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

NAME
       elf - format of Executable and Linking Format (ELF) files

SYNOPSIS
       #include 

DESCRIPTION
       The  header file  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 pro‐
       gram 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  particulari‐
       ties of the file.

       This  header  file describes the above mentioned headers as C structures and also includes
       structures for dynamic sections, relocation sections and symbol tables.

       The following types are used for N-bit architectures (N=32,64, ElfN stands  for  Elf32  or
       Elf64, uintN_t stands for uint32_t or uint64_t):

           ElfN_Addr       Unsigned program address, uintN_t
           ElfN_Off        Unsigned file offset, uintN_t
           ElfN_Section    Unsigned section index, uint16_t
           ElfN_Versym     Unsigned version symbol information, uint16_t
           Elf_Byte        unsigned char
           ElfN_Half       uint16_t
           ElfN_Sword      int32_t
           ElfN_Word       uint32_t
           ElfN_Sxword     int64_t
           ElfN_Xword      uint64_t

       (Note:  The  *BSD  terminology  is a bit different.  There Elf64_Half is twice as large as
       Elf32_Half, and Elf64Quarter is used for uint16_t.  In  order  to  avoid  confusion  these
       types are replaced by explicit ones in the below.)

       All  data  structures that the file format defines follow the "natural" size and alignment
       guidelines for the relevant class.  If necessary, data structures contain explicit padding
       to  ensure  4-byte alignment for 4-byte objects, to force structure sizes to a multiple of
       4, and so on.

       The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:

           #define EI_NIDENT 16

           typedef struct {
               unsigned char e_ident[EI_NIDENT];
               uint16_t      e_type;
               uint16_t      e_machine;
               uint32_t      e_version;
               ElfN_Addr     e_entry;
               ElfN_Off      e_phoff;
               ElfN_Off      e_shoff;
               uint32_t      e_flags;
               uint16_t      e_ehsize;
               uint16_t      e_phentsize;
               uint16_t      e_phnum;
               uint16_t      e_shentsize;
               uint16_t      e_shnum;
               uint16_t      e_shstrndx;
           } ElfN_Ehdr;

       The fields have the following meanings:

       e_ident     This array of bytes specifies how to interpret the file,  independent  of  the
                   processor  or  the file's remaining contents.  Within this array everything is
                   named by macros, which start with the prefix EI_ and may contain values  which
                   start with the prefix ELF.  The following macros are defined:

                   EI_MAG0     The  first  byte of the magic number.  It must be filled with ELF‐
                               MAG0.  (0: 0x7f)

                   EI_MAG1     The second byte of the magic number.  It must be filled with  ELF‐
                               MAG1.  (1: 'E')

                   EI_MAG2     The  third  byte of the magic number.  It must be filled with ELF‐
                               MAG2.  (2: 'L')

                   EI_MAG3     The fourth byte of the magic number.  It must be filled with  ELF‐
                               MAG3.  (3: 'F')

                   EI_CLASS    The fifth byte identifies the architecture for this binary:

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

                   EI_DATA     The  sixth  byte specifies the data encoding of the processor-spe‐
                               cific 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  inter‐
                               pretation 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  ver‐
                               sion  number  is  dependent  on the ABI identified by the EI_OSABI
                               field.  Applications conforming  to  this  specification  use  the
                               value 0.

                   EI_PAD      Start of padding.  These bytes are reserved and set to zero.  Pro‐
                               grams which read them should ignore them.  The  value  for  EI_PAD
                               will  change  in  the  future  if currently unused bytes are given
                               meanings.

                   EI_NIDENT   The size of the e_ident array.

       e_type      This member of the structure identifies the object file type:

                   ET_NONE     An unknown type.
                   ET_REL      A relocatable file.
                   ET_EXEC     An executable file.
                   ET_DYN      A shared object.
                   ET_CORE     A core file.

       e_machine   This member specifies the required architecture for an individual  file.   For
                   example:

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

       e_version   This member identifies the file version:

                   EV_NONE     Invalid version.
                   EV_CURRENT  Current version.

       e_entry     This member gives the virtual address to which the system first transfers con‐
                   trol, 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 ini‐
                   tial entry in section header table.  Otherwise, the sh_info member of the ini‐
                   tial entry contains the value zero.

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

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

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

                   If the number of entries in the section header table is larger than  or  equal
                   to SHN_LORESERVE (0xff00), e_shnum holds the value zero and the real number of
                   entries in the section header table is held in the sh_size member of the  ini‐
                   tial entry in section header table.  Otherwise, the sh_size member of the ini‐
                   tial 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.

