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ELF(5)                    FreeBSD File Formats Manual                   ELF(5)

NAME
     elf -- format of ELF executable binary files

SYNOPSIS
     #include <elf.h>

DESCRIPTION
     The header file <elf.h> defines the format of ELF executable binary
     files.  Amongst these files are normal executable files, relocatable
     object 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
     architecture only should include elf.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''.  Appli-
     cations written this way can be compiled on any architecture, regardless
     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 include both sys/elf32.h and
     sys/elf64.h instead of elf.h.  Furthermore, all types and structures need
     to be identified by either ``Elf32_xxx'' or ``Elf64_xxx''.  The macros
     need to be identified by ``ELF32_xxx'' or ``ELF64_xxx''.

     Whatever the system's architecture is, it will always include
     sys/elf_common.h as well as sys/elf_generic.h.

     These header files describe the above mentioned headers as C structures
     and also include structures for dynamic sections, relocation sections and
     symbol tables.

     The following types are being 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
           Elf32_Size      Unsigned object size

     For 64-bit architectures we have the following types:

           Elf64_Addr      Unsigned program address
           Elf64_Half      Unsigned halfword field
           Elf64_Off       Unsigned file offset
           Elf64_Sword     Signed large integer
           Elf64_Word      Field or unsigned large integer
           Elf64_Size      Unsigned object size
           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 architec-
                                       ture for this binary:

                                       ELFCLASSNONE  This class is invalid.
                                       ELFCLASS32    This defines the 32-bit
                                                     architecture.  It sup-
                                                     ports machines with files
                                                     and virtual address spa-
                                                     ces up to 4 Gigabytes.
                                       ELFCLASS64    This defines the 64-bit
                                                     architecture.
                        EI_DATA        The sixth byte specifies the data
                                       encoding of the processor-specific data
                                       in the file.  Currently these encodings
                                       are supported:

                                       ELFDATANONE  Unknown data format.
                                       ELFDATA2LSB  Two's complement, little-
                                                    endian.
                                       ELFDATA2MSB  Two's complement, big-
                                                    endian.
                        EI_VERSION     The version number of the ELF specifi-
                                       cation:

                                       EV_NONE     Invalid version.
                                       EV_CURRENT  Current version.
                        EI_OSABI       This byte identifies the operating sys-
                                       tem and ABI to which the object is tar-
                                       geted.  Some fields in other ELF struc-
                                       tures have flags and values that have
                                       platform specific meanings; the inter-
                                       pretation of those fields is determined
                                       by the value of this byte.  The follow-
                                       ing values are currently defined:

                                       ELFOSABI_SYSV        UNIX System V ABI.
                                       ELFOSABI_HPUX        HP-UX operating
                                                            system ABI.
                                       ELFOSABI_NETBSD      NetBSD operating
                                                            system ABI.
                                       ELFOSABI_LINUX       GNU/Linux operat-
                                                            ing system ABI.
                                       ELFOSABI_HURD        GNU/Hurd operating
                                                            system ABI.
                                       ELFOSABI_86OPEN      86Open Common IA32
                                                            ABI.
                                       ELFOSABI_SOLARIS     Solaris operating
                                                            system ABI.
                                       ELFOSABI_MONTEREY    Monterey project
                                                            ABI.
                                       ELFOSABI_IRIX        IRIX operating
                                                            system ABI.
                                       ELFOSABI_FREEBSD     FreeBSD operating
                                                            system ABI.
                                       ELFOSABI_TRU64       TRU64 UNIX operat-
                                                            ing system ABI.
                                       ELFOSABI_ARM         ARM architecture
                                                            ABI.
                                       ELFOSABI_STANDALONE  Standalone (embed-
                                                            ded) ABI.
                        EI_ABIVERSION  This byte identifies the version of the
                                       ABI to which the object is targeted.
                                       This field is used to distinguish among
                                       incompatible versions of an ABI.  The
                                       interpretation of this version number
                                       is dependent on the ABI identified by
                                       the EI_OSABI field.  Applications con-
                                       forming to this specification use the
                                       value 0.
                        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_PPC          PowerPC.
                        EM_ALPHA        Compaq [DEC] Alpha.

           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
                        process.  If the file has no associated entry point,
                        this member holds zero.
           e_phoff      This member holds the program header table's file off-
                        set in bytes.  If the file has no program header ta-
                        ble, this member holds zero.
           e_shoff      This member holds the section header table's file off-
                        set 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
                        defined.
           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 ta-
                        ble.  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 indexes.
                        SHN_LOPROC     This value up to and including
                                       SHN_HIPROC are reserved for processor-
                                       specific semantics.
                        SHN_HIPROC     This value down to and including
                                       SHN_LOPROC are 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 the range of reserved
                                       indices between SHN_LORESERVE and
                                       SHN_HIRESERVE, inclusive; 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_Size      p_filesz;
                   Elf32_Size      p_memsz;
                   Elf32_Word      p_flags;
                   Elf32_Size      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_Size      p_filesz;
                   Elf64_Size      p_memsz;
                   Elf64_Size      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
                     array 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
                                 defined 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
                                 meaningful 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 unspec-
                                 ified 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.  More-
                                 over, it may only occur if the program header
                                 table is part of the memory image of the pro-
                                 gram.  If it is present it must precede any
                                 loadable segment entry.
                     PT_LOPROC   This value up to and including PT_HIPROC are
                                 reserved for processor-specific semantics.
                     PT_HIPROC   This value down to and including PT_LOPROC
                                 are reserved for processor-specific seman-
                                 tics.

