers are exported for gas; coff_swap_aux_out, coff_swap_sym_out, coff_swap_lineno_out, coff_swap_reloc_out, coff_swap_filehdr_out, coff_swap_aouthdr_out, coff_swap_scnhdr_out. Gas currently keeps track of all the symbol table and reloc drudgery itself, thereby saving the internal BFD overhead, but uses BFD to swap things on the way out, making cross ports much safer. Doing so also allows BFD (and thus the linker) to use the same header files as gas, which makes one avenue to disaster disappear. Symbol reading

The simple canonical form for symbols used by BFD is not rich enough to keep all the information available in a coff symbol table. The back end gets around this problem by keeping the original symbol table around, "behind the scenes".

When a symbol table is requested (through a call to bfd_canonicalize_symtab), a request gets through to coff_get_normalized_symtab. This reads the symbol table from the coff file and swaps all the structures inside into the internal form. It also fixes up all the pointers in the table (represented in the file by offsets from the first symbol in the table) into physical pointers to elements in the new internal table. This involves some work since the meanings of fields change depending upon context: a field that is a pointer to another structure in the symbol table at one moment may be the size in bytes of a structure at the next. Another pass is made over the table. All symbols which mark file names (C_FILE symbols) are modified so that the internal string points to the value in the auxent (the real filename) rather than the normal text associated with the symbol (".file").

At this time the symbol names are moved around. Coff stores all symbols less than nine characters long physically within the symbol table; longer strings are kept at the end of the file in the string table. This pass moves all strings into memory and replaces them with pointers to the strings.

The symbol table is massaged once again, this time to create the canonical table used by the BFD application. Each symbol is inspected in turn, and a decision made (using the sclass field) about the various flags to set in the asymbol. See Symbols. The generated canonical table shares strings with the hidden internal symbol table.

Any linenumbers are read from the coff file too, and attached to the symbols which own the functions the linenumbers belong to. Symbol writing

Writing a symbol to a coff file which didn't come from a coff file will lose any debugging information. The asymbol structure remembers the BFD from which the symbol was taken, and on output the back end makes sure that the same destination target as source target is present.

When the symbols have come from a coff file then all the debugging information is preserved.

Symbol tables are provided for writing to the back end in a vector of pointers to pointers. This allows applications like the linker to accumulate and output large symbol tables without having to do too much byte copying.

This function runs through the provided symbol table and patches each symbol marked as a file place holder (C_FILE) to point to the next file place holder in the list. It also marks each offset field in the list with the offset from the first symbol of the current symbol.

Another function of this procedure is to turn the canonical value form of BFD into the form used by coff. Internally, BFD expects symbol values to be offsets from a section base; so a symbol physically at 0x120, but in a section starting at 0x100, would have the value 0x20. Coff expects symbols to contain their final value, so symbols have their values changed at this point to reflect their sum with their owning section. This transformation uses the output_section field of the asymbol's asection See Sections.

This routine runs though the provided symbol table and uses the offsets generated by the previous pass and the pointers generated when the symbol table was read in to create the structured hierarchy required by coff. It changes each pointer to a symbol into the index into the symbol table of the asymbol. This routine runs through the symbol table and patches up the symbols from their internal form into the coff way, calls the bit twiddlers, and writes out the table to the file. coff_symbol_type

The hidden information for an asymbol is described in a combined_entry_type:

