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NAME

       elf - format of Executable and Linking Format (ELF) 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.

       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;
               ElfN_Off      e_phoff;
               ElfN_Off      e_shoff;
               uint32_t      e_flags;
               uint16_t      e_ehsize;
               uint16_t      e_phentsize;
               uint16_t      e_phnum;
               uint16_t      e_shentsize;
               uint16_t      e_shnum;
               uint16_t      e_shstrndx;
           } ElfN_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  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
                                             architecture.     It     supports
                                             machines  with  files and virtual
                                             address spaces 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 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
                               determined 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.
                               ELFOSABI_LINUX      Linux ABI.
                               ELFOSABI_SOLARIS    Solaris ABI.
                               ELFOSABI_IRIX       IRIX ABI.
                               ELFOSABI_FREEBSD    FreeBSD ABI.
                               ELFOSABI_TRU64      TRU64 UNIX ABI.
                               ELFOSABI_ARM        ARM architecture ABI.
                               ELFOSABI_STANDALONE Stand-alone (embedded) 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 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_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.  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_phoff     This member holds the program header table’s file offset in
                   bytes.   If  the  file  has  no  program header table, this
                   member holds zero.

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

       e_flags     This member holds processor-specific flags associated  with
                   the  file.   Flag  names  take  the form EF_‘machine_flag’.
                   Currently no flags have been 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  table.   If
                   the  file  has  no  section  name string table, this member
                   holds the value SHN_UNDEF.

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

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

                   SHN_LOPROC    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 absolute values for  the
                                 corresponding    reference.    For   example,
                                 symbols 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_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  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.

       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 segment,
                               described  by  p_filesz and p_memsz.  The bytes
                               from the file are mapped to  the  beginning  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 pathname 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
                               unspecified  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.
                   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  a  bitmask  of  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
                   segments  must  have  congruent  values  for  p_vaddr   and
                   p_offset,  modulo  the  page  size.  Values of zero and one
                   mean no alignment is required.  Otherwise,  p_align  should
                   be  a  positive,  integral power of two, and p_vaddr should
                   equal p_offset, modulo p_align.

       A file’s section header table lets one locate all the file’s  sections.
       The  section  header  table  is  an  array  of Elf32_Shdr or Elf64_Shdr
       structures.  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
       section header table indices are reserved.  An  object  file  does  not
       have sections for these special indices:

       SHN_UNDEF     This  value  marks  an  undefined, missing, irrelevant or
                     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  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.   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 {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint32_t   sh_flags;
               Elf32_Addr sh_addr;
               Elf32_Off  sh_offset;
               uint32_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               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
       headers.

       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
                                inactive.   It  does  not  have  an associated
                                section.  Other members of the section  header
                                have undefined values.

                 SHT_PROGBITS   This  section holds information defined by the
                                program,  whose   format   and   meaning   are
                                determined 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
                                linking.  As a complete symbol table,  it  may
                                contain  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
                                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 SHT_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 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
                                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 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_offset This member’s value holds the byte offset from the  beginning
                 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
                 nonzero 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 doubleword
                 alignment  for  the  entire  section.   That is, the value of
                 sh_addr must be  congruent  to  zero,  modulo  the  value  of
                 sh_addralign.   Only zero and positive integral powers of two
                 are allowed.  Values of zero or one mean the section  has  no
                 alignment constraints.

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

       Various sections hold program and control information:

       .bss      This section holds uninitialized data that contributes to the
                 program’s   memory   image.    By   definition,   the  system
                 initializes the data with zeros when the  program  begins  to
                 run.   This  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++
                 constructor functions.  This section is of type SHT_PROGBITS.
                 The attribute types are SHF_ALLOC and SHF_WRITE.

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

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

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

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

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

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

       .dynsym   This  section  holds  the dynamic linking symbol table.  This
                 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.

       .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 definitons, 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
                 table 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
                 described below.  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
                 This section is used in  Linux  object  files  for  declaring
                 stack attributes.  This section is of type SHT_PROGBITS.  The
                 only attribute used is SHF_EXECINSTR.  This indicates to  the
                 GNU linker that the object file requires an executable stack.

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

       .relNAME  This section holds relocation information as described below.
                 If  the file has a loadable segment that includes relocation,
                 the section’s attributes  will  include  the  SHF_ALLOC  bit.
                 Otherwise  the  bit  will  be  off.  By convention, "NAME" is
                 supplied 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
                 supplied 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 holds read-only data that typically  contributes
                 to a non-writable segment in the process image.  This section
                 is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

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

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

       .symtab   This section holds  a  symbol  table.   If  the  file  has  a
                 loadable   segment   that  includes  the  symbol  table,  the
                 section’s  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  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
       table index is a subscript into this array.

           typedef struct {
               uint32_t      st_name;
               Elf32_Addr    st_value;
               uint32_t      st_size;
               unsigned char st_info;
               unsigned char st_other;
               uint16_t      st_shndx;
           } Elf32_Sym;

           typedef struct {
               uint32_t      st_name;
               unsigned char st_info;
               unsigned char st_other;
               uint16_t      st_shndx;
               Elf64_Addr    st_value;
               uint64_t      st_size;
           } Elf64_Sym;

       The 32-bit and 64-bit  versions  have  the  same  members,  just  in  a
       different 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
                             undefined reference 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 is
                             reserved for processor-specific semantics.

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

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

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

                             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
       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;
               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;
               uint64_t   r_info;
               int64_t    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.

       The .dynamic section contains a series of structures that hold relevant
       dynamic  linking  information.    The   d_tag   member   controls   the
       interpretation 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_STRSZ    Size in bytes of string table

                 DT_SYMENT   Size in bytes of a symbol table entry

                 DT_INIT     Address of the initialization function

                 DT_FINI     Address of the termination function

                 DT_SONAME   String table offset to name of shared object

                 DT_RPATH    String   table  offset  to  library  search  path
                             (deprecated)

                 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 non-writable 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
                 interpretations.

       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.

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).

COLOPHON

       This  page  is  part of release 2.77 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.