MirOS Manual: elf(5)

ELF(5)                       BSD Reference Manual                       ELF(5)

NAME

     elf - format of ELF executable binary files

SYNOPSIS

     #include <elf_abi.h>

DESCRIPTION

     The header file <elf_abi.h> defines the format of ELF executable binary
     files. Amongst these files are normal executable files, relocatable ob-
     ject files, core files and shared libraries.

     An executable file using the ELF file format consists of an ELF header,
     followed by a program header table or a section header table, or both.
     The ELF header is always at offset zero of the file. The program header
     table and the section header table's offset in the file are defined in
     the ELF header. The two tables describe the rest of the particularities
     of the file.

     Applications which wish to process ELF binary files for their native ar-
     chitecture only should include <elf_abi.h> in their source code. These
     applications should need to refer to all the types and structures by
     their generic names "Elf_xxx" and to the macros by "ELF_xxx". Applica-
     tions written this way can be compiled on any architecture, regardless of
     whether the host is 32-bit or 64-bit.

     Should an application need to process ELF files of an unknown architec-
     ture, then the application needs to explicitly use either "Elf32_xxx" or
     "Elf64_xxx" type and structure names. Likewise, the macros need to be
     identified by "ELF32_xxx" or "ELF64_xxx".

     This header file describes the above mentioned headers as C structures
     and also includes structures for dynamic sections, relocation sections
     and symbol tables.

     The following types are used for 32-bit architectures:

           Elf32_Addr      Unsigned program address
           Elf32_Half      Unsigned halfword field
           Elf32_Off       Unsigned file offset
           Elf32_Sword     Signed large integer
           Elf32_Word      Field or unsigned large integer

     And the following types are used for 64-bit architectures:

           Elf64_Addr      Unsigned program address
           Elf64_Shalf     Signed halfword field
           Elf64_Half      Unsigned halfword field
           Elf64_Off       Unsigned file offset
           Elf64_Sword     Signed large integer
           Elf64_Word      Field or unsigned large integer
           Elf64_Xword     Unsigned object size or alignment
           Elf64_Sxword    Signed object size or alignment
           Elf64_Quarter   Unsigned quarterword field

     All data structures that the file format defines follow the "natural"
     size and alignment guidelines for the relevant class. If necessary, data
     structures contain explicit padding to ensure 4-byte alignment for 4-byte
     objects, to force structure sizes to a multiple of 4, etc.

     The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:

           typedef struct {
                   unsigned char   e_ident[EI_NIDENT];
                   Elf32_Half      e_type;
                   Elf32_Half      e_machine;
                   Elf32_Word      e_version;
                   Elf32_Addr      e_entry;
                   Elf32_Off       e_phoff;
                   Elf32_Off       e_shoff;
                   Elf32_Word      e_flags;
                   Elf32_Half      e_ehsize;
                   Elf32_Half      e_phentsize;
                   Elf32_Half      e_phnum;
                   Elf32_Half      e_shentsize;
                   Elf32_Half      e_shnum;
                   Elf32_Half      e_shstrndx;
           } Elf32_Ehdr;

           typedef struct {
                   unsigned char   e_ident[EI_NIDENT];
                   Elf64_Quarter   e_type;
                   Elf64_Quarter   e_machine;
                   Elf64_Half      e_version;
                   Elf64_Addr      e_entry;
                   Elf64_Off       e_phoff;
                   Elf64_Off       e_shoff;
                   Elf64_Half      e_flags;
                   Elf64_Quarter   e_ehsize;
                   Elf64_Quarter   e_phentsize;
                   Elf64_Quarter   e_phnum;
                   Elf64_Quarter   e_shentsize;
                   Elf64_Quarter   e_shnum;
                   Elf64_Quarter   e_shstrndx;
           } Elf64_Ehdr;

     The fields have the following meanings:

           e_ident      This array of bytes specifies to interpret the file,
                        independent of the processor or the file's remaining
                        contents. Within this array everything is named by
                        macros, which start with the prefix EI_ and may con-
                        tain values which start with the prefix ELF. The fol-
                        lowing macros are defined:

                        EI_MAG0     The first byte of the magic number. It
                                    must be filled with ELFMAG0.