     typedef struct coff_ptr_struct
       /* Remembers the offset from the first symbol in the file for
          this symbol. Generated by coff_renumber_symbols. */
       unsigned int offset;
       /* Should the value of this symbol be renumbered.  Used for
          XCOFF C_BSTAT symbols.  Set by coff_slurp_symbol_table.  */
       unsigned int fix_value : 1;
       /* Should the tag field of this symbol be renumbered.
          Created by coff_pointerize_aux. */
       unsigned int fix_tag : 1;
       /* Should the endidx field of this symbol be renumbered.
          Created by coff_pointerize_aux. */
       unsigned int fix_end : 1;
       /* Should the x_csect.x_scnlen field be renumbered.
          Created by coff_pointerize_aux. */
       unsigned int fix_scnlen : 1;
       /* Fix up an XCOFF C_BINCL/C_EINCL symbol.  The value is the
          index into the line number entries.  Set by coff_slurp_symbol_table.  */
       unsigned int fix_line : 1;
       /* The container for the symbol structure as read and translated
          from the file. */
         union internal_auxent auxent;
         struct internal_syment syment;
       } u;
     } combined_entry_type;
     /* Each canonical asymbol really looks like this: */
     typedef struct coff_symbol_struct
       /* The actual symbol which the rest of BFD works with */
       asymbol symbol;
       /* A pointer to the hidden information for this symbol */
       combined_entry_type *native;
       /* A pointer to the linenumber information for this symbol */
       struct lineno_cache_entry *lineno;
       /* Have the line numbers been relocated yet ? */
       bfd_boolean done_lineno;
     } coff_symbol_type; bfd_coff_backend_data
     /* COFF symbol classifications.  */
     enum coff_symbol_classification
       /* Global symbol.  */
       /* Common symbol.  */
       /* Undefined symbol.  */
       /* Local symbol.  */
       /* PE section symbol.  */

Special entry points for gdb to swap in coff symbol table parts:

