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BFD Internals

This document describes some BFD internal information which may be helpful when working on BFD. It is very incomplete.

This document is not updated regularly, and may be out of date.

The initial version of this document was written by Ian Lance Taylor ian@cygnus.com.


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BFD overview

BFD is a library which provides a single interface to read and write object files, executables, archive files, and core files in any format.


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BFD library interfaces

One way to look at the BFD library is to divide it into four parts by type of interface.

The first interface is the set of generic functions which programs using the BFD library will call. These generic function normally translate directly or indirectly into calls to routines which are specific to a particular object file format. Many of these generic functions are actually defined as macros in bfd.h. These functions comprise the official BFD interface.

The second interface is the set of functions which appear in the target vectors. This is the bulk of the code in BFD. A target vector is a set of function pointers specific to a particular object file format. The target vector is used to implement the generic BFD functions. These functions are always called through the target vector, and are never called directly. The target vector is described in detail in BFD target vector. The set of functions which appear in a particular target vector is often referred to as a BFD backend.

The third interface is a set of oddball functions which are typically specific to a particular object file format, are not generic functions, and are called from outside of the BFD library. These are used as hooks by the linker and the assembler when a particular object file format requires some action which the BFD generic interface does not provide. These functions are typically declared in bfd.h, but in many cases they are only provided when BFD is configured with support for a particular object file format. These functions live in a grey area, and are not really part of the official BFD interface.

The fourth interface is the set of BFD support functions which are called by the other BFD functions. These manage issues like memory allocation, error handling, file access, hash tables, swapping, and the like. These functions are never called from outside of the BFD library.


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BFD library users

Another way to look at the BFD library is to divide it into three parts by the manner in which it is used.

The first use is to read an object file. The object file readers are programs like `gdb', `nm', `objdump', and `objcopy'. These programs use BFD to view an object file in a generic form. The official BFD interface is normally fully adequate for these programs.

The second use is to write an object file. The object file writers are programs like `gas' and `objcopy'. These programs use BFD to create an object file. The official BFD interface is normally adequate for these programs, but for some object file formats the assembler needs some additional hooks in order to set particular flags or other information. The official BFD interface includes functions to copy private information from one object file to another, and these functions are used by `objcopy' to avoid information loss.

The third use is to link object files. There is only one object file linker, `ld'. Originally, `ld' was an object file reader and an object file writer, and it did the link operation using the generic BFD structures. However, this turned out to be too slow and too memory intensive.

The official BFD linker functions were written to permit specific BFD backends to perform the link without translating through the generic structures, in the normal case where all the input files and output file have the same object file format. Not all of the backends currently implement the new interface, and there are default linking functions within BFD which use the generic structures and which work with all backends.

For several object file formats the linker needs additional hooks which are not provided by the official BFD interface, particularly for dynamic linking support. These functions are typically called from the linker emulation template.


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The BFD view of a file

BFD uses generic structures to manage information. It translates data into the generic form when reading files, and out of the generic form when writing files.

BFD describes a file as a pointer to the `bfd' type. A `bfd' is composed of the following elements. The BFD information can be displayed using the `objdump' program with various options.

general information
The object file format, a few general flags, the start address.
architecture
The architecture, including both a general processor type (m68k, MIPS etc.) and a specific machine number (m68000, R4000, etc.).
sections
A list of sections.
symbols
A symbol table.

BFD represents a section as a pointer to the `asection' type. Each section has a name and a size. Most sections also have an associated block of data, known as the section contents. Sections also have associated flags, a virtual memory address, a load memory address, a required alignment, a list of relocations, and other miscellaneous information.

BFD represents a relocation as a pointer to the `arelent' type. A relocation describes an action which the linker must take to modify the section contents. Relocations have a symbol, an address, an addend, and a pointer to a howto structure which describes how to perform the relocation. For more information, see BFD relocation handling.

BFD represents a symbol as a pointer to the `asymbol' type. A symbol has a name, a pointer to a section, an offset within that section, and some flags.

Archive files do not have any sections or symbols. Instead, BFD represents an archive file as a file which contains a list of `bfd's. BFD also provides access to the archive symbol map, as a list of symbol names. BFD provides a function to return the `bfd' within the archive which corresponds to a particular entry in the archive symbol map.


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BFD loses information

Most object file formats have information which BFD can not represent in its generic form, at least as currently defined.

There is often explicit information which BFD can not represent. For example, the COFF version stamp, or the ELF program segments. BFD provides special hooks to handle this information when copying, printing, or linking an object file. The BFD support for a particular object file format will normally store this information in private data and handle it using the special hooks.

In some cases there is also implicit information which BFD can not represent. For example, the MIPS processor distinguishes small and large symbols, and requires that all small symbls be within 32K of the GP register. This means that the MIPS assembler must be able to mark variables as either small or large, and the MIPS linker must know to put small symbols within range of the GP register. Since BFD can not represent this information, this means that the assembler and linker must have information that is specific to a particular object file format which is outside of the BFD library.

This loss of information indicates areas where the BFD paradigm breaks down. It is not actually possible to represent the myriad differences among object file formats using a single generic interface, at least not in the manner which BFD does it today.

Nevertheless, the BFD library does greatly simplify the task of dealing with object files, and particular problems caused by information loss can normally be solved using some sort of relatively constrained hook into the library.


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BFD programming guidelines

There is a lot of poorly written and confusing code in BFD. New BFD code should be written to a higher standard. Merely because some BFD code is written in a particular manner does not mean that you should emulate it.

Here are some general BFD programming guidelines:


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BFD target vector

BFD supports multiple object file formats by using the target vector. This is simply a set of function pointers which implement behaviour that is specific to a particular object file format.

In this section I list all of the entries in the target vector and describe what they do.


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Miscellaneous constants

The target vector starts with a set of constants.