                        EI_MAG1     The second byte of the magic number. It
                                    must be filled with ELFMAG1.

                        EI_MAG2     The third byte of the magic number. It
                                    must be filled with ELFMAG2.

                        EI_MAG3     The fourth byte of the magic number. It
                                    must be filled with ELFMAG3.

                        EI_CLASS    The fifth byte identifies the architecture
                                    for this binary:

                                    ELFCLASSNONE  This class is invalid.
                                    ELFCLASS32    This defines the 32-bit ar-
                                                  chitecture. It supports
                                                  machines with files and vir-
                                                  tual address spaces up to 4
                                                  Gigabytes.
                                    ELFCLASS64    This defines the 64-bit ar-
                                                  chitecture.

                        EI_DATA     The sixth byte specifies the data encoding
                                    of the processor-specific data in the
                                    file. Currently these encodings are sup-
                                    ported:

                                    ELFDATANONE  Unknown data format.
                                    ELFDATA2LSB  Two's complement, little-
                                                 endian.
                                    ELFDATA2MSB  Two's complement, big-endian.

                        EI_VERSION  The version number of the ELF specifica-
                                    tion:

                                    EV_NONE     Invalid version.
                                    EV_CURRENT  Current version.

                        EI_PAD      Start of padding. These bytes are reserved
                                    and set to zero. Programs which read them
                                    should ignore them. The value for EI_PAD
                                    will change in the future if currently
                                    unused bytes are given meanings.

                        EI_BRAND    Start of architecture identification.

                        EI_NIDENT   The size of the e_ident array.

           e_type       This member of the structure identifies the object
                        file type:

                        ET_NONE  An unknown type.
                        ET_REL   A relocatable file.
                        ET_EXEC  An executable file.
                        ET_DYN   A shared object.
                        ET_CORE  A core file.

           e_machine    This member specifies the required architecture for an
                        individual file:

                        EM_NONE         An unknown machine.
                        EM_M32          AT&T WE 32100.
                        EM_SPARC        Sun Microsystems SPARC.
                        EM_386          Intel 80386.
                        EM_68K          Motorola 68000.
                        EM_88K          Motorola 88000.
                        EM_486          Intel 80486.
                        EM_860          Intel 80860.
                        EM_MIPS         MIPS RS3000 (big-endian only).
                        EM_MIPS_RS4_BE  MIPS RS4000 (big-endian only).
                        EM_SPARC64      SPARC v9 64-bit (unofficial).
                        EM_PARISC       HPPA.
                        EM_SPARC32PLUS  SPARC with enhanced instruction set.
                        EM_PPC          PowerPC.
                        EM_ALPHA        Compaq [DEC] Alpha.
                        EM_SPARCV9      SPARC v9 64-bit.
                        EM_ALPHA_EXP    Compaq [DEC] Alpha with enhanced in-
                                        struction set.
                        EM_VAX          DEC Vax.

           e_version    This member identifies the file version:

                        EV_NONE     Invalid version.
                        EV_CURRENT  Current version.

           e_entry      This member gives the virtual address to which the
                        system first transfers control, thus starting the pro-
                        cess. If the file has no associated entry point, this
                        member holds zero.

           e_phoff      This member holds the program header table's file
                        offset in bytes. If the file has no program header
                        table, this member holds zero.

           e_shoff      This member holds the section header table's file
                        offset in bytes. If the file has no section header
                        table this member holds zero.

           e_flags      This member holds processor-specific flags associated
                        with the file. Flag names take the form
                        EF_`machine_flag'. Currently no flags have been de-
                        fined.

           e_ehsize     This member holds the ELF header's size in bytes.

           e_phentsize  This member holds the size in bytes of one entry in
                        the file's program header table; all entries are the
                        same size.

           e_phnum      This member holds the number of entries in the program
                        header table. Thus the product of e_phentsize and
                        e_phnum gives the table's size in bytes. If a file has
                        no program header, e_phnum holds the value zero.

           e_shentsize  This member holds a sections header's size in bytes. A
                        section header is one entry in the section header
                        table; all entries are the same size.

           e_shnum      This member holds the number of entries in the section
                        header table. Thus the product of e_shentsize and
                        e_shnum gives the section header table's size in
                        bytes. If a file has no section header table, e_shnum
                        holds the value of zero.

           e_shstrndx   This member holds the section header table index of
                        the entry associated with the section name string
                        table. If the file has no section name string table,
                        this member holds the value SHN_UNDEF.