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

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

                   SHN_LOPROC    Values greater than or equal to SHN_HIPROC are reserved for pro‐
                                 cessor-specific semantics.

                   SHN_HIPROC    Values less than or equal to SHN_LOPROC are reserved for proces‐
                                 sor-specific semantics.

                   SHN_ABS       This  value specifies absolute values for the corresponding ref‐
                                 erence.  For example, symbols defined relative to section number
                                 SHN_ABS have absolute values and are not affected by relocation.

                   SHN_COMMON    Symbols  defined  relative  to  this section are common symbols,
                                 such as Fortran COMMON or unallocated C external variables.

                   SHN_HIRESERVE This value specifies the upper bound of the  range  of  reserved
                                 indices  between SHN_LORESERVE and SHN_HIRESERVE, inclusive; the
                                 values do not reference the section header table.  That is,  the
                                 section  header  table does not contain entries for the reserved
                                 indices.

       An executable or shared object file's program header table is an array of structures, each
       describing a segment or other information the system needs to prepare the program for exe‐
       cution.  An object file segment contains one or more sections.  Program headers are  mean‐
       ingful  only  for  executable  and  shared object files.  A file specifies its own program
       header size with the ELF header's e_phentsize and e_phnum members.  The ELF program header
       is described by the type Elf32_Phdr or Elf64_Phdr depending on the architecture:

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

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

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

       p_type      This member of the Phdr struct tells what kind of segment this  array  element
                   describes or how to interpret the array element's information.

                   PT_NULL     The  array  element  is  unused  and the other members' values are
                               undefined.  This lets the program header have ignored entries.

                   PT_LOAD     The array element  specifies  a  loadable  segment,  described  by
                               p_filesz  and  p_memsz.  The bytes from the file are mapped to the
                               beginning of the memory segment.  If  the  segment's  memory  size
                               p_memsz  is  larger than the file size p_filesz, the "extra" bytes
                               are defined to hold the value 0 and to follow the  segment's  ini‐
                               tialized  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-termi‐
                               nated pathname to invoke as an interpreter.  This segment type  is
                               meaningful  only  for  executable  files  (though it may occur for
                               shared objects).  However it may not occur more  than  once  in  a
                               file.   If  it  is  present,  it must precede any loadable segment
                               entry.

                   PT_NOTE     The array element specifies the location and  size  for  auxiliary
                               information.

                   PT_SHLIB    This segment type is reserved but has unspecified semantics.  Pro‐
                               grams 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  mem‐
                               ory  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   Values greater than or equal to PT_HIPROC are reserved for proces‐
                               sor-specific semantics.

                   PT_HIPROC   Values less than or equal to PT_LOPROC are reserved for processor-
                               specific semantics.

                   PT_GNU_STACK
                               GNU extension which is used by the Linux  kernel  to  control  the
                               state of the stack via the flags set in the p_flags member.

       p_offset    This member holds the offset from the beginning of the file at which the first
                   byte of the segment resides.

       p_vaddr     This member holds the virtual address at which the first byte of  the  segment
                   resides in memory.

       p_paddr     On  systems for which physical addressing is relevant, this member is reserved
                   for the segment's physical address.  Under BSD this member  is  not  used  and
                   must be zero.

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

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

       p_flags     This member holds a 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_X, PF_W and PF_R.

       p_align     This member holds the value to which the segments are aligned in memory and in
                   the  file.   Loadable  process segments must have congruent values for p_vaddr
                   and p_offset, modulo the page size.  Values of zero and one mean no  alignment
                   is  required.  Otherwise, p_align should be a positive, integral power of two,
                   and p_vaddr should equal p_offset, modulo p_align.

       A file's section header table lets one locate all the file's sections.  The section header
       table is an array of Elf32_Shdr or Elf64_Shdr structures.  The ELF header's e_shoff member
       gives the byte offset from the beginning of the file to the section header table.  e_shnum
       holds the number of entries the section header table contains.  e_shentsize holds the size
       in bytes of each entry.

       A section header table index is a subscript into this array.  Some  section  header  table
       indices  are  reserved:  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    Values greater than or equal to SHN_HIPROC are reserved  for  proces‐
                            sor-specific semantics.

              SHN_HIPROC    Values  less  than or equal to SHN_LOPROC are reserved for processor-
                            specific semantics.

              SHN_ABS       This value specifies the absolute value for the corresponding  refer‐
                            ence.   For  example,  a  symbol  defined  relative to section number
                            SHN_ABS has an absolute value and is not affected by relocation.

              SHN_COMMON    Symbols defined relative to this section are common symbols, such  as
                            FORTRAN COMMON or unallocated C external variables.