`name'
The name of the target vector. This is an arbitrary string. This is how the target vector is named in command line options for tools which use BFD, such as the `--oformat' linker option.
`flavour'
A general description of the type of target. The following flavours are currently defined:
`bfd_target_unknown_flavour'
Undefined or unknown.
`bfd_target_aout_flavour'
a.out.
`bfd_target_coff_flavour'
COFF.
`bfd_target_ecoff_flavour'
ECOFF.
`bfd_target_elf_flavour'
ELF.
`bfd_target_ieee_flavour'
IEEE-695.
`bfd_target_nlm_flavour'
NLM.
`bfd_target_oasys_flavour'
OASYS.
`bfd_target_tekhex_flavour'
Tektronix hex format.
`bfd_target_srec_flavour'
Motorola S-record format.
`bfd_target_ihex_flavour'
Intel hex format.
`bfd_target_som_flavour'
SOM (used on HP/UX).
`bfd_target_os9k_flavour'
os9000.
`bfd_target_versados_flavour'
VERSAdos.
`bfd_target_msdos_flavour'
MS-DOS.
`bfd_target_evax_flavour'
openVMS.
`bfd_target_mmo_flavour'
Donald Knuth's MMIXware object format.

`byteorder'
The byte order of data in the object file. One of `BFD_ENDIAN_BIG', `BFD_ENDIAN_LITTLE', or `BFD_ENDIAN_UNKNOWN'. The latter would be used for a format such as S-records which do not record the architecture of the data.
`header_byteorder'
The byte order of header information in the object file. Normally the same as the `byteorder' field, but there are certain cases where it may be different.
`object_flags'
Flags which may appear in the `flags' field of a BFD with this format.
`section_flags'
Flags which may appear in the `flags' field of a section within a BFD with this format.
`symbol_leading_char'
A character which the C compiler normally puts before a symbol. For example, an a.out compiler will typically generate the symbol `_foo' for a function named `foo' in the C source, in which case this field would be `_'. If there is no such character, this field will be `0'.
`ar_pad_char'
The padding character to use at the end of an archive name. Normally `/'.
`ar_max_namelen'
The maximum length of a short name in an archive. Normally `14'.
`backend_data'
A pointer to constant backend data. This is used by backends to store whatever additional information they need to distinguish similar target vectors which use the same sets of functions.


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Swapping functions

Every target vector has function pointers used for swapping information in and out of the target representation. There are two sets of functions: one for data information, and one for header information. Each set has three sizes: 64-bit, 32-bit, and 16-bit. Each size has three actual functions: put, get unsigned, and get signed.

These 18 functions are used to convert data between the host and target representations.


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Format type dependent functions

Every target vector has three arrays of function pointers which are indexed by the BFD format type. The BFD format types are as follows:

`bfd_unknown'
Unknown format. Not used for anything useful.
`bfd_object'
Object file.
`bfd_archive'
Archive file.
`bfd_core'
Core file.

The three arrays of function pointers are as follows:

`bfd_check_format'
Check whether the BFD is of a particular format (object file, archive file, or core file) corresponding to this target vector. This is called by the `bfd_check_format' function when examining an existing BFD. If the BFD matches the desired format, this function will initialize any format specific information such as the `tdata' field of the BFD. This function must be called before any other BFD target vector function on a file opened for reading.
`bfd_set_format'
Set the format of a BFD which was created for output. This is called by the `bfd_set_format' function after creating the BFD with a function such as `bfd_openw'. This function will initialize format specific information required to write out an object file or whatever of the given format. This function must be called before any other BFD target vector function on a file opened for writing.
`bfd_write_contents'
Write out the contents of the BFD in the given format. This is called by `bfd_close' function for a BFD opened for writing. This really should not be an array selected by format type, as the `bfd_set_format' function provides all the required information. In fact, BFD will fail if a different format is used when calling through the `bfd_set_format' and the `bfd_write_contents' arrays; fortunately, since `bfd_close' gets it right, this is a difficult error to make.


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`BFD_JUMP_TABLE' macros

Most target vectors are defined using `BFD_JUMP_TABLE' macros. These macros take a single argument, which is a prefix applied to a set of functions. The macros are then used to initialize the fields in the target vector.

For example, the `BFD_JUMP_TABLE_RELOCS' macro defines three functions: `_get_reloc_upper_bound', `_canonicalize_reloc', and `_bfd_reloc_type_lookup'. A reference like `BFD_JUMP_TABLE_RELOCS (foo)' will expand into three functions prefixed with `foo': `foo_get_reloc_upper_bound', etc. The `BFD_JUMP_TABLE_RELOCS' macro will be placed such that those three functions initialize the appropriate fields in the BFD target vector.

This is done because it turns out that many different target vectors can share certain classes of functions. For example, archives are similar on most platforms, so most target vectors can use the same archive functions. Those target vectors all use `BFD_JUMP_TABLE_ARCHIVE' with the same argument, calling a set of functions which is defined in archive.c.

Each of the `BFD_JUMP_TABLE' macros is mentioned below along with the description of the function pointers which it defines. The function pointers will be described using the name without the prefix which the `BFD_JUMP_TABLE' macro defines. This name is normally the same as the name of the field in the target vector structure. Any differences will be noted.


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Generic functions

The `BFD_JUMP_TABLE_GENERIC' macro is used for some catch all functions which don't easily fit into other categories.

`_close_and_cleanup'
Free any target specific information associated with the BFD. This is called when any BFD is closed (the `bfd_write_contents' function mentioned earlier is only called for a BFD opened for writing). Most targets use `bfd_alloc' to allocate all target specific information, and therefore don't have to do anything in this function. This function pointer is typically set to `_bfd_generic_close_and_cleanup', which simply returns true.
`_bfd_free_cached_info'
Free any cached information associated with the BFD which can be recreated later if necessary. This is used to reduce the memory consumption required by programs using BFD. This is normally called via the `bfd_free_cached_info' macro. It is used by the default archive routines when computing the archive map. Most targets do not do anything special for this entry point, and just set it to `_bfd_generic_free_cached_info', which simply returns true.
`_new_section_hook'
This is called from `bfd_make_section_anyway' whenever a new section is created. Most targets use it to initialize section specific information. This function is called whether or not the section corresponds to an actual section in an actual BFD.
`_get_section_contents'
Get the contents of a section. This is called from `bfd_get_section_contents'. Most targets set this to `_bfd_generic_get_section_contents', which does a `bfd_seek' based on the section's `filepos' field and a `bfd_bread'. The corresponding field in the target vector is named `_bfd_get_section_contents'.
`_get_section_contents_in_window'
Set a `bfd_window' to hold the contents of a section. This is called from `bfd_get_section_contents_in_window'. The `bfd_window' idea never really caught on, and I don't think this is ever called. Pretty much all targets implement this as `bfd_generic_get_section_contents_in_window', which uses `bfd_get_section_contents' to do the right thing. The corresponding field in the target vector is named `_bfd_get_section_contents_in_window'.