           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
                     address.  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.

     An 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
     beginning 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 sec-
     tion 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.
     SHN_LORESERVE  This value specifies the lower bound of the range of
                    reserved indices.
     SHN_LOPROC     This value up to and including SHN_HIPROC are reserved for
                    processor-specific semantics.
     SHN_HIPROC     This value down to and including SHN_LOPROC are reserved
                    for processor-specific semantics.
     SHN_ABS        This value specifies absolute values for the corresponding
                    reference.  For example, symbols defined relative to sec-
                    tion number SHN_ABS have absolute values and are 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
                    indices.

     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_Size      sh_size;
                   Elf32_Word      sh_link;
                   Elf32_Word      sh_info;
                   Elf32_Size      sh_addralign;
                   Elf32_Size      sh_entsize;
           } Elf32_Shdr;

           typedef struct {
                   Elf64_Half      sh_name;
                   Elf64_Half      sh_type;
                   Elf64_Size      sh_flags;
                   Elf64_Addr      sh_addr;
                   Elf64_Off       sh_offset;
                   Elf64_Size      sh_size;
                   Elf64_Half      sh_link;
                   Elf64_Half      sh_info;
                   Elf64_Size      sh_addralign;
                   Elf64_Size      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,
                   giving 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 sec-
                                 tion.  Other members of the section header
                                 have undefined values.
                   SHT_PROGBITS  The 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.  Typi-
                                 cally, SHT_SYMTAB provides symbols for link
                                 editing, though it may also be used for
                                 dynamic linking.  As a complete symbol table,
                                 it may contain 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.  All
                                 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
                                 contain a SHN_SYMTAB section.
                   SHT_LOPROC    This value up to and including SHT_HIPROC are
                                 reserved for processor-specific semantics.
                   SHT_HIPROC    This value down to and including SHT_LOPROC
                                 are reserved for processor-specific seman-
                                 tics.
                   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
                                 future 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      The section occupies memory during process
                                  execution.  Some control sections do not
                                  reside in the memory image of an object
                                  file.  This attribute is off for those sec-
                                  tions.
                   SHF_EXECINSTR  The section contains executable machine
                                  instructions.
                   SHF_MASKPROC   All bits included in this mask are reserved
                                  for processor-specific semantics.

     sh_addr       If the section will appear in the memory image of a
                   process, this member holds the address at which the sec-
                   tion'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
                   section 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
                   section holds a doubleword, the system must ensure double-
                   word 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 attributes 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.
     .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.
     .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
                section 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
                attribute types are used.
     .plt       This section holds the procedure linkage table.  This section
                is of type SHT_PROGBITS.  The attributes are processor-spe-
                cific.
     .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 sup-
                plied by the section to which the relocations apply.  Thus a
                relocation 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 sup-
                plied by the section to which the relocations apply.  Thus a
                relocation 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 hold 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
                attributes 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
                attributes 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
     relocate 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_Size      st_size;
                   unsigned char   st_info;
                   unsigned char   st_other;
                   Elf32_Half      st_shndx;
           } Elf32_Sym;

           typedef struct {
                   Elf64_Half      st_name;
                   unsigned char   st_info;
                   unsigned char   st_other;
                   Elf64_Quarter   st_shndx;
                   Elf64_Addr      st_value;
                   Elf64_Size      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
                            relocation 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 are
                            reserved for processor-specific semantics.
               STT_HIPROC   This value down to and including STT_LOPROC are
                            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 file without
                           interfering with each other.
               STB_GLOBAL  Global symbols are visible to all object files
                           being 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
                           definitions have lower precedence.
               STB_LOPROC  This value up to and including STB_HIPROC are
                           reserved for processor-specific semantics.
               STB_HIPROC  This value down to and including STB_LOPROC are
                           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
               action.  This member holds the relevant section header table
               index.

     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
     object 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_Addr      r_offset;
                   Elf64_Size      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_Addr      r_offset;
                   Elf64_Size      r_info;
                   Elf64_Off       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
     The ELF header files made their appearance in FreeBSD 2.2.6.  ELF in
     itself first appeared in AT&T System V UNIX.  The ELF format is an
     adopted standard.

AUTHORS
     This manual page was written by Jeroen Ruigrok van der Werven
     <asmodai@FreeBSD.org> with inspiration from BSDi's BSD/OS elf(5) manpage.

FreeBSD 4.10                     July 31, 1999                    FreeBSD 4.10

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | HISTORY | AUTHORS

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