     typedef struct
       void (*_bfd_coff_swap_aux_in)
         (bfd *, void *, int, int, int, int, void *);
       void (*_bfd_coff_swap_sym_in)
         (bfd *, void *, void *);
       void (*_bfd_coff_swap_lineno_in)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_aux_out)
         (bfd *, void *, int, int, int, int, void *);
       unsigned int (*_bfd_coff_swap_sym_out)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_lineno_out)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_reloc_out)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_filehdr_out)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_aouthdr_out)
         (bfd *, void *, void *);
       unsigned int (*_bfd_coff_swap_scnhdr_out)
         (bfd *, void *, void *);
       unsigned int _bfd_filhsz;
       unsigned int _bfd_aoutsz;
       unsigned int _bfd_scnhsz;
       unsigned int _bfd_symesz;
       unsigned int _bfd_auxesz;
       unsigned int _bfd_relsz;
       unsigned int _bfd_linesz;
       unsigned int _bfd_filnmlen;
       bfd_boolean _bfd_coff_long_filenames;
       bfd_boolean _bfd_coff_long_section_names;
       unsigned int _bfd_coff_default_section_alignment_power;
       bfd_boolean _bfd_coff_force_symnames_in_strings;
       unsigned int _bfd_coff_debug_string_prefix_length;
       void (*_bfd_coff_swap_filehdr_in)
         (bfd *, void *, void *);
       void (*_bfd_coff_swap_aouthdr_in)
         (bfd *, void *, void *);
       void (*_bfd_coff_swap_scnhdr_in)
         (bfd *, void *, void *);
       void (*_bfd_coff_swap_reloc_in)
         (bfd *abfd, void *, void *);
       bfd_boolean (*_bfd_coff_bad_format_hook)
         (bfd *, void *);
       bfd_boolean (*_bfd_coff_set_arch_mach_hook)
         (bfd *, void *);
       void * (*_bfd_coff_mkobject_hook)
         (bfd *, void *, void *);
       bfd_boolean (*_bfd_styp_to_sec_flags_hook)
         (bfd *, void *, const char *, asection *, flagword *);
       void (*_bfd_set_alignment_hook)
         (bfd *, asection *, void *);
       bfd_boolean (*_bfd_coff_slurp_symbol_table)
         (bfd *);
       bfd_boolean (*_bfd_coff_symname_in_debug)
         (bfd *, struct internal_syment *);
       bfd_boolean (*_bfd_coff_pointerize_aux_hook)
         (bfd *, combined_entry_type *, combined_entry_type *,
                 unsigned int, combined_entry_type *);
       bfd_boolean (*_bfd_coff_print_aux)
         (bfd *, FILE *, combined_entry_type *, combined_entry_type *,
                 combined_entry_type *, unsigned int);
       void (*_bfd_coff_reloc16_extra_cases)
         (bfd *, struct bfd_link_info *, struct bfd_link_order *, arelent *,
                bfd_byte *, unsigned int *, unsigned int *);
       int (*_bfd_coff_reloc16_estimate)
         (bfd *, asection *, arelent *, unsigned int,
                 struct bfd_link_info *);
       enum coff_symbol_classification (*_bfd_coff_classify_symbol)
         (bfd *, struct internal_syment *);
       bfd_boolean (*_bfd_coff_compute_section_file_positions)
         (bfd *);
       bfd_boolean (*_bfd_coff_start_final_link)
         (bfd *, struct bfd_link_info *);
       bfd_boolean (*_bfd_coff_relocate_section)
         (bfd *, struct bfd_link_info *, bfd *, asection *, bfd_byte *,
                 struct internal_reloc *, struct internal_syment *, asection **);
       reloc_howto_type *(*_bfd_coff_rtype_to_howto)
         (bfd *, asection *, struct internal_reloc *,
                 struct coff_link_hash_entry *, struct internal_syment *,
                 bfd_vma *);
       bfd_boolean (*_bfd_coff_adjust_symndx)
         (bfd *, struct bfd_link_info *, bfd *, asection *,
                 struct internal_reloc *, bfd_boolean *);
       bfd_boolean (*_bfd_coff_link_add_one_symbol)
         (struct bfd_link_info *, bfd *, const char *, flagword,
                 asection *, bfd_vma, const char *, bfd_boolean, bfd_boolean,
                 struct bfd_link_hash_entry **);
       bfd_boolean (*_bfd_coff_link_output_has_begun)
         (bfd *, struct coff_final_link_info *);
       bfd_boolean (*_bfd_coff_final_link_postscript)
         (bfd *, struct coff_final_link_info *);
     } bfd_coff_backend_data;
     #define coff_backend_info(abfd) \
       ((bfd_coff_backend_data *) (abfd)->xvec->backend_data)
     #define bfd_coff_swap_aux_in(a,e,t,c,ind,num,i) \
       ((coff_backend_info (a)->_bfd_coff_swap_aux_in) (a,e,t,c,ind,num,i))
     #define bfd_coff_swap_sym_in(a,e,i) \
       ((coff_backend_info (a)->_bfd_coff_swap_sym_in) (a,e,i))
     #define bfd_coff_swap_lineno_in(a,e,i) \
       ((coff_backend_info ( a)->_bfd_coff_swap_lineno_in) (a,e,i))
     #define bfd_coff_swap_reloc_out(abfd, i, o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_out) (abfd, i, o))
     #define bfd_coff_swap_lineno_out(abfd, i, o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_lineno_out) (abfd, i, o))
     #define bfd_coff_swap_aux_out(a,i,t,c,ind,num,o) \
       ((coff_backend_info (a)->_bfd_coff_swap_aux_out) (a,i,t,c,ind,num,o))
     #define bfd_coff_swap_sym_out(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_sym_out) (abfd, i, o))
     #define bfd_coff_swap_scnhdr_out(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_out) (abfd, i, o))
