elf


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.

       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  N-bit  architectures  (N=32,64,  ElfN
       stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t):

           ElfN_Addr       Unsigned program address, uintN_t
           ElfN_Off        Unsigned file offset, uintN_t
           ElfN_Section    Unsigned section index, uint16_t
           ElfN_Versym     Unsigned version symbol information, uint16_t
           Elf_Byte        unsigned char
           ElfN_Half       uint16_t
           ElfN_Sword      int32_t
           ElfN_Word       uint32_t
           ElfN_Sxword     int64_t
           ElfN_Xword      uint64_t

       (Note:  The  *BSD  terminology is a bit different.  There Elf64_Half is
       twice as large as Elf32_Half, and Elf64Quarter is  used  for  uint16_t.
       In  order  to avoid confusion these types are replaced by explicit ones
       in the below.)

       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:

           #define EI_NIDENT 16

           typedef struct {
               unsigned char e_ident[EI_NIDENT];
               uint16_t      e_type;
               uint16_t      e_machine;
               uint32_t      e_version;
               ElfN_Addr     e_entry;

       e_ident     This  array of bytes specifies to interpret the file, inde-
                   pendent 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  contain  values  which
                   start  with  the  prefix  ELF.   The  following  macros are
                   defined:

                   EI_MAG0     The first byte of the magic number.  It must be
                               filled with ELFMAG0.  (0: 0x7f)

                   EI_MAG1     The  second  byte of the magic number.  It must
                               be filled with ELFMAG1.  (1: 'E')

                   EI_MAG2     The third byte of the magic number.  It must be
                               filled with ELFMAG2.  (2: 'L')

                   EI_MAG3     The  fourth  byte of the magic number.  It must
                               be filled with ELFMAG3.  (3: 'F')

                   EI_CLASS    The fifth byte identifies the architecture  for
                               this binary:

                               ELFCLASSNONE  This class is invalid.
                               ELFCLASS32    This defines the 32-bit architec-
                                             ture.  It supports machines  with
                                             files  and virtual address spaces
                                             up to 4 Gigabytes.
                               ELFCLASS64    This defines the 64-bit architec-
                                             ture.

                   EI_DATA     The  sixth  byte specifies the data encoding of
                               the processor-specific data in the file.   Cur-
                               rently 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 specification:
                               EV_NONE       Invalid version.
                               EV_CURRENT    Current version.

                   EI_OSABI    This  byte  identifies the operating system and
                               ABI to which  the  object  is  targeted.   Some
                               fields  in  other ELF structures have flags and
                               values that  have  platform-specific  meanings;
                               the  interpretation  of  those fields is deter-
                               mined by the value of this byte.  E.g.:

                               ELFOSABI_NONE       Same as ELFOSABI_SYSV
                               ELFOSABI_SYSV       UNIX System V ABI.
                               ELFOSABI_HPUX       HP-UX ABI.
                               ELFOSABI_NETBSD     NetBSD ABI.
                               of  an ABI.  The interpretation of this version
                               number is dependent on the  ABI  identified  by
                               the EI_OSABI field.  Applications conforming 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_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.  E.g.:

                   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_860      Intel 80860.
                   EM_MIPS     MIPS RS3000 (big-endian only).
                   EM_PARISC   HP/PA.
                   EM_SPARC32PLUS
                               SPARC with enhanced instruction set.
                   EM_PPC      PowerPC.
                   EM_PPC64    PowerPC 64-bit.
                   EM_S390     IBM S/390
                   EM_ARM      Advanced RISC Machines
                   EM_SH       Renesas SuperH
                   EM_SPARCV9  SPARC v9 64-bit.
                   EM_IA_64    Intel Itanium
                   EM_X86_64   AMD x86-64
                   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 process.  If the
                   file has no associated entry point, this member holds zero.

       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.

                   If the number of entries in the  program  header  table  is
                   larger than or equal to PN_XNUM (0xffff), this member holds
                   PN_XNUM (0xffff) and the real number of entries in the pro-
                   gram header table is held in the sh_info member of the ini-
                   tial entry in section header table.  Otherwise, the sh_info
                   member of the initial entry contains the value zero.

                   PN_XNUM  This  is  defined  as  0xffff,  the largest number
                            e_phnum can have, specifying where the actual num-
                            ber of program headers is assigned.

       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.

                   If  the  number  of  entries in the section header table is
                   larger than or equal  to  SHN_LORESERVE  (0xff00),  e_shnum
                   holds  the value zero and the real number of entries in the
                   section header table is held in the sh_size member  of  the
                   initial  entry  in  section  header  table.  Otherwise, the
                   sh_size member of the initial entry in the  section  header
                   table holds the value 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.

                   If the index of section name string table section is larger
                   than  or equal to SHN_LORESERVE (0xff00), this member holds
                   SHN_XINDEX (0xffff) and the real index of the section  name
                   string  table  section is held in the sh_link member of the
                   initial entry  in  section  header  table.  Otherwise,  the
                   sh_link member of the initial entry in section header table
                   contains the value zero.