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Copy functions

The `BFD_JUMP_TABLE_COPY' macro is used for functions which are called when copying BFDs, and for a couple of functions which deal with internal BFD information.

`_bfd_copy_private_bfd_data'
This is called when copying a BFD, via `bfd_copy_private_bfd_data'. If the input and output BFDs have the same format, this will copy any private information over. This is called after all the section contents have been written to the output file. Only a few targets do anything in this function.
`_bfd_merge_private_bfd_data'
This is called when linking, via `bfd_merge_private_bfd_data'. It gives the backend linker code a chance to set any special flags in the output file based on the contents of the input file. Only a few targets do anything in this function.
`_bfd_copy_private_section_data'
This is similar to `_bfd_copy_private_bfd_data', but it is called for each section, via `bfd_copy_private_section_data'. This function is called before any section contents have been written. Only a few targets do anything in this function.
`_bfd_copy_private_symbol_data'
This is called via `bfd_copy_private_symbol_data', but I don't think anything actually calls it. If it were defined, it could be used to copy private symbol data from one BFD to another. However, most BFDs store extra symbol information by allocating space which is larger than the `asymbol' structure and storing private information in the extra space. Since `objcopy' and other programs copy symbol information by copying pointers to `asymbol' structures, the private symbol information is automatically copied as well. Most targets do not do anything in this function.
`_bfd_set_private_flags'
This is called via `bfd_set_private_flags'. It is basically a hook for the assembler to set magic information. For example, the PowerPC ELF assembler uses it to set flags which appear in the e_flags field of the ELF header. Most targets do not do anything in this function.
`_bfd_print_private_bfd_data'
This is called by `objdump' when the `-p' option is used. It is called via `bfd_print_private_data'. It prints any interesting information about the BFD which can not be otherwise represented by BFD and thus can not be printed by `objdump'. Most targets do not do anything in this function.


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Core file support functions

The `BFD_JUMP_TABLE_CORE' macro is used for functions which deal with core files. Obviously, these functions only do something interesting for targets which have core file support.

`_core_file_failing_command'
Given a core file, this returns the command which was run to produce the core file.
`_core_file_failing_signal'
Given a core file, this returns the signal number which produced the core file.
`_core_file_matches_executable_p'
Given a core file and a BFD for an executable, this returns whether the core file was generated by the executable.


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Archive functions

The `BFD_JUMP_TABLE_ARCHIVE' macro is used for functions which deal with archive files. Most targets use COFF style archive files (including ELF targets), and these use `_bfd_archive_coff' as the argument to `BFD_JUMP_TABLE_ARCHIVE'. Some targets use BSD/a.out style archives, and these use `_bfd_archive_bsd'. (The main difference between BSD and COFF archives is the format of the archive symbol table). Targets with no archive support use `_bfd_noarchive'. Finally, a few targets have unusual archive handling.

`_slurp_armap'
Read in the archive symbol table, storing it in private BFD data. This is normally called from the archive `check_format' routine. The corresponding field in the target vector is named `_bfd_slurp_armap'.
`_slurp_extended_name_table'
Read in the extended name table from the archive, if there is one, storing it in private BFD data. This is normally called from the archive `check_format' routine. The corresponding field in the target vector is named `_bfd_slurp_extended_name_table'.
`construct_extended_name_table'
Build and return an extended name table if one is needed to write out the archive. This also adjusts the archive headers to refer to the extended name table appropriately. This is normally called from the archive `write_contents' routine. The corresponding field in the target vector is named `_bfd_construct_extended_name_table'.
`_truncate_arname'
This copies a file name into an archive header, truncating it as required. It is normally called from the archive `write_contents' routine. This function is more interesting in targets which do not support extended name tables, but I think the GNU `ar' program always uses extended name tables anyhow. The corresponding field in the target vector is named `_bfd_truncate_arname'.
`_write_armap'
Write out the archive symbol table using calls to `bfd_bwrite'. This is normally called from the archive `write_contents' routine. The corresponding field in the target vector is named `write_armap' (no leading underscore).
`_read_ar_hdr'
Read and parse an archive header. This handles expanding the archive header name into the real file name using the extended name table. This is called by routines which read the archive symbol table or the archive itself. The corresponding field in the target vector is named `_bfd_read_ar_hdr_fn'.
`_openr_next_archived_file'
Given an archive and a BFD representing a file stored within the archive, return a BFD for the next file in the archive. This is called via `bfd_openr_next_archived_file'. The corresponding field in the target vector is named `openr_next_archived_file' (no leading underscore).
`_get_elt_at_index'
Given an archive and an index, return a BFD for the file in the archive corresponding to that entry in the archive symbol table. This is called via `bfd_get_elt_at_index'. The corresponding field in the target vector is named `_bfd_get_elt_at_index'.
`_generic_stat_arch_elt'
Do a stat on an element of an archive, returning information read from the archive header (modification time, uid, gid, file mode, size). This is called via `bfd_stat_arch_elt'. The corresponding field in the target vector is named `_bfd_stat_arch_elt'.
`_update_armap_timestamp'
After the entire contents of an archive have been written out, update the timestamp of the archive symbol table to be newer than that of the file. This is required for a.out style archives. This is normally called by the archive `write_contents' routine. The corresponding field in the target vector is named `_bfd_update_armap_timestamp'.


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Symbol table functions

The `BFD_JUMP_TABLE_SYMBOLS' macro is used for functions which deal with symbols.

