elf

ELF(5)                     Linux Programmer's Manual                    ELF(5)

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  con-
                 tents.   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  Giga-
                                        bytes.
                          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  depen-
                          dent  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  indi-
                 vidual 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'.   Cur-
                 rently, 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 pro-
                 gram header table is held in the sh_info member of  the  ini-
                 tial  entry  in section header table.  Otherwise, the sh_info
                 member of the initial entry contains the value zero.

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

       e_shentsize
                 This member holds a sections header's size in bytes.  A  sec-
                 tion  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  sec-
                 tion 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 sys-
       tem needs to prepare the program for execution.  An object file segment
       contains one or more sections.  Program headers are meaningful only for
       executable and shared object files.  A file specifies its  own  program
       header size with the ELF header's e_phentsize and e_phnum members.  The
       ELF program header is described by the type  Elf32_Phdr  or  Elf64_Phdr
       depending on the architecture:

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

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

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

       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  mem-
                             bers'  values  are undefined.  This lets the pro-
                             gram 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 ele-
                             ment 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 seg-
                 ment:

                 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.

   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 struc-
       tures.   The ELF header's e_shoff member gives the byte offset from the
       beginning of the file to the section header table.  e_shnum  holds  the
       number of entries the section header table contains.  e_shentsize holds
       the size in bytes of each entry.

       A section header table index is a subscript into this array.  Some sec-
       tion  header  table  indices  are  reserved:  the initial entry and the
       indices between SHN_LORESERVE and SHN_HIRESERVE.  The initial entry  is
       used  in  ELF  extensions  for  e_phnum, e_shnum and e_strndx; in other
       cases, each field in the initial entry is set to zero.  An object  file
       does not have sections for these special indices:

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

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

       SHN_LOPROC, 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 head-
       ers.

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

       sh_type   This member categorizes the section's contents and semantics.

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

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

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

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

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

                 SHT_HASH       This  section  holds  a symbol hash table.  An
                                object participating in dynamic  linking  must
                                contain  a  symbol hash table.  An object file
                                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 con-
                                tain a SHT_SYMTAB section.

                 SHT_LOPROC, SHT_HIPROC
                                Values in  the  inclusive  range  [SHT_LOPROC,
                                SHT_HIPROC]  are  reserved  for processor-spe-
                                cific semantics.

                 SHT_LOUSER     This value specifies the lower  bound  of  the
                                range of indices reserved for application pro-
                                grams.

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

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

                 SHF_WRITE      This section  contains  data  that  should  be
                                writable during process execution.

                 SHF_ALLOC      This  section  occupies  memory during process
                                execution.   Some  control  sections  do   not
                                reside  in the memory image of an object file.
                                This attribute is off for those sections.

                 SHF_EXECINSTR  This  section  contains   executable   machine
                                instructions.

                 SHF_MASKPROC   All  bits  included  in this mask are reserved
                                for processor-specific semantics.

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

       sh_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 sec-
                 tion holds a doubleword, the system  must  ensure  doubleword
                 alignment  for  the  entire  section.   That is, the value of
                 sh_addr must be  congruent  to  zero,  modulo  the  value  of
                 sh_addralign.   Only zero and positive integral powers of two
                 are allowed.  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 initial-
                 izes the data with zeros when  the  program  begins  to  run.
                 This  section is of type SHT_NOBITS.  The attribute types are
                 SHF_ALLOC and SHF_WRITE.

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

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

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

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

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

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

       .dynamic  This  section  holds  dynamic  linking information.  The sec-
                 tion's attributes will include the  SHF_ALLOC  bit.   Whether
                 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 ta-
                 ble of ElfN_Verneed structures.   This  section  is  of  type
                 SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

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

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

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

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

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

       .note     This section holds 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 runtime ABI of
                 the ELF image.  It may include the operating system name  and
                 its runtime 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 compatibil-
                 ity 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-spe-
                 cific.

       .relNAME  This section holds relocation information as described below.
                 If  the file has a loadable segment that includes relocation,
                 the section's attributes  will  include  the  SHF_ALLOC  bit.
                 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  load-
                 able  segment  that  includes the symbol table, the section's
                 attributes will include the SHF_ALLOC  bit.   Otherwise,  the
                 bit will be off.  This section is of type SHT_SYMTAB.

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

   String and symbol tables
       String  table  sections  hold null-terminated character sequences, com-
       monly called strings.  The object file uses these strings to  represent
       symbol and section names.  One references a string as an index into the
       string table section.  The first byte, which is index zero, is  defined
       to  hold  a null byte ('\0').  Similarly, a string table's last byte is
       defined to hold a null byte, ensuring null termination for all strings.

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

           typedef struct {
               uint32_t      st_name;
               Elf32_Addr    st_value;
               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 differ-
       ent order.

       st_name   This  member  holds  an  index  into the object file's symbol
                 string table, which holds character  representations  of  the
                 symbol  names.   If  the  value  is  nonzero, it represents a
                 string table index that gives the  symbol  name.   Otherwise,
                 the symbol 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  unde-
                             fined 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 inter-
                                 posed by definitions in other modules.
                 STV_INTERNAL    Processor-specific hidden class.
                 STV_HIDDEN      Symbol is unavailable to other modules;  ref-
                                 erences 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 sym-
       bolic  definitions.   Relocatable  files  must  have  information  that
       describes  how  to  modify  their  section contents, thus allowing exe-
       cutable and shared object files to hold the  right  information  for  a
       process's program image.  Relocation entries are these data.

       Relocation structures that do not need an addend:

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

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

       Relocation structures that need an addend:

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

           typedef struct {
               Elf64_Addr r_offset;
               uint64_t   r_info;
               int64_t    r_addend;
           } Elf64_Rela;

       r_offset  This  member gives the location at which to apply the reloca-
                 tion 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 interpreta-
       tion of d_un.

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

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

       d_tag     This member may have any of the following values:

                 DT_NULL     Marks end of dynamic section

                 DT_NEEDED   String table offset to name of a needed library

                 DT_PLTRELSZ Size in bytes of PLT 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 library search path (dep-
                             recated)

                 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  reloca-
                             tion  entries  should apply to a nonwritable seg-
                             ment

                 DT_JMPREL   Address of relocation entries  associated  solely
                             with the PLT

                 DT_BIND_NOW Instruct  dynamic  linker  to process all reloca-
                             tions before transferring  control  to  the  exe-
                             cutable

                 DT_RUNPATH  String table offset to library search path

                 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 interpre-
                 tations.

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

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

   Notes (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 usu-
       ally 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  operat-
                      ing  system  or  architecture specific and often require
                      close coordination with kernels, C libraries, and debug-
                      gers.   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 com-
                                           parator 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:

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

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

                             o word 0: number of entries
                             o word 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_strndx respectively are Linux
       extensions.  Sun, BSD and AMD64 also support them; for further informa-
       tion, look under SEE ALSO.

SEE ALSO
       as(1),  elfedit(1),  gdb(1),  ld(1),  nm(1),  objdump(1),   readelf(1),
       size(1), strings(1), strip(1), execve(2), dl_iterate_phdr(3), 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).

       Sun Microsystems, Linker and Libraries Guide.

       AMD64 ABI Draft, System V Application Binary Interface AMD64  Architec-
       ture Processor Supplement.

COLOPHON
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                            ELF(5)
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