     #define bfd_coff_swap_filehdr_out(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_out) (abfd, i, o))
     #define bfd_coff_swap_aouthdr_out(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_out) (abfd, i, o))
     #define bfd_coff_filhsz(abfd) (coff_backend_info (abfd)->_bfd_filhsz)
     #define bfd_coff_aoutsz(abfd) (coff_backend_info (abfd)->_bfd_aoutsz)
     #define bfd_coff_scnhsz(abfd) (coff_backend_info (abfd)->_bfd_scnhsz)
     #define bfd_coff_symesz(abfd) (coff_backend_info (abfd)->_bfd_symesz)
     #define bfd_coff_auxesz(abfd) (coff_backend_info (abfd)->_bfd_auxesz)
     #define bfd_coff_relsz(abfd)  (coff_backend_info (abfd)->_bfd_relsz)
     #define bfd_coff_linesz(abfd) (coff_backend_info (abfd)->_bfd_linesz)
     #define bfd_coff_filnmlen(abfd) (coff_backend_info (abfd)->_bfd_filnmlen)
     #define bfd_coff_long_filenames(abfd) \
       (coff_backend_info (abfd)->_bfd_coff_long_filenames)
     #define bfd_coff_long_section_names(abfd) \
       (coff_backend_info (abfd)->_bfd_coff_long_section_names)
     #define bfd_coff_default_section_alignment_power(abfd) \
       (coff_backend_info (abfd)->_bfd_coff_default_section_alignment_power)
     #define bfd_coff_swap_filehdr_in(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_filehdr_in) (abfd, i, o))
     #define bfd_coff_swap_aouthdr_in(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_aouthdr_in) (abfd, i, o))
     #define bfd_coff_swap_scnhdr_in(abfd, i,o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_scnhdr_in) (abfd, i, o))
     #define bfd_coff_swap_reloc_in(abfd, i, o) \
       ((coff_backend_info (abfd)->_bfd_coff_swap_reloc_in) (abfd, i, o))
     #define bfd_coff_bad_format_hook(abfd, filehdr) \
       ((coff_backend_info (abfd)->_bfd_coff_bad_format_hook) (abfd, filehdr))
     #define bfd_coff_set_arch_mach_hook(abfd, filehdr)\
       ((coff_backend_info (abfd)->_bfd_coff_set_arch_mach_hook) (abfd, filehdr))
     #define bfd_coff_mkobject_hook(abfd, filehdr, aouthdr)\
       ((coff_backend_info (abfd)->_bfd_coff_mkobject_hook)\
        (abfd, filehdr, aouthdr))
     #define bfd_coff_styp_to_sec_flags_hook(abfd, scnhdr, name, section, flags_ptr)\
       ((coff_backend_info (abfd)->_bfd_styp_to_sec_flags_hook)\
        (abfd, scnhdr, name, section, flags_ptr))
     #define bfd_coff_set_alignment_hook(abfd, sec, scnhdr)\
       ((coff_backend_info (abfd)->_bfd_set_alignment_hook) (abfd, sec, scnhdr))
     #define bfd_coff_slurp_symbol_table(abfd)\
       ((coff_backend_info (abfd)->_bfd_coff_slurp_symbol_table) (abfd))
     #define bfd_coff_symname_in_debug(abfd, sym)\
       ((coff_backend_info (abfd)->_bfd_coff_symname_in_debug) (abfd, sym))
     #define bfd_coff_force_symnames_in_strings(abfd)\
       (coff_backend_info (abfd)->_bfd_coff_force_symnames_in_strings)
     #define bfd_coff_debug_string_prefix_length(abfd)\
       (coff_backend_info (abfd)->_bfd_coff_debug_string_prefix_length)
     #define bfd_coff_print_aux(abfd, file, base, symbol, aux, indaux)\
       ((coff_backend_info (abfd)->_bfd_coff_print_aux)\
        (abfd, file, base, symbol, aux, indaux))
     #define bfd_coff_reloc16_extra_cases(abfd, link_info, link_order,\
                                          reloc, data, src_ptr, dst_ptr)\
       ((coff_backend_info (abfd)->_bfd_coff_reloc16_extra_cases)\
        (abfd, link_info, link_order, reloc, data, src_ptr, dst_ptr))
     #define bfd_coff_reloc16_estimate(abfd, section, reloc, shrink, link_info)\
       ((coff_backend_info (abfd)->_bfd_coff_reloc16_estimate)\
        (abfd, section, reloc, shrink, link_info))
     #define bfd_coff_classify_symbol(abfd, sym)\
       ((coff_backend_info (abfd)->_bfd_coff_classify_symbol)\
        (abfd, sym))
     #define bfd_coff_compute_section_file_positions(abfd)\
       ((coff_backend_info (abfd)->_bfd_coff_compute_section_file_positions)\
     #define bfd_coff_start_final_link(obfd, info)\
       ((coff_backend_info (obfd)->_bfd_coff_start_final_link)\
        (obfd, info))
     #define bfd_coff_relocate_section(obfd,info,ibfd,o,con,rel,isyms,secs)\
       ((coff_backend_info (ibfd)->_bfd_coff_relocate_section)\
        (obfd, info, ibfd, o, con, rel, isyms, secs))
     #define bfd_coff_rtype_to_howto(abfd, sec, rel, h, sym, addendp)\
       ((coff_backend_info (abfd)->_bfd_coff_rtype_to_howto)\
        (abfd, sec, rel, h, sym, addendp))
     #define bfd_coff_adjust_symndx(obfd, info, ibfd, sec, rel, adjustedp)\
       ((coff_backend_info (abfd)->_bfd_coff_adjust_symndx)\
        (obfd, info, ibfd, sec, rel, adjustedp))
     #define bfd_coff_link_add_one_symbol(info, abfd, name, flags, section,\
                                          value, string, cp, coll, hashp)\
       ((coff_backend_info (abfd)->_bfd_coff_link_add_one_symbol)\
        (info, abfd, name, flags, section, value, string, cp, coll, hashp))
     #define bfd_coff_link_output_has_begun(a,p) \
       ((coff_backend_info (a)->_bfd_coff_link_output_has_begun) (a, p))
     #define bfd_coff_final_link_postscript(a,p) \
       ((coff_backend_info (a)->_bfd_coff_final_link_postscript) (a, p)) Writing relocations