`_get_symtab_upper_bound'
Return a sensible upper bound on the amount of memory which will be required to read the symbol table. In practice most targets return the amount of memory required to hold `asymbol' pointers for all the symbols plus a trailing `NULL' entry, and store the actual symbol information in BFD private data. This is called via `bfd_get_symtab_upper_bound'. The corresponding field in the target vector is named `_bfd_get_symtab_upper_bound'.
`_canonicalize_symtab'
Read in the symbol table. This is called via `bfd_canonicalize_symtab'. The corresponding field in the target vector is named `_bfd_canonicalize_symtab'.
`_make_empty_symbol'
Create an empty symbol for the BFD. This is needed because most targets store extra information with each symbol by allocating a structure larger than an `asymbol' and storing the extra information at the end. This function will allocate the right amount of memory, and return what looks like a pointer to an empty `asymbol'. This is called via `bfd_make_empty_symbol'. The corresponding field in the target vector is named `_bfd_make_empty_symbol'.
`_print_symbol'
Print information about the symbol. This is called via `bfd_print_symbol'. One of the arguments indicates what sort of information should be printed:
`bfd_print_symbol_name'
Just print the symbol name.
`bfd_print_symbol_more'
Print the symbol name and some interesting flags. I don't think anything actually uses this.
`bfd_print_symbol_all'
Print all information about the symbol. This is used by `objdump' when run with the `-t' option.
The corresponding field in the target vector is named `_bfd_print_symbol'.
`_get_symbol_info'
Return a standard set of information about the symbol. This is called via `bfd_symbol_info'. The corresponding field in the target vector is named `_bfd_get_symbol_info'.
`_bfd_is_local_label_name'
Return whether the given string would normally represent the name of a local label. This is called via `bfd_is_local_label' and `bfd_is_local_label_name'. Local labels are normally discarded by the assembler. In the linker, this defines the difference between the `-x' and `-X' options.
`_get_lineno'
Return line number information for a symbol. This is only meaningful for a COFF target. This is called when writing out COFF line numbers.
`_find_nearest_line'
Given an address within a section, use the debugging information to find the matching file name, function name, and line number, if any. This is called via `bfd_find_nearest_line'. The corresponding field in the target vector is named `_bfd_find_nearest_line'.
`_bfd_make_debug_symbol'
Make a debugging symbol. This is only meaningful for a COFF target, where it simply returns a symbol which will be placed in the `N_DEBUG' section when it is written out. This is called via `bfd_make_debug_symbol'.
`_read_minisymbols'
Minisymbols are used to reduce the memory requirements of programs like `nm'. A minisymbol is a cookie pointing to internal symbol information which the caller can use to extract complete symbol information. This permits BFD to not convert all the symbols into generic form, but to instead convert them one at a time. This is called via `bfd_read_minisymbols'. Most targets do not implement this, and just use generic support which is based on using standard `asymbol' structures.
`_minisymbol_to_symbol'
Convert a minisymbol to a standard `asymbol'. This is called via `bfd_minisymbol_to_symbol'.


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Relocation support

The `BFD_JUMP_TABLE_RELOCS' macro is used for functions which deal with relocations.

`_get_reloc_upper_bound'
Return a sensible upper bound on the amount of memory which will be required to read the relocations for a section. In practice most targets return the amount of memory required to hold `arelent' pointers for all the relocations plus a trailing `NULL' entry, and store the actual relocation information in BFD private data. This is called via `bfd_get_reloc_upper_bound'.
`_canonicalize_reloc'
Return the relocation information for a section. This is called via `bfd_canonicalize_reloc'. The corresponding field in the target vector is named `_bfd_canonicalize_reloc'.
`_bfd_reloc_type_lookup'
Given a relocation code, return the corresponding howto structure (see BFD relocation codes). This is called via `bfd_reloc_type_lookup'. The corresponding field in the target vector is named `reloc_type_lookup'.


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Output functions

The `BFD_JUMP_TABLE_WRITE' macro is used for functions which deal with writing out a BFD.

`_set_arch_mach'
Set the architecture and machine number for a BFD. This is called via `bfd_set_arch_mach'. Most targets implement this by calling `bfd_default_set_arch_mach'. The corresponding field in the target vector is named `_bfd_set_arch_mach'.
`_set_section_contents'
Write out the contents of a section. This is called via `bfd_set_section_contents'. The corresponding field in the target vector is named `_bfd_set_section_contents'.


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Linker functions

The `BFD_JUMP_TABLE_LINK' macro is used for functions called by the linker.

`_sizeof_headers'
Return the size of the header information required for a BFD. This is used to implement the `SIZEOF_HEADERS' linker script function. It is normally used to align the first section at an efficient position on the page. This is called via `bfd_sizeof_headers'. The corresponding field in the target vector is named `_bfd_sizeof_headers'.
`_bfd_get_relocated_section_contents'
Read the contents of a section and apply the relocation information. This handles both a final link and a relocatable link; in the latter case, it adjust the relocation information as well. This is called via `bfd_get_relocated_section_contents'. Most targets implement it by calling `bfd_generic_get_relocated_section_contents'.
`_bfd_relax_section'
Try to use relaxation to shrink the size of a section. This is called by the linker when the `-relax' option is used. This is called via `bfd_relax_section'. Most targets do not support any sort of relaxation.
`_bfd_link_hash_table_create'
Create the symbol hash table to use for the linker. This linker hook permits the backend to control the size and information of the elements in the linker symbol hash table. This is called via `bfd_link_hash_table_create'.
`_bfd_link_add_symbols'
Given an object file or an archive, add all symbols into the linker symbol hash table. Use callbacks to the linker to include archive elements in the link. This is called via `bfd_link_add_symbols'.
`_bfd_final_link'
Finish the linking process. The linker calls this hook after all of the input files have been read, when it is ready to finish the link and generate the output file. This is called via `bfd_final_link'.
`_bfd_link_split_section'
I don't know what this is for. Nothing seems to call it. The only non-trivial definition is in som.c.


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Dynamic linking information functions

The `BFD_JUMP_TABLE_DYNAMIC' macro is used for functions which read dynamic linking information.

`_get_dynamic_symtab_upper_bound'
Return a sensible upper bound on the amount of memory which will be required to read the dynamic symbol table. In practice most targets return the amount of memory required to hold `asymbol' pointers for all the symbols plus a trailing `NULL' entry, and store the actual symbol information in BFD private data. This is called via `bfd_get_dynamic_symtab_upper_bound'. The corresponding field in the target vector is named `_bfd_get_dynamic_symtab_upper_bound'.
`_canonicalize_dynamic_symtab'
Read the dynamic symbol table. This is called via `bfd_canonicalize_dynamic_symtab'. The corresponding field in the target vector is named `_bfd_canonicalize_dynamic_symtab'.
`_get_dynamic_reloc_upper_bound'
Return a sensible upper bound on the amount of memory which will be required to read the dynamic relocations. In practice most targets return the amount of memory required to hold `arelent' pointers for all the relocations plus a trailing `NULL' entry, and store the actual relocation information in BFD private data. This is called via `bfd_get_dynamic_reloc_upper_bound'. The corresponding field in the target vector is named `_bfd_get_dynamic_reloc_upper_bound'.
`_canonicalize_dynamic_reloc'
Read the dynamic relocations. This is called via `bfd_canonicalize_dynamic_reloc'. The corresponding field in the target vector is named `_bfd_canonicalize_dynamic_reloc'.


