ELF(5)



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  de-
       fined  in the ELF header.  The two tables describe the rest of the par-
       ticularities 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, inde-
              pendent of the  processor  or  the  file's  remaining  contents.
              Within  this  array  everything  is named by macros, which start
              with the prefix EI_ and may contain values which start with  the
              prefix ELF.  The following macros are defined:

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

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

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

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

              EI_CLASS
                     The  fifth  byte identifies the architecture for this bi-
                     nary:

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

              EI_DATA
                     The sixth byte specifies the data encoding of the proces-
                     sor-specific  data  in the file.  Currently, these encod-
                     ings 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  speci-
                     fication:

                     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  distin-
                     guish  among incompatible versions of an ABI.  The inter-
                     pretation of this version number is dependent on the  ABI
                     identified  by the EI_OSABI field.  Applications conform-
                     ing 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 individ-
              ual file.  For example:

              EM_NONE         An unknown machine
              EM_M32          AT&T WE 32100
              EM_SPARC        Sun Microsystems SPARC
              EM_386          Intel 80386
              EM_68K          Motorola 68000
              EM_88K          Motorola 88000
              EM_860          Intel 80860
              EM_MIPS         MIPS RS3000 (big-endian only)
              EM_PARISC       HP/PA
              EM_SPARC32PLUS  SPARC with enhanced instruction set
              EM_PPC          PowerPC
              EM_PPC64        PowerPC 64-bit
              EM_S390         IBM S/390
              EM_ARM          Advanced RISC Machines
              EM_SH           Renesas SuperH
              EM_SPARCV9      SPARC v9 64-bit
              EM_IA_64        Intel Itanium
              EM_X86_64       AMD x86-64
              EM_VAX          DEC Vax

       e_version
              This member identifies the file version:

              EV_NONE         Invalid version
              EV_CURRENT      Current version

       e_entry
              This member gives the virtual address to which the system  first
              transfers  control,  thus starting the process.  If the file has
              no associated entry point, this member holds zero.

       e_phoff
              This member holds the program  header  table's  file  offset  in
              bytes.   If  the  file  has no program header table, this member
              holds zero.

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

       e_flags
              This member holds processor-specific flags associated  with  the
              file.   Flag  names take the form EF_`machine_flag'.  Currently,
              no flags have been defined.

       e_ehsize
              This member holds the ELF header's size in bytes.

       e_phentsize
              This member holds the size in bytes of one entry in  the  file's
              program header table; all entries are the same size.

       e_phnum
              This  member  holds  the number of entries in the program header
              table.  Thus the product of e_phentsize and  e_phnum  gives  the
              table's size in bytes.  If a file has no program header, e_phnum
              holds the value zero.

              If the number of entries in the program header table  is  larger
              than  or  equal  to  PN_XNUM (0xffff), this member holds PN_XNUM
              (0xffff) and the real number of entries in  the  program  header
              table is held in the sh_info member of the initial entry in sec-
              tion 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 head-
                     ers is assigned.

       e_shentsize
              This  member holds a sections header's size in bytes.  A section
              header is one entry in the section header table; all entries are
              the same size.

       e_shnum
              This  member  holds  the number of entries in the section header
              table.  Thus the product of e_shentsize and  e_shnum  gives  the
              section  header table's size in bytes.  If a file has no section
              header table, e_shnum holds the value of zero.

              If the number of entries in the section header table  is  larger
              than or equal to SHN_LORESERVE (0xff00), e_shnum holds the value
              zero and the real number of entries in the section header  table
              is  held  in  the sh_size member of the initial entry in section
              header table.  Otherwise, the sh_size member of the initial  en-
              try 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 ini-
              tial 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 members'
                        values are undefined.  This lets  the  program  header
                        have ignored entries.

                 PT_LOAD
                        The  array  element  specifies a loadable segment, de-
                        scribed by p_filesz and p_memsz.  The bytes  from  the
                        file  are  mapped  to the beginning of the memory seg-
                        ment.  If the segment's memory size p_memsz is  larger
                        than the file size p_filesz, the "extra" bytes are de-
                        fined 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  informa-
                        tion.

                 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).   How-
                        ever it may not occur more than once in a file.  If it
                        is present, it must precede any loadable  segment  en-
                        try.