To write relocations, the back end steps though the canonical relocation table and create an internal_reloc. The symbol index to use is removed from the offset field in the symbol table supplied. The address comes directly from the sum of the section base address and the relocation offset; the type is dug directly from the howto field. Then the internal_reloc is swapped into the shape of an external_reloc and written out to disk. Reading linenumbers

Creating the linenumber table is done by reading in the entire coff linenumber table, and creating another table for internal use.

A coff linenumber table is structured so that each function is marked as having a line number of 0. Each line within the function is an offset from the first line in the function. The base of the line number information for the table is stored in the symbol associated with the function.

Note: The PE format uses line number 0 for a flag indicating a new source file.

The information is copied from the external to the internal table, and each symbol which marks a function is marked by pointing its...

How does this work ? Reading relocations

Coff relocations are easily transformed into the internal BFD form (arelent).

Reading a coff relocation table is done in the following stages:

Next: , Previous: coff, Up: BFD back ends


ELF backends

BFD support for ELF formats is being worked on. Currently, the best supported back ends are for sparc and i386 (running svr4 or Solaris 2).

Documentation of the internals of the support code still needs to be written. The code is changing quickly enough that we haven't bothered yet. bfd_elf_find_section


     struct elf_internal_shdr *bfd_elf_find_section (bfd *abfd, char *name);

Helper functions for GDB to locate the string tables. Since BFD hides string tables from callers, GDB needs to use an internal hook to find them. Sun's .stabstr, in particular, isn't even pointed to by the .stab section, so ordinary mechanisms wouldn't work to find it, even if we had some.

Previous: elf, Up: BFD back ends

3.4 mmo backend

The mmo object format is used exclusively together with Professor Donald E. Knuth's educational 64-bit processor MMIX. The simulator mmix which is available at understands this format. That package also includes a combined assembler and linker called mmixal. The mmo format has no advantages feature-wise compared to e.g. ELF. It is a simple non-relocatable object format with no support for archives or debugging information, except for symbol value information and line numbers (which is not yet implemented in BFD). See for more information about MMIX. The ELF format is used for intermediate object files in the BFD implementation.

Next: , Previous: mmo, Up: mmo

3.4.1 File layout

The mmo file contents is not partitioned into named sections as with e.g. ELF. Memory areas is formed by specifying the location of the data that follows. Only the memory area `0x0000...00' to `0x01ff...ff' is executable, so it is used for code (and constants) and the area `0x2000...00' to `0x20ff...ff' is used for writable data. See mmo section mapping.

There is provision for specifying “special data” of 65536 different types. We use type 80 (decimal), arbitrarily chosen the same as the ELF e_machine number for MMIX, filling it with section information normally found in ELF objects. See mmo section mapping.