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BFD generated files

BFD contains several automatically generated files. This section describes them. Some files are created at configure time, when you configure BFD. Some files are created at make time, when you build BFD. Some files are automatically rebuilt at make time, but only if you configure with the `--enable-maintainer-mode' option. Some files live in the object directory—the directory from which you run configure—and some live in the source directory. All files that live in the source directory are checked into the CVS repository.

bfd.h
Lives in the object directory. Created at make time from bfd-in2.h via bfd-in3.h. bfd-in3.h is created at configure time from bfd-in2.h. There are automatic dependencies to rebuild bfd-in3.h and hence bfd.h if bfd-in2.h changes, so you can normally ignore bfd-in3.h, and just think about bfd-in2.h and bfd.h.

bfd.h is built by replacing a few strings in bfd-in2.h. To see them, search for `@' in bfd-in2.h. They mainly control whether BFD is built for a 32 bit target or a 64 bit target.

bfd-in2.h
Lives in the source directory. Created from bfd-in.h and several other BFD source files. If you configure with the `--enable-maintainer-mode' option, bfd-in2.h is rebuilt automatically when a source file changes.
elf32-target.h
elf64-target.h
Live in the object directory. Created from elfxx-target.h. These files are versions of elfxx-target.h customized for either a 32 bit ELF target or a 64 bit ELF target.
libbfd.h
Lives in the source directory. Created from libbfd-in.h and several other BFD source files. If you configure with the `--enable-maintainer-mode' option, libbfd.h is rebuilt automatically when a source file changes.
libcoff.h
Lives in the source directory. Created from libcoff-in.h and coffcode.h. If you configure with the `--enable-maintainer-mode' option, libcoff.h is rebuilt automatically when a source file changes.
targmatch.h
Lives in the object directory. Created at make time from config.bfd. This file is used to map configuration triplets into BFD target vector variable names at run time.


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Files compiled multiple times in BFD

Several files in BFD are compiled multiple times. By this I mean that there are header files which contain function definitions. These header files are included by other files, and thus the functions are compiled once per file which includes them.

Preprocessor macros are used to control the compilation, so that each time the files are compiled the resulting functions are slightly different. Naturally, if they weren't different, there would be no reason to compile them multiple times.

This is a not a particularly good programming technique, and future BFD work should avoid it.

The following is a list of the files which are compiled multiple times.

aout-target.h
Describes a few functions and the target vector for a.out targets. This is used by individual a.out targets with different definitions of `N_TXTADDR' and similar a.out macros.
aoutf1.h
Implements standard SunOS a.out files. In principle it supports 64 bit a.out targets based on the preprocessor macro `ARCH_SIZE', but since all known a.out targets are 32 bits, this code may or may not work. This file is only included by a few other files, and it is difficult to justify its existence.
aoutx.h
Implements basic a.out support routines. This file can be compiled for either 32 or 64 bit support. Since all known a.out targets are 32 bits, the 64 bit support may or may not work. I believe the original intention was that this file would only be included by `aout32.c' and `aout64.c', and that other a.out targets would simply refer to the functions it defined. Unfortunately, some other a.out targets started including it directly, leading to a somewhat confused state of affairs.
coffcode.h
Implements basic COFF support routines. This file is included by every COFF target. It implements code which handles COFF magic numbers as well as various hook functions called by the generic COFF functions in coffgen.c. This file is controlled by a number of different macros, and more are added regularly.
coffswap.h
Implements COFF swapping routines. This file is included by coffcode.h, and thus by every COFF target. It implements the routines which swap COFF structures between internal and external format. The main control for this file is the external structure definitions in the files in the include/coff directory. A COFF target file will include one of those files before including coffcode.h and thus coffswap.h. There are a few other macros which affect coffswap.h as well, mostly describing whether certain fields are present in the external structures.
ecoffswap.h
Implements ECOFF swapping routines. This is like coffswap.h, but for ECOFF. It is included by the ECOFF target files (of which there are only two). The control is the preprocessor macro `ECOFF_32' or `ECOFF_64'.
elfcode.h
Implements ELF functions that use external structure definitions. This file is included by two other files: elf32.c and elf64.c. It is controlled by the `ARCH_SIZE' macro which is defined to be `32' or `64' before including it. The `NAME' macro is used internally to give the functions different names for the two target sizes.
elfcore.h
Like elfcode.h, but for functions that are specific to ELF core files. This is included only by elfcode.h.
elfxx-target.h
This file is the source for the generated files elf32-target.h and elf64-target.h, one of which is included by every ELF target. It defines the ELF target vector.
freebsd.h
Presumably intended to be included by all FreeBSD targets, but in fact there is only one such target, `i386-freebsd'. This defines a function used to set the right magic number for FreeBSD, as well as various macros, and includes aout-target.h.
netbsd.h
Like freebsd.h, except that there are several files which include it.
nlm-target.h
Defines the target vector for a standard NLM target.
nlmcode.h
Like elfcode.h, but for NLM targets. This is only included by nlm32.c and nlm64.c, both of which define the macro `ARCH_SIZE' to an appropriate value. There are no 64 bit NLM targets anyhow, so this is sort of useless.
nlmswap.h
Like coffswap.h, but for NLM targets. This is included by each NLM target, but I think it winds up compiling to the exact same code for every target, and as such is fairly useless.
peicode.h
Provides swapping routines and other hooks for PE targets. coffcode.h will include this rather than coffswap.h for a PE target. This defines PE specific versions of the COFF swapping routines, and also defines some macros which control coffcode.h itself.


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BFD relocation handling

The handling of relocations is one of the more confusing aspects of BFD. Relocation handling has been implemented in various different ways, all somewhat incompatible, none perfect.


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BFD relocation concepts

A relocation is an action which the linker must take when linking. It describes a change to the contents of a section. The change is normally based on the final value of one or more symbols. Relocations are created by the assembler when it creates an object file.