                        SHN_UNDEF      This value marks an undefined, missing,
                                       irrelevant, or otherwise meaningless
                                       section reference. For example, a sym-
                                       bol "defined" relative to section
                                       number SHN_UNDEF is an undefined sym-
                                       bol.

                        SHN_LORESERVE  This value specifies the lower bound of
                                       the range of reserved indices.

                        SHN_LOPROC     This value up to and including
                                       SHN_HIPROC is reserved for processor-
                                       specific semantics.

                        SHN_HIPROC     This value down to and including
                                       SHN_LOPROC is reserved for processor-
                                       specific semantics.

                        SHN_ABS        This value specifies absolute values
                                       for the corresponding reference. For
                                       example, symbols defined relative to
                                       section number SHN_ABS have absolute
                                       values and are not affected by reloca-
                                       tion.

                        SHN_COMMON     Symbols defined relative to this sec-
                                       tion are common symbols, such as For-
                                       tran COMMON or unallocated C external
                                       variables.

                        SHN_HIRESERVE  This value specifies the upper bound of
                                       the range of reserved indices between
                                       SHN_LORESERVE and SHN_HIRESERVE, in-
                                       clusive; the values do not reference
                                       the section header table. That is, the
                                       section header table does not contain
                                       entries for the reserved indices.

     An executable or shared object file's program header table is an array of
     structures, each describing a segment or other information the system
     needs to prepare the program for execution. An object file segment con-
     tains one or more sections. Program headers are meaningful only for exe-
     cutable and shared object files. A file specifies its own program header
     size with the ELF header's e_phentsize and e_phnum members. As with the
     ELF executable header, the program header also has different versions
     depending on the architecture:

           typedef struct {
                   Elf32_Word      p_type;
                   Elf32_Off       p_offset;
                   Elf32_Addr      p_vaddr;
                   Elf32_Addr      p_paddr;
                   Elf32_Word      p_filesz;
                   Elf32_Word      p_memsz;
                   Elf32_Word      p_flags;
                   Elf32_Word      p_align;
           } Elf32_Phdr;

           typedef struct {
                   Elf64_Half      p_type;
                   Elf64_Half      p_flags;
                   Elf64_Off       p_offset;
                   Elf64_Addr      p_vaddr;
                   Elf64_Addr      p_paddr;
                   Elf64_Xword     p_filesz;
                   Elf64_Xword     p_memsz;
                   Elf64_Xword     p_align;
           } Elf64_Phdr;

     The main difference between the 32-bit and the 64-bit program header lies
     only in the location of a p_flags member in the total struct.

           p_type    This member of the Phdr struct tells what kind of segment
                     this array element describes or how to interpret the ar-
                     ray element's information.

                     PT_NULL     The array element is unused and the other
                                 members' values are undefined. This lets the
                                 program header have ignored entries.

                     PT_LOAD     The array element specifies a loadable seg-
                                 ment, described by p_filesz and p_memsz. The
                                 bytes from the file are mapped to the begin-
                                 ning of the memory segment. If the segment's
                                 memory size (p_memsz) is larger than the file
                                 size (p_filesz), the "extra" bytes are de-
                                 fined to hold the value 0 and to follow the
                                 segment's initialized area. The file size may
                                 not be larger than the memory size. Loadable
                                 segment entries in the program header table
                                 appear in ascending order, sorted on the
                                 p_vaddr member.

                     PT_DYNAMIC  The array element specifies dynamic linking
                                 information.

                     PT_INTERP   The array element specifies the location and
                                 size of a null-terminated path name to invoke
                                 as an interpreter. This segment type is mean-
                                 ingful only for executable files (though it
                                 may occur for shared objects). However it may
                                 not occur more than once in a file. If it is
                                 present, it must precede any loadable segment
                                 entry.

                     PT_NOTE     The array element specifies the location and
                                 size for auxiliary information.