                   SHN_UNDEF     This  value  marks  an  undefined,   missing,
                   SHN_HIPROC    Values  less  than or equal to SHN_LOPROC are
                                 reserved for processor-specific semantics.

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

                   SHN_COMMON    Symbols defined relative to this section  are
                                 common  symbols,  such  as  Fortran COMMON or
                                 unallocated C external variables.

                   SHN_HIRESERVE This value specifies the upper bound  of  the
                                 range  of  reserved indices between SHN_LORE-
                                 SERVE and SHN_HIRESERVE, inclusive; the  val-
                                 ues  do  not reference the section header ta-
                                 ble.  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  sys-
       tem needs to prepare the program for execution.  An object file segment
       contains one or more sections.  Program headers are meaningful only for
       executable  and  shared object files.  A file specifies its own program
       header size with the ELF header's e_phentsize and e_phnum members.  The
       ELF  program  header  is described by the type Elf32_Phdr or Elf64_Phdr
       depending on the architecture:

           typedef struct {
               uint32_t   p_type;
               Elf32_Off  p_offset;
               Elf32_Addr p_vaddr;
               Elf32_Addr p_paddr;
               uint32_t   p_filesz;
               uint32_t   p_memsz;
               uint32_t   p_flags;
               uint32_t   p_align;
           } Elf32_Phdr;

           typedef struct {
               uint32_t   p_type;
               uint32_t   p_flags;
               Elf64_Off  p_offset;
               Elf64_Addr p_vaddr;
               Elf64_Addr p_paddr;
               uint64_t   p_filesz;
               uint64_t   p_memsz;
               uint64_t   p_align;
           } Elf64_Phdr;

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

                               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 ini-
                               tialized 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 pathname to invoke as
                               an interpreter.  This segment type is  meaning-
                               ful  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  unspeci-
                               fied 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 loadable
                               segment entry.

                   PT_LOPROC   Values greater than or equal to  PT_HIPROC  are
                               reserved for processor-specific semantics.

                   PT_HIPROC   Values  less  than  or  equal  to PT_LOPROC are
                               reserved  for   processor-specific   semantics.
                               PT_GNU_STACK GNU extension which is used by the
                               Linux kernel to control the state of the  stack
                               via the flags set in the p_flags member.

       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.
                   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 pos-
                   itive, 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
       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:  the initial entry and the
       indices between SHN_LORESERVE and SHN_HIRESERVE.  The initial entry  is
       used  in  ELF  extensions  for  e_phnum, e_shnum and e_strndx; in other
       cases, each field in the initial entry is set to zero.  An object  file
       does not have sections for these special indices:

              SHN_UNDEF     This value marks an undefined, missing, irrelevant
                            or otherwise meaningless section reference.

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

              SHN_LOPROC    Values  greater  than  or  equal to SHN_HIPROC are
                            reserved for processor-specific semantics.

              SHN_HIPROC    Values  less  than  or  equal  to  SHN_LOPROC  are
                            reserved for processor-specific semantics.

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

              SHN_COMMON    Symbols defined relative to this section are  com-
                            mon symbols, 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,  inclu-
                            sive.   The  section header table does not contain
                            entries for the reserved indices.

               uint32_t   sh_addralign;
               uint32_t   sh_entsize;
           } Elf32_Shdr;

           typedef struct {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint64_t   sh_flags;
               Elf64_Addr sh_addr;
               Elf64_Off  sh_offset;
               uint64_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               uint64_t   sh_addralign;
               uint64_t   sh_entsize;
           } Elf64_Shdr;

       No real differences exist between the 32-bit and 64-bit  section  head-
       ers.

       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 semantics.

                 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 unde-
                                fined 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
                                SHT_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
                                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 SHT_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 pro-
                                grams.

                 SHT_HIUSER     This value specifies the upper  bound  of  the
                                range of indices reserved for application pro-
                                grams.  Section types between  SHT_LOUSER  and
                                SHT_HIUSER  may  be  used  by the application,
                                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      This  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 sections.

                 SHF_EXECINSTR  This  section  contains   executable   machine
                                instructions.

                 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_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 initial-
                 izes the data with zeros when  the  program  begins  to  run.
                 This  section 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++ construc-
                 tor 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_PROG-
                 BITS.  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_PROG-
                 BITS.  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_PROG-
                 BITS.  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 sec-
                 tion's attributes will include the  SHF_ALLOC  bit.   Whether

       .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.

       .gnu.version
                 This  section  holds  the  version  symbol table, an array of
                 ElfN_Half elements.  This section is of type  SHT_GNU_versym.
                 The attribute type used is SHF_ALLOC.

       .gnu.version_d
                 This section holds the version symbol definitions, a table of
                 ElfN_Verdef   structures.    This   section   is   of    type
                 SHT_GNU_verdef.  The attribute type used is SHF_ALLOC.

       .gnu.version_r
                 This  section holds the version symbol needed elements, a ta-
                 ble of ElfN_Verneed structures.   This  section  is  of  type
                 SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

       .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.
                 Otherwise,  that  bit  will  be off.  This section is of type
                 SHT_PROGBITS.