Most relocations are simple. A typical simple relocation is to set 32 bits at a given offset in a section to the value of a symbol. This type of relocation would be generated for code like int *p = &i; where `p' and `i' are global variables. A relocation for the symbol `i' would be generated such that the linker would initialize the area of memory which holds the value of `p' to the value of the symbol `i'.

Slightly more complex relocations may include an addend, which is a constant to add to the symbol value before using it. In some cases a relocation will require adding the symbol value to the existing contents of the section in the object file. In others the relocation will simply replace the contents of the section with the symbol value. Some relocations are PC relative, so that the value to be stored in the section is the difference between the value of a symbol and the final address of the section contents.

In general, relocations can be arbitrarily complex. For example, relocations used in dynamic linking systems often require the linker to allocate space in a different section and use the offset within that section as the value to store. In the IEEE object file format, relocations may involve arbitrary expressions.

When doing a relocatable link, the linker may or may not have to do anything with a relocation, depending upon the definition of the relocation. Simple relocations generally do not require any special action.


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BFD relocation functions

In BFD, each section has an array of `arelent' structures. Each structure has a pointer to a symbol, an address within the section, an addend, and a pointer to a `reloc_howto_struct' structure. The howto structure has a bunch of fields describing the reloc, including a type field. The type field is specific to the object file format backend; none of the generic code in BFD examines it.

Originally, the function `bfd_perform_relocation' was supposed to handle all relocations. In theory, many relocations would be simple enough to be described by the fields in the howto structure. For those that weren't, the howto structure included a `special_function' field to use as an escape.

While this seems plausible, a look at `bfd_perform_relocation' shows that it failed. The function has odd special cases. Some of the fields in the howto structure, such as `pcrel_offset', were not adequately documented.

The linker uses `bfd_perform_relocation' to do all relocations when the input and output file have different formats (e.g., when generating S-records). The generic linker code, which is used by all targets which do not define their own special purpose linker, uses `bfd_get_relocated_section_contents', which for most targets turns into a call to `bfd_generic_get_relocated_section_contents', which calls `bfd_perform_relocation'. So `bfd_perform_relocation' is still widely used, which makes it difficult to change, since it is difficult to test all possible cases.

The assembler used `bfd_perform_relocation' for a while. This turned out to be the wrong thing to do, since `bfd_perform_relocation' was written to handle relocations on an existing object file, while the assembler needed to create relocations in a new object file. The assembler was changed to use the new function `bfd_install_relocation' instead, and `bfd_install_relocation' was created as a copy of `bfd_perform_relocation'.

Unfortunately, the work did not progress any farther, so `bfd_install_relocation' remains a simple copy of `bfd_perform_relocation', with all the odd special cases and confusing code. This again is difficult to change, because again any change can affect any assembler target, and so is difficult to test.

The new linker, when using the same object file format for all input files and the output file, does not convert relocations into `arelent' structures, so it can not use `bfd_perform_relocation' at all. Instead, users of the new linker are expected to write a `relocate_section' function which will handle relocations in a target specific fashion.

There are two helper functions for target specific relocation: `_bfd_final_link_relocate' and `_bfd_relocate_contents'. These functions use a howto structure, but they do not use the `special_function' field. Since the functions are normally called from target specific code, the `special_function' field adds little; any relocations which require special handling can be handled without calling those functions.

So, if you want to add a new target, or add a new relocation to an existing target, you need to do the following:


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BFD relocation codes

BFD has another way of describing relocations besides the howto structures described above: the enum `bfd_reloc_code_real_type'.

Every known relocation type can be described as a value in this enumeration. The enumeration contains many target specific relocations, but where two or more targets have the same relocation, a single code is used. For example, the single value `BFD_RELOC_32' is used for all simple 32 bit relocation types.

The main purpose of this relocation code is to give the assembler some mechanism to create `arelent' structures. In order for the assembler to create an `arelent' structure, it has to be able to obtain a howto structure. The function `bfd_reloc_type_lookup', which simply calls the target vector entry point `reloc_type_lookup', takes a relocation code and returns a howto structure.

The function `bfd_get_reloc_code_name' returns the name of a relocation code. This is mainly used in error messages.

Using both howto structures and relocation codes can be somewhat confusing. There are many processor specific relocation codes. However, the relocation is only fully defined by the howto structure. The same relocation code will map to different howto structures in different object file formats. For example, the addend handling may be different.

Most of the relocation codes are not really general. The assembler can not use them without already understanding what sorts of relocations can be used for a particular target. It might be possible to replace the relocation codes with something simpler.


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BFD relocation future

Clearly the current BFD relocation support is in bad shape. A wholescale rewrite would be very difficult, because it would require thorough testing of every BFD target. So some sort of incremental change is required.

My vague thoughts on this would involve defining a new, clearly defined, howto structure. Some mechanism would be used to determine which type of howto structure was being used by a particular format.

The new howto structure would clearly define the relocation behaviour in the case of an assembly, a relocatable link, and a final link. At least one special function would be defined as an escape, and it might make sense to define more.

One or more generic functions similar to `bfd_perform_relocation' would be written to handle the new howto structure.

This should make it possible to write a generic version of the relocate section functions used by the new linker. The target specific code would provide some mechanism (a function pointer or an initial conversion) to convert target specific relocations into howto structures.

Ideally it would be possible to use this generic relocate section function for the generic linker as well. That is, it would replace the `bfd_generic_get_relocated_section_contents' function which is currently normally used.

For the special case of ELF dynamic linking, more consideration needs to be given to writing ELF specific but ELF target generic code to handle special relocation types such as GOT and PLT.


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BFD ELF support

The ELF object file format is defined in two parts: a generic ABI and a processor specific supplement. The ELF support in BFD is split in a similar fashion. The processor specific support is largely kept within a single file. The generic support is provided by several other files. The processor specific support provides a set of function pointers and constants used by the generic support.


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ELF sections and segments

The ELF ABI permits a file to have either sections or segments or both. Relocateable object files conventionally have only sections. Executables conventionally have both. Core files conventionally have only program segments.

ELF sections are similar to sections in other object file formats: they have a name, a VMA, file contents, flags, and other miscellaneous information. ELF relocations are stored in sections of a particular type; BFD automatically converts these sections into internal relocation information.