                 PT_NOTE
                        The  array  element  specifies  the  location of notes
                        (ElfN_Nhdr).

                 PT_SHLIB
                        This segment type is reserved but has unspecified  se-
                        mantics.   Programs  that  contain an array element of
                        this type do not conform to the ABI.

                 PT_PHDR
                        The array element, if present, specifies the  location
                        and  size  of the program header table itself, both in
                        the file and in the memory image of the program.  This
                        segment  type  may not occur more than once in a file.
                        Moreover, it may occur only if the program header  ta-
                        ble is part of the memory image of the program.  If it
                        is present, it must precede any loadable  segment  en-
                        try.

                 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  mem-
              ber  is  reserved for the segment's physical address.  Under BSD
              this member is not used and must be zero.

       p_filesz
              This member holds the number of bytes in the file image  of  the
              segment.  It may be zero.

       p_memsz
              This member holds the number of bytes in the memory image of the
              segment.  It may be zero.

       p_flags
              This member holds a bit mask of flags relevant to the segment:

              PF_X   An executable segment.
              PF_W   A writable segment.
              PF_R   A readable segment.

              A text segment commonly has the flags PF_X  and  PF_R.   A  data
              segment commonly has PF_W and PF_R.

       p_align
              This member holds the value to which the segments are aligned in
              memory and in the file.  Loadable  process  segments  must  have
              congruent values for p_vaddr and p_offset, modulo the page size.
              Values of zero and one mean no alignment  is  required.   Other-
              wise,  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  in-
       dices  between  SHN_LORESERVE  and SHN_HIRESERVE.  The initial entry is
       used in ELF extensions for e_phnum, e_shnum and  e_shstrndx;  in  other
       cases,  each field in the initial entry is set to zero.  An object file
       does not have sections for these special indices:

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

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

       SHN_LOPROC, SHN_HIPROC
              Values greater in the inclusive range  [SHN_LOPROC,  SHN_HIPROC]
              are reserved for processor-specific semantics.

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

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

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

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

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

              SHT_RELA
                     This section holds relocation entries with  explicit  ad-
                     dends,  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 par-
                     ticipating 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 con-
                     ceptual 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 reloca-
                     tion sections.

              SHT_SHLIB
                     This section is reserved but has unspecified semantics.

              SHT_DYNSYM
                     This section holds a minimal set of dynamic linking  sym-
                     bols.   An object file can also contain a SHT_SYMTAB sec-
                     tion.

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

              SHT_LOUSER
                     This  value specifies the lower bound of the range of in-
                     dices reserved for application programs.

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

       sh_flags
              Sections support one-bit flags that describe  miscellaneous  at-
              tributes.   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  sec-
                     tions.

              SHF_EXECINSTR
                     This section contains executable machine instructions.

              SHF_MASKPROC
                     All  bits  included in this mask are reserved for proces-
                     sor-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 sec-
              tion 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  in-
              terpretation depends on the section type.

       sh_info
              This  member  holds  extra information, whose interpretation de-
              pends on the section type.

       sh_addralign
              Some sections have address alignment constraints.  If a  section
              holds  a doubleword, the system must ensure doubleword alignment
              for the entire section.  That is, the value of sh_addr  must  be
              congruent  to zero, modulo the value of sh_addralign.  Only zero
              and positive integral powers of two are allowed.  The value 0 or
              1 means that the section has no alignment constraints.

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

       Various sections hold program and control information:

       .bss   This section holds uninitialized data that  contributes  to  the
              program's  memory  image.  By definition, the system initializes
              the data with zeros when the program begins to run.   This  sec-
              tion  is  of type SHT_NOBITS.  The attribute types are SHF_ALLOC
              and SHF_WRITE.

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

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

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

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

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

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

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

       .dynstr
              This section holds strings needed for dynamic linking, most com-
              monly  the strings that represent the names associated with sym-
              bol table entries.  This section is of type SHT_STRTAB.  The at-
              tribute type used is SHF_ALLOC.

       .dynsym
              This  section holds the dynamic linking symbol table.  This sec-
              tion 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  sec-
              tion is of type SHT_PROGBITS.  The attributes used are SHF_ALLOC
              and SHF_EXECINSTR.