                     PT_SHLIB    This segment type is reserved but has un-
                                 specified semantics. Programs that contain an
                                 array element of this type do not conform to
                                 the ABI.

                     PT_PHDR     The array element, if present, specifies the
                                 location and size of the program header table
                                 itself, both in the file and in the memory
                                 image of the program. This segment type may
                                 not occur more than once in a file. Moreover,
                                 it may only occur if the program header table
                                 is part of the memory image of the program.
                                 If it is present, it must precede any load-
                                 able segment entry.

                     PT_LOOS     This value up to and including PT_HIOS is
                                 reserverd for operating system-specific se-
                                 mantics.

                     PT_HIOS     This value down to and including PT_LOOS is
                                 reserved for operating system-specific seman-
                                 tics.

                     PT_LOPROC   This value up to and including PT_HIPROC is
                                 reserved for processor-specific semantics.

                     PT_HIPROC   This value down to and including PT_LOPROC is
                                 reserved for processor-specific semantics.

           p_offset  This member holds the offset from the beginning of the
                     file at which the first byte of the segment resides.

           p_vaddr   This member holds the virtual address at which the first
                     byte of the segment resides in memory.

           p_paddr   On systems for which physical addressing is relevant,
                     this member is reserved for the segment's physical ad-
                     dress. Under BSD this member is not used and must be
                     zero.

           p_filesz  This member holds the number of bytes in the file image
                     of the segment. It may be zero.

           p_memsz   This member holds the number of bytes in the memory image
                     of the segment. It may be zero.

           p_flags   This member holds flags relevant to the segment:

                     PF_X  An executable segment.
                     PF_W  A writable segment.
                     PF_R  A readable segment.

                     A text segment commonly has the flags PF_X and PF_R. A
                     data segment commonly has PF_X, PF_W and PF_R.

           p_align   This member holds the value to which the segments are
                     aligned in memory and in the file. Loadable process seg-
                     ments must have congruent values for p_vaddr and
                     p_offset, modulo the page size. Values of zero and one
                     mean no alignment is required. Otherwise, p_align should
                     be a positive, integral power of two, and p_vaddr should
                     equal p_offset, modulo p_align.

     A file's section header table lets one locate all the file's sections.
     The section header table is an array of Elf32_Shdr or Elf64_Shdr struc-
     tures. The ELF header's e_shoff member gives the byte offset from the be-
     ginning of the file to the section header table. e_shnum holds the number
     of entries the section header table contains. e_shentsize holds the size
     in bytes of each entry.

     A section header table index is a subscript into this array. Some section
     header table indices are reserved. An object file does not have sections
     for these special indices:

     SHN_UNDEF      This value marks an undefined, missing, irrelevant or oth-
                    erwise meaningless section reference. For example, a sym-
                    bol "defined" relative to section number SHN_UNDEF is an
                    undefined symbol.

     SHN_LORESERVE  This value specifies the lower bound of the range of
                    reserved indices.

     SHN_LOPROC     This value up to and including SHN_HIPROC is reserved for
                    processor-specific semantics.

     SHN_HIPROC     This value down to and including SHN_LOPROC is reserved
                    for processor-specific semantics.

     SHN_ABS        This value specifies the absolute value for the
                    corresponding reference. For example, a symbol defined re-
                    lative to section number SHN_ABS has an absolute value and
                    is not affected by relocation.

     SHN_COMMON     Symbols defined relative to this section are common sym-
                    bols, such as FORTRAN COMMON or unallocated C external
                    variables.

     SHN_HIRESERVE  This value specifies the upper bound of the range of
                    reserved indices. The system reserves indices between
                    SHN_LORESERVE and SHN_HIRESERVE, inclusive. The section
                    header table does not contain entries for the reserved in-
                    dices.