              SHN_HIRESERVE This  value  specifies  the  upper  bound  of  the  range of reserved
                            indices.  The  system  reserves  indices  between  SHN_LORESERVE  and
                            SHN_HIRESERVE,  inclusive.  The section header table does not contain
                            entries for the reserved indices.

       The section header has the following structure:

           typedef struct {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint32_t   sh_flags;
               Elf32_Addr sh_addr;
               Elf32_Off  sh_offset;
               uint32_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               uint32_t   sh_addralign;
               uint32_t   sh_entsize;
           } Elf32_Shdr;

           typedef struct {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint64_t   sh_flags;
               Elf64_Addr sh_addr;
               Elf64_Off  sh_offset;
               uint64_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               uint64_t   sh_addralign;
               uint64_t   sh_entsize;
           } Elf64_Shdr;

       No real differences exist between the 32-bit and 64-bit section headers.

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

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

                 SHT_NULL       This value marks the section header as  inactive.   It  does  not
                                have  an associated section.  Other members of the section header
                                have undefined values.

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

                 SHT_SYMTAB     This  section  holds  a symbol table.  Typically, SHT_SYMTAB pro‐
                                vides 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 mul‐
                                tiple string table sections.

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

                 SHT_HASH       This section holds a symbol hash table.  An object  participating
                                in  dynamic  linking must contain a symbol hash table.  An object
                                file may have only one hash table.

                 SHT_DYNAMIC    This section holds information for dynamic  linking.   An  object
                                file may have only one dynamic section.

                 SHT_NOTE       This section holds information that marks the file in some way.

                 SHT_NOBITS     A  section  of this type occupies no space in the file but other‐
                                wise resembles SHT_PROGBITS.  Although this section  contains  no
                                bytes, the sh_offset member contains the conceptual file offset.

                 SHT_REL        This  section  holds relocation offsets without explicit addends,
                                such as type Elf32_Rel for the 32-bit class of object files.   An
                                object file may have multiple relocation sections.

                 SHT_SHLIB      This section is reserved but has unspecified semantics.

                 SHT_DYNSYM     This  section holds a minimal set of dynamic linking symbols.  An
                                object file can also contain a SHT_SYMTAB section.

                 SHT_LOPROC     This value up to and including SHT_HIPROC is reserved for proces‐
                                sor-specific semantics.

                 SHT_HIPROC     This  value down to and including SHT_LOPROC is reserved for pro‐
                                cessor-specific semantics.

                 SHT_LOUSER     This value specifies the lower bound  of  the  range  of  indices
                                reserved for application programs.

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

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

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

                 SHF_ALLOC      This section occupies memory during process execution.  Some con‐
                                trol 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-spe‐
                                cific semantics.

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

       sh_offset This member's value holds the byte offset from the beginning of the file to  the
                 first  byte  in the section.  One section type, SHT_NOBITS, occupies no space in
                 the file, and its sh_offset member locates the conceptual placement in the file.

       sh_size   This member holds the section's size in  bytes.   Unless  the  section  type  is
                 SHT_NOBITS,  the  section occupies sh_size bytes in the file.  A section of type
                 SHT_NOBITS may have a nonzero size, but it occupies no space in the file.

       sh_link   This member holds a  section  header  table  index  link,  whose  interpretation
                 depends on the section type.

       sh_info   This member holds extra information, whose interpretation depends on the section
                 type.

       sh_addralign
                 Some sections have address alignment constraints.  If a section holds a  double‐
                 word,  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.  Val‐
                 ues of zero or one mean the section has no alignment constraints.

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

       Various sections hold program and control information:

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

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

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

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

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

       .debug    This section holds information for symbolic debugging.  The contents are unspec‐
                 ified.  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-spe‐
                 cific.  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 termi‐
                 nation  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 struc‐
                 tures.   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_PROG‐
                 BITS.  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  ini‐
                 tialization  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 sec‐
                 tion 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  debugging, which
                 describes the correspondence between the program source and  the  machine  code.
                 The  contents  are  unspecified.   This  section  is  of  type SHT_PROGBITS.  No
                 attribute types are used.

       .note     This section holds information in the "Note Section" format.  This section is of
                 type SHT_NOTE.  No attribute types are used.  OpenBSD native executables usually
                 contain a .note.openbsd.ident section to identify themselves, for the kernel  to
                 bypass any compatibility ELF binary emulation tests when loading the file.