       .line     This section  holds  line  number  information  for  symbolic
                 debugging,  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.
                 This  section  is  of  type SHT_NOTE.  No attribute types are
                 used.   OpenBSD  native   executables   usually   contain   a
                 .note.openbsd.ident  section  to identify themselves, for the
                 kernel to bypass any compatibility ELF binary emulation tests
                 when loading the file.

       .note.GNU-stack
                 Otherwise 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.
                 Otherwise 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 nonwritable 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 nonwritable 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  load-
                 able  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, com-
       monly 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 byte ('\0').  Similarly, a string table's last byte is
       defined to hold a null byte, 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 ta-
       ble index is a subscript into this array.

           typedef struct {
               uint32_t      st_name;
               Elf32_Addr    st_value;
               uint64_t      st_size;
           } Elf64_Sym;

       The 32-bit and 64-bit versions have the same members, just in a differ-
       ent order.

       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  nonzero, 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 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
                             being  combined.   One  file's  definition  of  a
                             global symbol will satisfy another  file's  unde-
                             fined reference to the same symbol.

                             ELF32_ST_BIND(info)     or    ELF64_ST_BIND(info)
                             extract a binding from an st_info value.

                             ELF32_ST_TYPE(info) or ELF64_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 defines the symbol visibility.

                 STV_DEFAULT     Default symbol visibility rules.
                 STV_INTERNAL    Processor-specific hidden class.
                 STV_HIDDEN      Symbol is unavailable in other modules.
                 STV_PROTECTED   Not preemptible, not exported.

                 There are macros for extracting the visibility type:

                 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

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

       Relocation structures that do not need an addend:

           typedef struct {
               Elf32_Addr r_offset;
               uint32_t   r_info;
           } Elf32_Rel;

           typedef struct {
               Elf64_Addr r_offset;
               uint64_t   r_info;
           } Elf64_Rel;

       Relocation structures that need an addend:

           typedef struct {
               Elf32_Addr r_offset;
               uint32_t   r_info;
               int32_t    r_addend;
           } Elf32_Rela;

           typedef struct {
               Elf64_Addr r_offset;
                   to which the relocation must be made and the type of  relo-
                   cation  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.

       The .dynamic section contains a series of structures that hold relevant
       dynamic linking information.  The d_tag member controls the interpreta-
       tion of d_un.

           typedef struct {
               Elf32_Sword    d_tag;
               union {
                   Elf32_Word d_val;
                   Elf32_Addr d_ptr;
               } d_un;
           } Elf32_Dyn;
           extern Elf32_Dyn _DYNAMIC[];

           typedef struct {
               Elf64_Sxword    d_tag;
               union {
                   Elf64_Xword d_val;
                   Elf64_Addr  d_ptr;
               } d_un;
           } Elf64_Dyn;
           extern Elf64_Dyn _DYNAMIC[];

       d_tag     This member may have any of the following values:

                 DT_NULL     Marks end of dynamic section

                 DT_NEEDED   String table offset to name of a needed library

                 DT_PLTRELSZ Size in bytes of PLT relocs

                 DT_PLTGOT   Address of PLT and/or GOT

                 DT_HASH     Address of symbol hash table

                 DT_STRTAB   Address of string table

                 DT_SYMTAB   Address of symbol table

                 DT_RELA     Address of Rela relocs table

                 DT_RELASZ   Size in bytes of Rela table

                 DT_RELAENT  Size in bytes of a Rela table entry


                 DT_SYMBOLIC Alert  linker to search this shared object before
                             the executable for symbols

                 DT_REL      Address of Rel relocs table

                 DT_RELSZ    Size in bytes of Rel table

                 DT_RELENT   Size in bytes of a Rel table entry

                 DT_PLTREL   Type of reloc the PLT refers (Rela or Rel)

                 DT_DEBUG    Undefined use for debugging

                 DT_TEXTREL  Absence of this indicates no relocs should  apply
                             to a nonwritable segment

                 DT_JMPREL   Address of reloc entries solely for the PLT

                 DT_BIND_NOW Instruct  dynamic  linker  to  process all relocs
                             before transferring control to the executable

                 DT_RUNPATH  String table offset to library search path

                 DT_LOPROC   Start of processor-specific semantics

                 DT_HIPROC   End of processor-specific semantics

       d_val     This member represents integer values with various  interpre-
                 tations.

       d_ptr     This  member  represents  program  virtual  addresses.   When
                 interpreting these addresses, the actual  address  should  be
                 computed  based  on  the  original file value and memory base
                 address.  Files do not contain relocation  entries  to  fixup
                 these addresses.

       _DYNAMIC  Array  containing  all the dynamic structures in the .dynamic
                 section.  This is automatically populated by the linker.

NOTES
       ELF first appeared in System V.  The ELF format is an adopted standard.

       The extensions for e_phnum, e_shnum and e_strndx respectively are Linux
       extensions.  Sun, BSD and AMD64 also support them; for further informa-
       tion, look under SEE ALSO.

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.




Linux                             2010-06-19                            ELF(5)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2017 Hurricane Electric. All Rights Reserved.