     The section header has the following structure:

           typedef struct {
                   Elf32_Word      sh_name;
                   Elf32_Word      sh_type;
                   Elf32_Word      sh_flags;
                   Elf32_Addr      sh_addr;
                   Elf32_Off       sh_offset;
                   Elf32_Word      sh_size;
                   Elf32_Word      sh_link;
                   Elf32_Word      sh_info;
                   Elf32_Word      sh_addralign;
                   Elf32_Word      sh_entsize;
           } Elf32_Shdr;

           typedef struct {
                   Elf64_Half      sh_name;
                   Elf64_Half      sh_type;
                   Elf64_Xword     sh_flags;
                   Elf64_Addr      sh_addr;
                   Elf64_Off       sh_offset;
                   Elf64_Xword     sh_size;
                   Elf64_Half      sh_link;
                   Elf64_Half      sh_info;
                   Elf64_Xword     sh_addralign;
                   Elf64_Xword     sh_entsize;
           } Elf64_Shdr;

     sh_name       This member specifies the name of the section. Its value is
                   an index into the section header string table section, giv-
                   ing the location of a null-terminated string.

     sh_type       This member categorizes the section's contents and seman-
                   tics.

                   SHT_NULL      This value marks the section header as inac-
                                 tive. It does not have an associated section.
                                 Other members of the section header have un-
                                 defined values.

                   SHT_PROGBITS  This section holds information defined by the
                                 program, whose format and meaning are deter-
                                 mined solely by the program.

                   SHT_SYMTAB    This section holds a symbol table. Typically,
                                 SHT_SYMTAB provides symbols for link editing,
                                 though it may also be used for dynamic link-
                                 ing. As a complete symbol table, it may con-
                                 tain many symbols unnecessary for dynamic
                                 linking. An object file can also contain a
                                 SHN_DYNSYM section.

                   SHT_STRTAB    This section holds a string table. An object
                                 file may have multiple string table sections.

                   SHT_RELA      This section holds relocation entries with
                                 explicit addends, such as type Elf32_Rela for
                                 the 32-bit class of object files. An object
                                 may have multiple relocation sections.

                   SHT_HASH      This section holds a symbol hash table. An
                                 object participating in dynamic linking must
                                 contain a symbol hash table. An object file
                                 may have only one hash table.

                   SHT_DYNAMIC   This section holds information for dynamic
                                 linking. An object file may have only one
                                 dynamic section.

                   SHT_NOTE      This section holds information that marks the
                                 file in some way.

                   SHT_NOBITS    A section of this type occupies no space in
                                 the file but otherwise resembles
                                 SHN_PROGBITS. Although this section contains
                                 no bytes, the sh_offset member contains the
                                 conceptual file offset.

                   SHT_REL       This section holds relocation offsets without
                                 explicit addends, such as type Elf32_Rel for
                                 the 32-bit class of object files. An object
                                 file may have multiple relocation sections.

                   SHT_SHLIB     This section is reserved but has unspecified
                                 semantics.

                   SHT_DYNSYM    This section holds a minimal set of dynamic
                                 linking symbols. An object file can also con-
                                 tain a SHN_SYMTAB section.

                   SHT_LOPROC    This value up to and including SHT_HIPROC is
                                 reserved for processor-specific semantics.

                   SHT_HIPROC    This value down to and including SHT_LOPROC
                                 is reserved for processor-specific semantics.

                   SHT_LOUSER    This value specifies the lower bound of the
                                 range of indices reserved for application
                                 programs.

                   SHT_HIUSER    This value specifies the upper bound of the
                                 range of indices reserved for application
                                 programs. Section types between SHT_LOUSER
                                 and SHT_HIUSER may be used by the applica-
                                 tion, without conflicting with current or fu-
                                 ture system-defined section types.

     sh_flags      Sections support one-bit flags that describe miscellaneous
                   attributes. If a flag bit is set in sh_flags, the attribute
                   is "on" for the section. Otherwise, the attribute is "off"
                   or does not apply. Undefined attributes are set to zero.