ELF program segments are intended for fast interpretation by a system loader. They have a type, a VMA, an LMA, file contents, and a couple of other fields. When an ELF executable is run on a Unix system, the system loader will examine the program segments to decide how to load it. The loader will ignore the section information. Loadable program segments (type `PT_LOAD') are directly loaded into memory. Other program segments are interpreted by the loader, and generally provide dynamic linking information.

When an ELF file has both program segments and sections, an ELF program segment may encompass one or more ELF sections, in the sense that the portion of the file which corresponds to the program segment may include the portions of the file corresponding to one or more sections. When there is more than one section in a loadable program segment, the relative positions of the section contents in the file must correspond to the relative positions they should hold when the program segment is loaded. This requirement should be obvious if you consider that the system loader will load an entire program segment at a time.

On a system which supports dynamic paging, such as any native Unix system, the contents of a loadable program segment must be at the same offset in the file as in memory, modulo the memory page size used on the system. This is because the system loader will map the file into memory starting at the start of a page. The system loader can easily remap entire pages to the correct load address. However, if the contents of the file were not correctly aligned within the page, the system loader would have to shift the contents around within the page, which is too expensive. For example, if the LMA of a loadable program segment is `0x40080' and the page size is `0x1000', then the position of the segment contents within the file must equal `0x80' modulo `0x1000'.

BFD has only a single set of sections. It does not provide any generic way to examine both sections and segments. When BFD is used to open an object file or executable, the BFD sections will represent ELF sections. When BFD is used to open a core file, the BFD sections will represent ELF program segments.

When BFD is used to examine an object file or executable, any program segments will be read to set the LMA of the sections. This is because ELF sections only have a VMA, while ELF program segments have both a VMA and an LMA. Any program segments will be copied by the `copy_private' entry points. They will be printed by the `print_private' entry point. Otherwise, the program segments are ignored. In particular, programs which use BFD currently have no direct access to the program segments.

When BFD is used to create an executable, the program segments will be created automatically based on the section information. This is done in the function `assign_file_positions_for_segments' in elf.c. This function has been tweaked many times, and probably still has problems that arise in particular cases.

There is a hook which may be used to explicitly define the program segments when creating an executable: the `bfd_record_phdr' function in bfd.c. If this function is called, BFD will not create program segments itself, but will only create the program segments specified by the caller. The linker uses this function to implement the `PHDRS' linker script command.


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BFD ELF generic support

In general, functions which do not read external data from the ELF file are found in elf.c. They operate on the internal forms of the ELF structures, which are defined in include/elf/internal.h. The internal structures are defined in terms of `bfd_vma', and so may be used for both 32 bit and 64 bit ELF targets.

The file elfcode.h contains functions which operate on the external data. elfcode.h is compiled twice, once via elf32.c with `ARCH_SIZE' defined as `32', and once via elf64.c with `ARCH_SIZE' defined as `64'. elfcode.h includes functions to swap the ELF structures in and out of external form, as well as a few more complex functions.

Linker support is found in elflink.c. The linker support is only used if the processor specific file defines `elf_backend_relocate_section', which is required to relocate the section contents. If that macro is not defined, the generic linker code is used, and relocations are handled via `bfd_perform_relocation'.

The core file support is in elfcore.h, which is compiled twice, for both 32 and 64 bit support. The more interesting cases of core file support only work on a native system which has the sys/procfs.h header file. Without that file, the core file support does little more than read the ELF program segments as BFD sections.

The BFD internal header file elf-bfd.h is used for communication among these files and the processor specific files.

The default entries for the BFD ELF target vector are found mainly in elf.c. Some functions are found in elfcode.h.

The processor specific files may override particular entries in the target vector, but most do not, with one exception: the `bfd_reloc_type_lookup' entry point is always processor specific.


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BFD ELF processor specific support

By convention, the processor specific support for a particular processor will be found in elfnn-cpu.c, where nn is either 32 or 64, and cpu is the name of the processor.


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Required processor specific support

When writing a elfnn-cpu.c file, you must do the following:

You must also add the magic number for this processor to the `prep_headers' function in elf.c.

You must also create a header file in the include/elf directory called cpu.h. This file should define any target specific information which may be needed outside of the BFD code. In particular it should use the `START_RELOC_NUMBERS', `RELOC_NUMBER', `FAKE_RELOC', `EMPTY_RELOC' and `END_RELOC_NUMBERS' macros to create a table mapping the number used to identify a relocation to a name describing that relocation.

While not a BFD component, you probably also want to make the binutils program `readelf' parse your ELF objects. For this, you need to add code for EM_cpu as appropriate in binutils/readelf.c.


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Processor specific linker support

The linker will be much more efficient if you define a relocate section function. This will permit BFD to use the ELF specific linker support.

If you do not define a relocate section function, BFD must use the generic linker support, which requires converting all symbols and relocations into BFD `asymbol' and `arelent' structures. In this case, relocations will be handled by calling `bfd_perform_relocation', which will use the howto structures you have defined. See BFD relocation handling.

In order to support linking into a different object file format, such as S-records, `bfd_perform_relocation' must work correctly with your howto structures, so you can't skip that step. However, if you define the relocate section function, then in the normal case of linking into an ELF file the linker will not need to convert symbols and relocations, and will be much more efficient.

To use a relocation section function, define the macro `elf_backend_relocate_section' as the name of a function which will take the contents of a section, as well as relocation, symbol, and other information, and modify the section contents according to the relocation information. In simple cases, this is little more than a loop over the relocations which computes the value of each relocation and calls `_bfd_final_link_relocate'. The function must check for a relocatable link, and in that case normally needs to do nothing other than adjust the addend for relocations against a section symbol.

The complex cases generally have to do with dynamic linker support. GOT and PLT relocations must be handled specially, and the linker normally arranges to set up the GOT and PLT sections while handling relocations. When generating a shared library, random relocations must normally be copied into the shared library, or converted to RELATIVE relocations when possible.


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Other processor specific support options

There are many other macros which may be defined in elfnn-cpu.c. These macros may be found in elfxx-target.h.

Macros may be used to override some of the generic ELF target vector functions.

Several processor specific hook functions which may be defined as macros. These functions are found as function pointers in the `elf_backend_data' structure defined in elf-bfd.h. In general, a hook function is set by defining a macro `elf_backend_name'.

