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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
       objects.

       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.

   Basic types
       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, and so on.

   ELF header (Ehdr)
       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  how  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 seventh byte is the version number of the ELF specification:

                     EV_NONE       Invalid version.
                     EV_CURRENT    Current version.

              EI_OSABI
                     The  eighth 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.  For example:

                     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
                     The ninth 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_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.   For
              example:

              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.

              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  program header table is held in the sh_info member of the initial 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 number 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.

   Program header (Phdr)
       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  structure  indicates 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 of notes (ElfN_Nhdr).

                 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  occur only 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
                 PT_HIPROC
                        Values  in  the  inclusive  range [PT_LOPROC, PT_HIPROC] 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 bit mask 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_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.

   Section header (Shdr)
       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:  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_shstrndx;  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
       SHN_HIPROC
              Values  greater  in  the  inclusive range [SHN_LOPROC, SHN_HIPROC] 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 notes (ElfN_Nhdr).

              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
              SHT_HIPROC
                     Values in the inclusive range  [SHT_LOPROC,  SHT_HIPROC]  are  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.  The value 0 or 1 means that
              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  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  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 various notes.  This section is of type SHT_NOTE.  No attribute
              types are used.

       .note.ABI-tag
              This section is used to declare the expected run-time ABI of the ELF image.  It may
              include  the  operating  system name and its run-time versions.  This section is of
              type SHT_NOTE.  The only attribute used is SHF_ALLOC.

       .note.gnu.build-id
              This section is used to hold an ID that uniquely identifies the contents of the ELF
              image.   Different  files with the same build ID should contain the same executable
              content.  See the --build-id option to the GNU linker (ld (1))  for  more  details.
              This section is of type SHT_NOTE.  The only attribute used is SHF_ALLOC.

       .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.

       .note.openbsd.ident
              OpenBSD  native  executables usually contain this section to identify themselves so
              the kernel can bypass any compatibility ELF binary emulation tests when loading the
              file.

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

       .relNAME
              This section holds relocation information as described below.  If the  file  has  a
              loadable  segment  that  includes relocation, the section's attributes will include
              the SHF_ALLOC bit.  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 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  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 and symbol tables
       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
              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
              STT_HIPROC
                     Values  in  the  inclusive  range  [STT_LOPROC, STT_HIPROC] are reserved for
                     processor-specific semantics.

              STB_LOCAL
                     Local symbols are not visible  outside  the  object  file  containing  their
                     definition.   Local  symbols  of  the  same name may exist in multiple 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
              STB_HIPROC
                     Values  in  the  inclusive  range  [STB_LOPROC, STB_HIPROC] are reserved for
                     processor-specific semantics.

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

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

              ELF32_ST_TYPE(info)
              ELF64_ST_TYPE(info)
                     Extract a type from an st_info value.

              ELF32_ST_INFO(bind, type)
              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.  Global and weak symbols are  available  to
                     other  modules;  references  in  the  local  module  can  be  interposed  by
                     definitions in other modules.
              STV_INTERNAL
                     Processor-specific hidden class.
              STV_HIDDEN
                     Symbol is unavailable to other  modules;  references  in  the  local  module
                     always  resolve to the local symbol (i.e., the symbol can't be interposed by
                     definitions in other modules).
              STV_PROTECTED
                     Symbol is available to other modules, but references  in  the  local  module
                     always resolve to the local symbol.

              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 entries (Rel & Rela)
       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'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;
               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.

   Dynamic tags (Dyn)
       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 relocation entries

              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 relocation table

              DT_RELASZ   Size in bytes of the Rela relocation table

              DT_RELAENT  Size in bytes of a Rela relocation 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  search  path for direct and indirect library
                          dependencies

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

              DT_REL      Address of Rel relocation table

              DT_RELSZ    Size in bytes of Rel relocation table

              DT_RELENT   Size in bytes of a Rel table entry

              DT_PLTREL   Type of relocation entry to which the PLT refers (Rela or Rel)

              DT_DEBUG    Undefined use for debugging

              DT_TEXTREL  Absence of this entry indicates that no relocation entries should apply
                          to a nonwritable segment

              DT_JMPREL   Address of relocation entries associated solely with the PLT

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

              DT_RUNPATH  String table offset to search path for direct library dependencies

              DT_LOPROC
              DT_HIPROC   Values  in  the inclusive range [DT_LOPROC, DT_HIPROC] are reserved for
                          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 (Nhdr)
       ELF notes allow for appending arbitrary information for  the  system  to  use.   They  are
       largely  used by core files (e_type of ET_CORE), but many projects define their own set of
       extensions.  For example, the GNU tool chain uses ELF notes to pass information  from  the
       linker to the C library.

       Note  sections contain a series of notes (see the struct definitions below).  Each note is
       followed by the name field  (whose  length  is  defined  in  n_namesz)  and  then  by  the
       descriptor  field (whose length is defined in n_descsz) and whose starting address has a 4
       byte alignment.  Neither field is defined in  the  note  struct  due  to  their  arbitrary
       lengths.