                   SHF_WRITE      This section contains data that should be
                                  writable during process execution.
                   SHF_ALLOC      This section occupies memory during process
                                  execution. Some control sections do not re-
                                  side in the memory image of an object file.
                                  This attribute is off for those sections.
                   SHF_EXECINSTR  This section contains executable machine in-
                                  structions.
                   SHF_MASKPROC   All bits included in this mask are reserved
                                  for processor-specific semantics.

     sh_addr       If this section appears in the memory image of a process,
                   this member holds the address at which the section's first
                   byte should reside. Otherwise, the member contains zero.

     sh_offset     This member's value holds the byte offset from the begin-
                   ning of the file to the first byte in the section. One sec-
                   tion type, SHT_NOBITS, occupies no space in the file, and
                   its sh_offset member locates the conceptual placement in
                   the file.

     sh_size       This member holds the section's size in bytes. Unless the
                   section type is SHT_NOBITS, the section occupies sh_size
                   bytes in the file. A section of type SHT_NOBITS may have a
                   non-zero size, but it occupies no space in the file.

     sh_link       This member holds a section header table index link, whose
                   interpretation depends on the section type.

     sh_info       This member holds extra information, whose interpretation
                   depends on the section type.

     sh_addralign  Some sections have address alignment constraints. If a sec-
                   tion holds a doubleword, the system must ensure doubleword
                   alignment for the entire section. That is, the value of
                   sh_addr must be congruent to zero, modulo the value of
                   sh_addralign. Only zero and positive integral powers of two
                   are allowed. Values of zero or one mean the section has no
                   alignment constraints.

     sh_entsize    Some sections hold a table of fixed-sized entries, such as
                   a symbol table. For such a section, this member gives the
                   size in bytes for each entry. This member contains zero if
                   the section does not hold a table of fixed-size entries.

     Various sections hold program and control information:

     .bss       This section holds uninitialized data that contributes to the
                program's memory image. By definition, the system initializes
                the data with zeros when the program begins to run. This sec-
                tion is of type SHT_NOBITS. The attribute types are SHF_ALLOC
                and SHF_WRITE.

     .comment   This section holds version control information. This section
                is of type SHT_PROGBITS. No attribute types are used.

     .ctors     This section holds initialized pointers to the C++ constructor
                functions. This section is of type SHT_PROGBITS. The attribute
                types are SHF_ALLOC and SHF_WRITE.

     .data      This section holds initialized data that contribute to the
                program's memory image. This section is of type SHT_PROGBITS.
                The attribute types are SHF_ALLOC and SHF_WRITE.

     .data1     This section holds initialized data that contribute to the
                program's memory image. This section is of type SHT_PROGBITS.
                The attribute types are SHF_ALLOC and SHF_WRITE.

     .debug     This section holds information for symbolic debugging. The
                contents are unspecified. This section is of type
                SHT_PROGBITS. No attribute types are used.

     .dtors     This section holds initialized pointers to the C++ destructor
                functions. This section is of type SHT_PROGBITS. The attribute
                types are SHF_ALLOC and SHF_WRITE.

     .dynamic   This section holds dynamic linking information. The section's
                attributes will include the SHF_ALLOC bit. Whether the
                SHF_WRITE bit is set is processor-specific. This section is of
                type SHT_DYNAMIC. See the attributes above.

     .dynstr    This section holds strings needed for dynamic linking, most
                commonly the strings that represent the names associated with
                symbol table entries. This section is of type SHT_STRTAB. The
                attribute type used is SHF_ALLOC.

     .dynsym    This section holds the dynamic linking symbol table. This sec-
                tion is of type SHT_DYNSYM. The attribute used is SHF_ALLOC.

     .fini      This section holds executable instructions that contribute to
                the process termination code. When a program exits normally
                the system arranges to execute the code in this section. This
                section is of type SHT_PROGBITS. The attributes used are
                SHF_ALLOC and SHF_EXECINSTR.

     .got       This section holds the global offset table. This section is of
                type SHT_PROGBITS. The attributes are processor-specific.

     .hash      This section holds a symbol hash table. This section is of
                type SHT_HASH. The attribute used is SHF_ALLOC.

     .init      This section holds executable instructions that contribute to
                the process initialization code. When a program starts to run
                the system arranges to execute the code in this section before
                calling the main program entry point. This section is of type
                SHT_PROGBITS. The attributes used are SHF_ALLOC and
                SHF_EXECINSTR.

     .interp    This section holds the pathname of a program interpreter. If
                the file has a loadable segment that includes the section, the
                section's attributes will include the SHF_ALLOC bit. Other-
                wise, that bit will be off. This section is of type
                SHT_PROGBITS.