Contents is entered as 32-bit words, xor:ed over previous contents, always zero-initialized. A word that starts with the byte `0x98' forms a command called a `lopcode', where the next byte distinguished between the thirteen lopcodes. The two remaining bytes, called the `Y' and `Z' fields, or the `YZ' field (a 16-bit big-endian number), are used for various purposes different for each lopcode. As documented in, the lopcodes are:

0x98000001. The next word is contents, regardless of whether it starts with 0x98 or not.
0x9801YYZZ, where `Z' is 1 or 2. This is a location directive, setting the location for the next data to the next 32-bit word (for Z = 1) or 64-bit word (for Z = 2), plus Y * 2^56. Normally `Y' is 0 for the text segment and 2 for the data segment.
0x9802YYZZ. Increase the current location by `YZ' bytes.
0x9803YYZZ, where `Z' is 1 or 2. Store the current location as 64 bits into the location pointed to by the next 32-bit (Z = 1) or 64-bit (Z = 2) word, plus Y * 2^56.
0x9804YYZZ. `YZ' is stored into the current location plus 2 - 4 * YZ.
0x980500ZZ. `Z' is 16 or 24. A value `L' derived from the following 32-bit word are used in a manner similar to `YZ' in lop_fixr: it is xor:ed into the current location minus 4 * L. The first byte of the word is 0 or 1. If it is 1, then L = (lowest 24 bits of word) - 2^Z, if 0, then L = (lowest 24 bits of word).
0x9806YYZZ. `Y' is the file number, `Z' is count of 32-bit words. Set the file number to `Y' and the line counter to 0. The next Z * 4 bytes contain the file name, padded with zeros if the count is not a multiple of four. The same `Y' may occur multiple times, but `Z' must be 0 for all but the first occurrence.
0x9807YYZZ. `YZ' is the line number. Together with lop_file, it forms the source location for the next 32-bit word. Note that for each non-lopcode 32-bit word, line numbers are assumed incremented by one.
0x9808YYZZ. `YZ' is the type number. Data until the next lopcode other than lop_quote forms special data of type `YZ'. See mmo section mapping.

Other types than 80, (or type 80 with a content that does not parse) is stored in sections named .MMIX.spec_data.n where n is the `YZ'-type. The flags for such a sections say not to allocate or load the data. The vma is 0. Contents of multiple occurrences of special data n is concatenated to the data of the previous lop_spec ns. The location in data or code at which the lop_spec occurred is lost.

0x980901ZZ. The first lopcode in a file. The `Z' field forms the length of header information in 32-bit words, where the first word tells the time in seconds since `00:00:00 GMT Jan 1 1970'.
0x980a00ZZ. Z > 32. This lopcode follows after all content-generating lopcodes in a program. The `Z' field denotes the value of `rG' at the beginning of the program. The following 256 - Z big-endian 64-bit words are loaded into global registers `$G' ... `$255'.
0x980b0000. The next-to-last lopcode in a program. Must follow immediately after the lop_post lopcode and its data. After this lopcode follows all symbols in a compressed format (see Symbol-table).
0x980cYYZZ. The last lopcode in a program. It must follow the lop_stab lopcode and its data. The `YZ' field contains the number of 32-bit words of symbol table information after the preceding lop_stab lopcode.

Note that the lopcode "fixups"; lop_fixr, lop_fixrx and lop_fixo are not generated by BFD, but are handled. They are generated by mmixal.

This trivial one-label, one-instruction file:

      :Main TRAP 1,2,3

can be represented this way in mmo:

      0x98090101 - lop_pre, one 32-bit word with timestamp.
      0x98010002 - lop_loc, text segment, using a 64-bit address.
                   Note that mmixal does not emit this for the file above.
      0x00000000 - Address, high 32 bits.
      0x00000000 - Address, low 32 bits.
      0x98060002 - lop_file, 2 32-bit words for file-name.
      0x74657374 - "test"
      0x2e730000 - ".s\0\0"
      0x98070001 - lop_line, line 1.
      0x00010203 - TRAP 1,2,3
      0x980a00ff - lop_post, setting $255 to 0.
      0x980b0000 - lop_stab for ":Main" = 0, serial 1.
      0x203a4040   See Symbol-table.
      0x980c0005 - lop_end; symbol table contained five 32-bit words.

