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man: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 informa-
         tion,   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 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 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 =  mem-
         ory + 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), patchelf(1), read-
     elf(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 4.16 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 2018-04-30 ELF(5)

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