There are a few processor specific constants which may also be defined. These are again found in the `elf_backend_data' structure.

I will not define the various functions and constants here; see the comments in elf-bfd.h.

Normally any odd characteristic of a particular ELF processor is handled via a hook function. For example, the special `SHN_MIPS_SCOMMON' section number found in MIPS ELF is handled via the hooks `section_from_bfd_section', `symbol_processing', `add_symbol_hook', and `output_symbol_hook'.

Dynamic linking support, which involves processor specific relocations requiring special handling, is also implemented via hook functions.


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BFD ELF core files

On native ELF Unix systems, core files are generated without any sections. Instead, they only have program segments.

When BFD is used to read an ELF core file, the BFD sections will actually represent program segments. Since ELF program segments do not have names, BFD will invent names like `segmentn' where n is a number.

A single ELF program segment may include both an initialized part and an uninitialized part. The size of the initialized part is given by the `p_filesz' field. The total size of the segment is given by the `p_memsz' field. If `p_memsz' is larger than `p_filesz', then the extra space is uninitialized, or, more precisely, initialized to zero.

BFD will represent such a program segment as two different sections. The first, named `segmentna', will represent the initialized part of the program segment. The second, named `segmentnb', will represent the uninitialized part.

ELF core files store special information such as register values in program segments with the type `PT_NOTE'. BFD will attempt to interpret the information in these segments, and will create additional sections holding the information. Some of this interpretation requires information found in the host header file sys/procfs.h, and so will only work when BFD is built on a native system.

BFD does not currently provide any way to create an ELF core file. In general, BFD does not provide a way to create core files. The way to implement this would be to write `bfd_set_format' and `bfd_write_contents' routines for the `bfd_core' type; see BFD target vector format.


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BFD ELF future

The current dynamic linking support has too much code duplication. While each processor has particular differences, much of the dynamic linking support is quite similar for each processor. The GOT and PLT are handled in fairly similar ways, the details of -Bsymbolic linking are generally similar, etc. This code should be reworked to use more generic functions, eliminating the duplication.

Similarly, the relocation handling has too much duplication. Many of the `reloc_type_lookup' and `info_to_howto' functions are quite similar. The relocate section functions are also often quite similar, both in the standard linker handling and the dynamic linker handling. Many of the COFF processor specific backends share a single relocate section function (`_bfd_coff_generic_relocate_section'), and it should be possible to do something like this for the ELF targets as well.

The appearance of the processor specific magic number in `prep_headers' in elf.c is somewhat bogus. It should be possible to add support for a new processor without changing the generic support.

The processor function hooks and constants are ad hoc and need better documentation.

When a linker script uses `SIZEOF_HEADERS', the ELF backend must guess at the number of program segments which will be required, in `get_program_header_size'. This is because the linker calls `bfd_sizeof_headers' before it knows all the section addresses and sizes. The ELF backend may later discover, when creating program segments, that more program segments are required. This is currently reported as an error in `assign_file_positions_for_segments'.

In practice this makes it difficult to use `SIZEOF_HEADERS' except with a carefully defined linker script. Unfortunately, `SIZEOF_HEADERS' is required for fast program loading on a native system, since it permits the initial code section to appear on the same page as the program segments, saving a page read when the program starts running. Fortunately, native systems permit careful definition of the linker script. Still, ideally it would be possible to use relaxation to compute the number of program segments.


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BFD glossary

This is a short glossary of some BFD terms.

a.out
The a.out object file format. The original Unix object file format. Still used on SunOS, though not Solaris. Supports only three sections.
archive
A collection of object files produced and manipulated by the `ar' program.
backend
The implementation within BFD of a particular object file format. The set of functions which appear in a particular target vector.
BFD
The BFD library itself. Also, each object file, archive, or executable opened by the BFD library has the type `bfd *', and is sometimes referred to as a bfd.
COFF
The Common Object File Format. Used on Unix SVR3. Used by some embedded targets, although ELF is normally better.
DLL
A shared library on Windows.
dynamic linker
When a program linked against a shared library is run, the dynamic linker will locate the appropriate shared library and arrange to somehow include it in the running image.
dynamic object
Another name for an ELF shared library.
ECOFF
The Extended Common Object File Format. Used on Alpha Digital Unix (formerly OSF/1), as well as Ultrix and Irix 4. A variant of COFF.
ELF
The Executable and Linking Format. The object file format used on most modern Unix systems, including GNU/Linux, Solaris, Irix, and SVR4. Also used on many embedded systems.
executable
A program, with instructions and symbols, and perhaps dynamic linking information. Normally produced by a linker.
LMA
Load Memory Address. This is the address at which a section will be loaded. Compare with VMA, below.
NLM
NetWare Loadable Module. Used to describe the format of an object which be loaded into NetWare, which is some kind of PC based network server program.
object file
A binary file including machine instructions, symbols, and relocation information. Normally produced by an assembler.
object file format
The format of an object file. Typically object files and executables for a particular system are in the same format, although executables will not contain any relocation information.
PE
The Portable Executable format. This is the object file format used for Windows (specifically, Win32) object files. It is based closely on COFF, but has a few significant differences.
PEI
The Portable Executable Image format. This is the object file format used for Windows (specifically, Win32) executables. It is very similar to PE, but includes some additional header information.
relocations
Information used by the linker to adjust section contents. Also called relocs.
section
Object files and executable are composed of sections. Sections have optional data and optional relocation information.
shared library
A library of functions which may be used by many executables without actually being linked into each executable. There are several different implementations of shared libraries, each having slightly different features.
symbol
Each object file and executable may have a list of symbols, often referred to as the symbol table. A symbol is basically a name and an address. There may also be some additional information like the type of symbol, although the type of a symbol is normally something simple like function or object, and should be confused with the more complex C notion of type. Typically every global function and variable in a C program will have an associated symbol.
target vector
A set of functions which implement support for a particular object file format. The `bfd_target' structure.
Win32
The current Windows API, implemented by Windows 95 and later and Windows NT 3.51 and later, but not by Windows 3.1.
XCOFF
The eXtended Common Object File Format. Used on AIX. A variant of COFF, with a completely different symbol table implementation.
VMA
Virtual Memory Address. This is the address a section will have when an executable is run. Compare with LMA, above.


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Index

Table of Contents