     .line      This section holds line number information for symbolic debug-
                ging, which describes the correspondence between the program
                source and the machine code. The contents are unspecified.
                This section is of type SHT_PROGBITS. No attribute types are
                used.

     .note      This section holds information in the "Note Section" format
                described below. This section is of type SHT_NOTE. No attri-
                bute types are used. OpenBSD native executables usually con-
                tain a .note.openbsd.ident section to identify themselves, for
                the kernel to bypass any compatibility ELF binary emulation
                tests when loading the file.

     .plt       This section holds the procedure linkage table. This section
                is of type SHT_PROGBITS. The attributes are processor-
                specific.

     .relNAME   This section holds relocation information as described below.
                If the file has a loadable segment that includes relocation,
                the section's attributes will include the SHF_ALLOC bit. Oth-
                erwise the bit will be off. By convention, "NAME" is supplied
                by the section to which the relocations apply. Thus a reloca-
                tion section for .text normally would have the name .rel.text.
                This section is of type SHT_REL.

     .relaNAME  This section holds relocation information as described below.
                If the file has a loadable segment that includes relocation,
                the section's attributes will include the SHF_ALLOC bit. Oth-
                erwise the bit will be off. By convention, "NAME" is supplied
                by the section to which the relocations apply. Thus a reloca-
                tion section for .text normally would have the name
                .rela.text. This section is of type SHT_RELA.

     .rodata    This section holds read-only data that typically contributes
                to a non-writable segment in the process image. This section
                is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

     .rodata1   This section holds read-only data that typically contributes
                to a non-writable segment in the process image. This section
                is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.

     .shstrtab  This section holds section names. This section is of type
                SHT_STRTAB. No attribute types are used.

     .strtab    This section holds strings, most commonly the strings that
                represent the names associated with symbol table entries. If
                the file has a loadable segment that includes the symbol
                string table, the section's attributes will include the
                SHF_ALLOC bit. Otherwise the bit will be off. This section is
                of type SHT_STRTAB.

     .symtab    This section holds a symbol table. If the file has a loadable
                segment that includes the symbol table, the section's attri-
                butes will include the SHF_ALLOC bit. Otherwise the bit will
                be off. This section is of type SHT_SYMTAB.

     .text      This section holds the "text", or executable instructions, of
                a program. This section is of type SHT_PROGBITS. The attri-
                butes used are SHF_ALLOC and SHF_EXECINSTR.

     String table sections hold null-terminated character sequences, commonly
     called strings. The object file uses these strings to represent symbol
     and section names. One references a string as an index into the string
     table section. The first byte, which is index zero, is defined to hold a
     null character. Similarly, a string table's last byte is defined to hold
     a null character, ensuring null termination for all strings.

     An object file's symbol table holds information needed to locate and re-
     locate a program's symbolic definitions and references. A symbol table
     index is a subscript into this array.

           typedef struct {
                   Elf32_Word      st_name;
                   Elf32_Addr      st_value;
                   Elf32_Word      st_size;
                   unsigned char   st_info;
                   unsigned char   st_other;
                   Elf32_Half      st_shndx;
           } Elf32_Sym;

           typedef struct {
                   Elf64_Half      st_name;
                   Elf_Byte        st_info;
                   Elf_Byte        st_other;
                   Elf64_Quarter   st_shndx;
                   Elf64_Xword     st_value;
                   Elf64_Xword     st_size;
           } Elf64_Sym;

     st_name   This member holds an index into the object file's symbol string
               table, which holds character representations of the symbol
               names. If the value is non-zero, it represents a string table
               index that gives the symbol name. Otherwise, the symbol table
               has no name.

     st_value  This member gives the value of the associated symbol.

     st_size   Many symbols have associated sizes. This member holds zero if
               the symbol has no size or an unknown size.

     st_info   This member specifies the symbol's type and binding attributes:

               STT_NOTYPE   The symbol's type is not defined.

               STT_OBJECT   The symbol is associated with a data object.

               STT_FUNC     The symbol is associated with a function or other
                            executable code.

               STT_SECTION  The symbol is associated with a section. Symbol
                            table entries of this type exist primarily for re-
                            location and normally have STB_LOCAL bindings.