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

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

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

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

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

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

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

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

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

       The 32-bit and 64-bit versions have the same members, just in a different order.

       st_name   This  member  holds  an  index into the object file's symbol string table, which
                 holds character representations of the symbol names.  If the value  is  nonzero,
                 it  represents  a string table index that gives the symbol name.  Otherwise, the
                 symbol table has no name.

       st_value  This member gives the value of the associated symbol.

       st_size   Many symbols have associated sizes.  This member holds zero if the symbol has no
                 size or an unknown size.

       st_info   This member specifies the symbol's type and binding attributes:

                 STT_NOTYPE  The symbol's type is not defined.

                 STT_OBJECT  The symbol is associated with a data object.

                 STT_FUNC    The symbol is associated with a function or other executable code.

                 STT_SECTION The  symbol  is  associated with a section.  Symbol table entries of
                             this type exist primarily for relocation and normally have STB_LOCAL
                             bindings.

                 STT_FILE    By  convention,  the symbol's name gives the name of the source file
                             associated with the object file.  A file symbol has STB_LOCAL  bind‐
                             ings,  its  section  index  is  SHN_ABS,  and  it precedes the other
                             STB_LOCAL symbols of the file, if it is present.

                 STT_LOPROC  This value up to and including STT_HIPROC is reserved for processor-
                             specific semantics.

                 STT_HIPROC  This  value down to and including STT_LOPROC is reserved for proces‐
                             sor-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 mul‐
                             tiple files without interfering with each other.

                 STB_GLOBAL  Global symbols are visible to all object files being combined.   One
                             file's  definition  of  a  global symbol will satisfy another file's
                             undefined reference to the same symbol.

                 STB_WEAK    Weak symbols resemble global symbols,  but  their  definitions  have
                             lower precedence.

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

                 STB_HIPROC  This value down to and including STB_LOPROC is reserved for  proces‐
                             sor-specific semantics.

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

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

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

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

       st_other  This member defines the symbol visibility.

                 STV_DEFAULT     Default symbol visibility rules.
                 STV_INTERNAL    Processor-specific hidden class.
                 STV_HIDDEN      Symbol is unavailable in other modules.
                 STV_PROTECTED   Not preemptible, not exported.

                 There are macros for extracting the visibility type:

                 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

       Relocation is the process of connecting symbolic  references  with  symbolic  definitions.
       Relocatable  files  must  have information that describes how to modify their section con‐
       tents, 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  sec‐
                   tion  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 relo‐
                   cation must be made and the type of relocation to apply.  Relocation types are
                   processor-specific.   When  the text refers to a relocation entry's relocation
                   type or symbol table index, it means the result of applying  ELF[32|64]_R_TYPE
                   or ELF[32|64]_R_SYM, respectively, to the entry's r_info member.

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

       The .dynamic section contains a series of structures that hold  relevant  dynamic  linking
       information.  The d_tag member controls the interpretation of d_un.

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

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

       d_tag     This member may have any of the following values:

                 DT_NULL     Marks end of dynamic section

                 DT_NEEDED   String table offset to name of a needed library

                 DT_PLTRELSZ Size in bytes of PLT relocs

                 DT_PLTGOT   Address of PLT and/or GOT

                 DT_HASH     Address of symbol hash table

                 DT_STRTAB   Address of string table

                 DT_SYMTAB   Address of symbol table

                 DT_RELA     Address of Rela relocs table

                 DT_RELASZ   Size in bytes of Rela table

                 DT_RELAENT  Size in bytes of a Rela table entry

                 DT_STRSZ    Size in bytes of string table

                 DT_SYMENT   Size in bytes of a symbol table entry

                 DT_INIT     Address of the initialization function

                 DT_FINI     Address of the termination function

                 DT_SONAME   String table offset to name of shared object

                 DT_RPATH    String table offset to library search path (deprecated)

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

                 DT_REL      Address of Rel relocs table

                 DT_RELSZ    Size in bytes of Rel table

                 DT_RELENT   Size in bytes of a Rel table entry

                 DT_PLTREL   Type of reloc the PLT refers (Rela or Rel)

                 DT_DEBUG    Undefined use for debugging

                 DT_TEXTREL  Absence of this indicates no relocs should apply  to  a  nonwritable
                             segment

                 DT_JMPREL   Address of reloc entries solely for the PLT

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

                 DT_RUNPATH  String table offset to library search path

                 DT_LOPROC   Start of processor-specific semantics

                 DT_HIPROC   End of processor-specific semantics

       d_val     This member represents integer values with various interpretations.

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

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

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

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

SEE ALSO
       as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

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

       Sun Microsystems, Linker and Libraries Guide.

       AMD64  ABI  Draft, System V Application Binary Interface AMD64 Architecture Processor Sup‐
       plement.

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2013-04-17                                     ELF(5)

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