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

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

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

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

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

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

       .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  debug-
              ging,  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 at-
              tribute 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 iden-
              tify  themselves  so the kernel can bypass any compatibility ELF
              binary emulation tests when loading the file.

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

       .relNAME
              This  section  holds  relocation information as described below.
              If the file has a loadable segment that includes relocation, the
              section's attributes will include the SHF_ALLOC bit.  Otherwise,
              the bit will be off.  By convention, "NAME" is supplied  by  the
              section  to which the relocations apply.  Thus a relocation sec-
              tion 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 sec-
              tion 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  rep-
              resent  the  names associated with symbol table entries.  If the
              file has a loadable segment that includes the symbol string  ta-
              ble,  the  section's  attributes will include the SHF_ALLOC bit.
              Otherwise, the bit  will  be  off.   This  section  is  of  type
              SHT_STRTAB.

       .symtab
              This  section  holds a symbol table.  If the file has a loadable
              segment that includes the symbol table, the section's attributes
              will include the SHF_ALLOC bit.  Otherwise, the bit will be off.
              This section is of type SHT_SYMTAB.

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

   String and symbol tables
       String table sections hold null-terminated  character  sequences,  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 exe-
                     cutable 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 sym-
                     bol 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 com-
                     bined.   One  file's  definition  of a global symbol will
                     satisfy another file's undefined reference  to  the  same
                     symbol.

              STB_WEAK
                     Weak  symbols  resemble global symbols, but their defini-
                     tions have lower precedence.

              STB_LOPROC, STB_HIPROC
                     Values in the inclusive  range  [STB_LOPROC,  STB_HIPROC]
                     are reserved for processor-specific semantics.

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

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

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

              ELF32_ST_INFO(bind, type), ELF64_ST_INFO(bind, type)
                     Convert a binding and a type into an st_info value.

       st_other
              This member defines the symbol visibility.

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

              There are macros for extracting the visibility type:

              ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

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

       Relocation structures that do not need an addend:

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

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

       Relocation structures that need an addend:

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

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

       r_offset
              This member gives the location at which to apply the  relocation
              action.   For  a  relocatable file, the value is the byte offset
              from the beginning of the section to the storage  unit  affected
              by the relocation.  For an executable file or shared object, the
              value is the virtual address of the storage unit affected by the
              relocation.

       r_info This  member  gives  both the symbol table index with respect to
              which the relocation must be made and the type of relocation  to
              apply.   Relocation types are processor-specific.  When the text
              refers to a relocation entry's relocation type or  symbol  table
              index,  it  means  the  result  of applying ELF[32|64]_R_TYPE or
              ELF[32|64]_R_SYM, respectively, to the entry's r_info member.

       r_addend
              This member specifies a constant  addend  used  to  compute  the
              value to be stored into the relocatable field.

   Dynamic tags (Dyn)
       The .dynamic section contains a series of structures that hold relevant
       dynamic linking information.  The d_tag member controls the 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 (depre-
                          cated)

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

              DT_REL      Address of Rel relocation table

              DT_RELSZ    Size in bytes of Rel relocation table

              DT_RELENT   Size in bytes of a Rel table entry

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

              DT_DEBUG    Undefined use for debugging

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

              DT_JMPREL   Address of relocation entries associated solely with
                          the PLT

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

              DT_RUNPATH  String table offset to 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  interpreta-
              tions.

       d_ptr  This  member  represents program virtual addresses.  When inter-
              preting 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 sec-
              tion.  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  li-
       brary.

       Note sections contain a series of notes (see the struct definitions be-
       low).  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.   Nei-
       ther  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 imme-
              diately follow this note in memory.  The  name  is  null  termi-
              nated.  For example, if the name is "GNU", then n_namesz will be
              set to 4.

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

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

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

                   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 compara-
                                        tor 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  fol-
                          lowed  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  name-
                   space is unknown.

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

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

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

SEE ALSO
       as(1), elfedit(1), gdb(1), ld(1), nm(1), objdump(1), patchelf(1), read-
       elf(1), size(1), strings(1), strip(1),  execve(2),  dl_iterate_phdr(3),
       core(5), ld.so(8)

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

       UNIX System Laboratories, "Object Files", Executable and Linking Format
       (ELF).

       Sun Microsystems, Linker and Libraries Guide.

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

COLOPHON
       This  page  is  part of release 5.07 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/.

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