Next: , Previous: File layout, Up: mmo

3.4.2 Symbol table format

From mmixal.w (or really, the generated mmixal.tex) in “Symbols are stored and retrieved by means of a `ternary search trie', following ideas of Bentley and Sedgewick. (See ACM–SIAM Symp. on Discrete Algorithms `8' (1997), 360–369; R.Sedgewick, `Algorithms in C' (Reading, Mass. Addison–Wesley, 1998), `15.4'.) Each trie node stores a character, and there are branches to subtries for the cases where a given character is less than, equal to, or greater than the character in the trie. There also is a pointer to a symbol table entry if a symbol ends at the current node.”

So it's a tree encoded as a stream of bytes. The stream of bytes acts on a single virtual global symbol, adding and removing characters and signalling complete symbol points. Here, we read the stream and create symbols at the completion points.

First, there's a control byte m. If any of the listed bits in m is nonzero, we execute what stands at the right, in the listed order:

      0x40 - Traverse left trie.
             (Read a new command byte and recurse.)
      0x2f - Read the next byte as a character and store it in the
             current character position; increment character position.
             Test the bits of m:
             0x80 - The character is 16-bit (so read another byte,
                    merge into current character.
             0xf  - We have a complete symbol; parse the type, value
                    and serial number and do what should be done
                    with a symbol.  The type and length information
                    is in j = (m & 0xf).
                    j == 0xf: A register variable.  The following
                              byte tells which register.
                    j <= 8:   An absolute symbol.  Read j bytes as the
                              big-endian number the symbol equals.
                              A j = 2 with two zero bytes denotes an
                              unknown symbol.
                    j > 8:    As with j <= 8, but add (0x20 << 56)
                              to the value in the following j - 8
                    Then comes the serial number, as a variant of
                    uleb128, but better named ubeb128:
                    Read bytes and shift the previous value left 7
                    (multiply by 128).  Add in the new byte, repeat
                    until a byte has bit 7 set.  The serial number
                    is the computed value minus 128.
             0x20 - Traverse middle trie.  (Read a new command byte
                    and recurse.)  Decrement character position.
      0x10 - Traverse right trie.  (Read a new command byte and

Let's look again at the lop_stab for the trivial file (see File layout).

      0x980b0000 - lop_stab for ":Main" = 0, serial 1.

This forms the trivial trie (note that the path between “:” and “M” is redundant):

      203a     ":"
      40       /
      40      /
      10      \
      40      /
      40     /
      204d  "M"
      2061  "a"
      2069  "i"
      016e  "n" is the last character in a full symbol, and
            with a value represented in one byte.
      00    The value is 0.
      81    The serial number is 1.

Previous: Symbol-table, Up: mmo

3.4.3 mmo section mapping

The implementation in BFD uses special data type 80 (decimal) to encapsulate and describe named sections, containing e.g. debug information. If needed, any datum in the encapsulation will be quoted using lop_quote. First comes a 32-bit word holding the number of 32-bit words containing the zero-terminated zero-padded segment name. After the name there's a 32-bit word holding flags describing the section type. Then comes a 64-bit big-endian word with the section length (in bytes), then another with the section start address. Depending on the type of section, the contents might follow, zero-padded to 32-bit boundary. For a loadable section (such as data or code), the contents might follow at some later point, not necessarily immediately, as a lop_loc with the same start address as in the section description, followed by the contents. This in effect forms a descriptor that must be emitted before the actual contents. Sections described this way must not overlap.

For areas that don't have such descriptors, synthetic sections are formed by BFD. Consecutive contents in the two memory areas `0x0000...00' to `0x01ff...ff' and `0x2000...00' to `0x20ff...ff' are entered in sections named .text and .data respectively. If an area is not otherwise described, but would together with a neighboring lower area be less than `0x40000000' bytes long, it is joined with the lower area and the gap is zero-filled. For other cases, a new section is formed, named .MMIX.sec.n. Here, n is a number, a running count through the mmo file, starting at 0.