               STT_FILE     By convention, the symbol's name gives the name of
                            the source file associated with the object file. A
                            file symbol has STB_LOCAL bindings, its section
                            index is SHN_ABS, and it precedes the other
                            STB_LOCAL symbols of the file, if it is present.

               STT_LOPROC   This value up to and including STT_HIPROC is
                            reserved for processor-specific semantics.

               STT_HIPROC   This value down to and including STT_LOPROC is
                            reserved for processor-specific semantics.

               STB_LOCAL   Local symbols are not visible outside the object
                           file containing their definition. Local symbols of
                           the same name may exist in multiple files without
                           interfering with each other.

               STB_GLOBAL  Global symbols are visible to all object files be-
                           ing combined. One file's definition of a global
                           symbol will satisfy another file's undefined refer-
                           ence to the same symbol.

               STB_WEAK    Weak symbols resemble global symbols, but their de-
                           finitions have lower precedence.

               STB_LOPROC  This value up to and including STB_HIPROC is
                           reserved for processor-specific semantics.

               STB_HIPROC  This value down to and including STB_LOPROC is
                           reserved for processor-specific semantics.

                           There are macros for packing and unpacking the
                           binding and type fields:

                           ELF32_ST_BIND(info)        or ELF64_ST_BIND(info)
                                                      extract a binding from
                                                      an st_info value.
                           ELF64_ST_TYPE(info)        or ELF32_ST_TYPE(info)
                                                      extract a type from an
                                                      st_info value.
                           ELF32_ST_INFO(bind, type)  or ELF64_ST_INFO(bind,
                                                      type) convert a binding
                                                      and a type into an
                                                      st_info value.

     st_other  This member currently holds zero and has no defined meaning.

     st_shndx  Every symbol table entry is "defined" in relation to some sec-
               tion. This member holds the relevant section header table in-
               dex.

     Relocation is the process of connecting symbolic references with symbolic
     definitions. Relocatable files must have information that describes how
     to modify their section contents, thus allowing executable and shared ob-
     ject files to hold the right information for a process' program image.
     Relocation entries are these data.

     Relocation structures that do not need an addend:

           typedef struct {
                   Elf32_Addr      r_offset;
                   Elf32_Word      r_info;
           } Elf32_Rel;

           typedef struct {
                   Elf64_Xword     r_offset;
                   Elf64_Xword     r_info;
           } Elf64_Rel;

     Relocation structures that need an addend:

           typedef struct {
                   Elf32_Addr      r_offset;
                   Elf32_Word      r_info;
                   Elf32_Sword     r_addend;
           } Elf32_Rela;

           typedef struct {
                   Elf64_Xword     r_offset;
                   Elf64_Xword     r_info;
                   Elf64_Sxword    r_addend;
           } Elf64_Rela;

     r_offset  This member gives the location at which to apply the relocation
               action. For a relocatable file, the value is the byte offset
               from the beginning of the section to the storage unit affected
               by the relocation. For an executable file or shared object, the
               value is the virtual address of the storage unit affected by
               the relocation.

     r_info    This member gives both the symbol table index with respect to
               which the relocation must be made and the type of relocation to
               apply. Relocation types are processor-specific. When the text
               refers to a relocation entry's relocation type or symbol table
               index, it means the result of applying ELF_[32|64]_R_TYPE or
               ELF[32|64]_R_SYM, respectively, to the entry's r_info member.

     r_addend  This member specifies a constant addend used to compute the
               value to be stored into the relocatable field.

SEE ALSO

     as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)

     Hewlett-Packard, Elf-64 Object File Format.

     Santa Cruz Operation, System V Application Binary Interface.

     Unix System Laboratories, "Object Files", Executable and Linking Format
     (ELF).

HISTORY

     OpenBSD ELF support first appeared in OpenBSD 1.2, although not all sup-
     ported platforms use it as the native binary file format. ELF in itself
     first appeared in AT&TNon-Null System V UNIX. The ELF format is an adopt-
     ed standard.

AUTHORS

     This manual page was written by Jeroen Ruigrok van der Werven
     <asmodai@FreeBSD.org> with inspiration from BSDi's elf manpage.

MirOS BSD #10-current           July 31, 1999                               13

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