       An example for parsing out two consecutive notes should clarify their layout in memory:

           void *memory, *name, *desc;
           Elf64_Nhdr *note, *next_note;

           /* The buffer is pointing to the start of the section/segment. */
           note = memory;

           /* If the name is defined, it follows the note. */
           name = note->n_namesz == 0 ? NULL : memory + sizeof(*note);

           /* If the descriptor is defined, it follows the name
              (with alignment). */

           desc = note->n_descsz == 0 ? NULL :
                  memory + sizeof(*note) + ALIGN_UP(note->n_namesz, 4);

           /* The next note follows both (with alignment). */
           next_note = memory + sizeof(*note) +
                                ALIGN_UP(note->n_namesz, 4) +
                                ALIGN_UP(note->n_descsz, 4);

       Keep  in  mind  that  the interpretation of n_type depends on the namespace defined by the
       n_namesz field.  If the n_namesz field is not set (e.g., is 0), then there are two sets of
       notes:  one  for core files and one for all other ELF types.  If the namespace is unknown,
       then tools will usually fallback to these sets of notes as well.

           typedef struct {
               Elf32_Word n_namesz;
               Elf32_Word n_descsz;
               Elf32_Word n_type;
           } Elf32_Nhdr;

           typedef struct {
               Elf64_Word n_namesz;
               Elf64_Word n_descsz;
               Elf64_Word n_type;
           } Elf64_Nhdr;

       n_namesz
              The length of the name field in bytes.  The contents will immediately  follow  this
              note  in  memory.  The name is null terminated.  For example, if the name is "GNU",
              then n_namesz will be set to 4.

       n_descsz
              The length of the descriptor field in bytes.  The contents will immediately  follow
              the name field in memory.

       n_type Depending on the value of the name field, this member may have any of the following
              values:

              Core files (e_type = ET_CORE)
                   Notes  used  by  all  core  files.   These  are  highly  operating  system  or
                   architecture  specific  and  often  require close coordination with kernels, C
                   libraries, and debuggers.  These are used when the namespace  is  the  default
                   (i.e.,  n_namesz  will  be  set  to  0),  or  a fallback when the namespace is
                   unknown.

                   NT_PRSTATUS          prstatus struct
                   NT_FPREGSET          fpregset struct
                   NT_PRPSINFO          prpsinfo struct
                   NT_PRXREG            prxregset struct
                   NT_TASKSTRUCT        task structure
                   NT_PLATFORM          String from sysinfo(SI_PLATFORM)
                   NT_AUXV              auxv array
                   NT_GWINDOWS          gwindows struct
                   NT_ASRS              asrset struct
                   NT_PSTATUS           pstatus struct
                   NT_PSINFO            psinfo struct
                   NT_PRCRED            prcred struct
                   NT_UTSNAME           utsname struct
                   NT_LWPSTATUS         lwpstatus struct
                   NT_LWPSINFO          lwpinfo struct
                   NT_PRFPXREG          fprxregset struct
                   NT_SIGINFO           siginfo_t (size might increase over time)
                   NT_FILE              Contains information about mapped files
                   NT_PRXFPREG          user_fxsr_struct
                   NT_PPC_VMX           PowerPC Altivec/VMX registers
                   NT_PPC_SPE           PowerPC SPE/EVR registers
                   NT_PPC_VSX           PowerPC VSX registers
                   NT_386_TLS           i386 TLS slots (struct user_desc)
                   NT_386_IOPERM        x86 io permission bitmap (1=deny)
                   NT_X86_XSTATE        x86 extended state using xsave
                   NT_S390_HIGH_GPRS    s390 upper register halves
                   NT_S390_TIMER        s390 timer register
                   NT_S390_TODCMP       s390 time-of-day (TOD) clock comparator register
                   NT_S390_TODPREG      s390 time-of-day (TOD) programmable register
                   NT_S390_CTRS         s390 control registers
                   NT_S390_PREFIX       s390 prefix register
                   NT_S390_LAST_BREAK   s390 breaking event address
                   NT_S390_SYSTEM_CALL  s390 system call restart data
                   NT_S390_TDB          s390 transaction diagnostic block
                   NT_ARM_VFP           ARM VFP/NEON registers
                   NT_ARM_TLS           ARM TLS register
                   NT_ARM_HW_BREAK      ARM hardware breakpoint registers
                   NT_ARM_HW_WATCH      ARM hardware watchpoint registers
                   NT_ARM_SYSTEM_CALL   ARM system call number

              n_name = GNU
                   Extensions used by the GNU tool chain.

                   NT_GNU_ABI_TAG
                          Operating system (OS) ABI information.  The desc field will be 4 words:

                          [0]  OS descriptor (ELF_NOTE_OS_LINUX, ELF_NOTE_OS_GNU, and so on)`
                          [1]  major version of the ABI
                          [2]  minor version of the ABI
                          [3]  subminor version of the ABI

                   NT_GNU_HWCAP
                          Synthetic hwcap information.  The desc field begins with two words:

                          [0]  number of entries
                          [1]  bit mask of enabled entries

                          Then follow variable-length entries,  one  byte  followed  by  a  null-
                          terminated hwcap name string.  The byte gives the bit number to test if
                          enabled, (1U << bit) & bit mask.

                   NT_GNU_BUILD_ID
                          Unique build ID as generated by the GNU ld(1) --build-id  option.   The
                          desc consists of any nonzero number of bytes.

                   NT_GNU_GOLD_VERSION
                          The desc contains the GNU Gold linker version used.

              Default/unknown namespace (e_type != ET_CORE)
                   These  are  used when the namespace is the default (i.e., n_namesz will be set
                   to 0), or a fallback when the namespace is unknown.

                   NT_VERSION  A version string of some sort.
                   NT_ARCH     Architecture information.

NOTES

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

       The extensions for e_phnum, e_shnum, and e_shstrndx  respectively  are  Linux  extensions.
       Sun, BSD, and AMD64 also support them; for further information, look under SEE ALSO.

SEE ALSO

       as(1),  elfedit(1), gdb(1), ld(1), nm(1), objcopy(1), objdump(1), patchelf(1), readelf(1),
       size(1), strings(1), strip(1), execve(2), dl_iterate_phdr(3), core(5), ld.so(8)

       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).

       Sun Microsystems, Linker and Libraries Guide.

       AMD64 ABI Draft, System  V  Application  Binary  Interface  AMD64  Architecture  Processor
       Supplement.