A loadable section specified as:

      .section secname,"ax"
      TETRA 1,2,3,4,-1,-2009
      BYTE 80

and linked to address `0x4', is represented by the sequence:

      0x98080050 - lop_spec 80
      0x00000002 - two 32-bit words for the section name
      0x7365636e - "secn"
      0x616d6500 - "ame\0"
      0x00000033 - flags CODE, READONLY, LOAD, ALLOC
      0x00000000 - high 32 bits of section length
      0x0000001c - section length is 28 bytes; 6 * 4 + 1 + alignment to 32 bits
      0x00000000 - high 32 bits of section address
      0x00000004 - section address is 4
      0x98010002 - 64 bits with address of following data
      0x00000000 - high 32 bits of address
      0x00000004 - low 32 bits: data starts at address 4
      0x00000001 - 1
      0x00000002 - 2
      0x00000003 - 3
      0x00000004 - 4
      0xffffffff - -1
      0xfffff827 - -2009
      0x50000000 - 80 as a byte, padded with zeros.

Note that the lop_spec wrapping does not include the section contents. Compare this to a non-loaded section specified as:

      .section thirdsec
      TETRA 200001,100002
      BYTE 38,40

This, when linked to address `0x200000000000001c', is represented by:

      0x98080050 - lop_spec 80
      0x00000002 - two 32-bit words for the section name
      0x7365636e - "thir"
      0x616d6500 - "dsec"
      0x00000010 - flag READONLY
      0x00000000 - high 32 bits of section length
      0x0000000c - section length is 12 bytes; 2 * 4 + 2 + alignment to 32 bits
      0x20000000 - high 32 bits of address
      0x0000001c - low 32 bits of address 0x200000000000001c
      0x00030d41 - 200001
      0x000186a2 - 100002
      0x26280000 - 38, 40 as bytes, padded with zeros

For the latter example, the section contents must not be loaded in memory, and is therefore specified as part of the special data. The address is usually unimportant but might provide information for e.g. the DWARF 2 debugging format.

Next: , Previous: BFD back ends, Up: Top

Appendix A GNU Free Documentation License

Version 1.1, March 2000
     Copyright (C) 2000, 2003 Free Software Foundation, Inc.
     51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.


    The purpose of this License is to make a manual, textbook, or other written document “free” in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


    This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you.”

    A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

    A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

    The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

    The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

    A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not “Transparent” is called “Opaque.”

    Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

    The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.


    You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

    You may also lend copies, under the same conditions stated above, and you may publicly display copies.


    If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.


    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).
    C. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    D. Preserve all the copyright notices of the Document.
    E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
    H. Include an unaltered copy of this License.
    I. Preserve the section entitled “History”, and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    K. In any section entitled “Acknowledgements” or “Dedications”, preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    M. Delete any section entitled “Endorsements.” Such a section may not be included in the Modified Version.
    N. Do not retitle any existing section as “Endorsements” or to conflict in title with any Invariant Section.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

    You may add a section entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties–for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.


    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections entitled “History” in the various original documents, forming one section entitled “History”; likewise combine any sections entitled “Acknowledgements”, and any sections entitled “Dedications.” You must delete all sections entitled “Endorsements.”


    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an “aggregate”, and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.


    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.


    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

     Copyright (C)  year  your name.
     Permission is granted to copy, distribute and/or modify this document
     under the terms of the GNU Free Documentation License, Version 1.1
     or any later version published by the Free Software Foundation;
     with the Invariant Sections being list their titles, with the
     Front-Cover Texts being list, and with the Back-Cover Texts being list.
     A copy of the license is included in the section entitled "GNU
     Free Documentation License."

If you have no Invariant Sections, write “with no Invariant Sections” instead of saying which ones are invariant. If you have no Front-Cover Texts, write “no Front-Cover Texts” instead of “Front-Cover Texts being list”; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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