noble (5) elf.5.gz

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NAME

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

SYNOPSIS

       #include <elf.h>

DESCRIPTION

       The  header  file  <elf.h>  defines  the  format of ELF executable binary files.  Amongst these files are
       normal executable files, relocatable object files, core files, and shared objects.

       An executable file using the ELF file format consists of an ELF header,  followed  by  a  program  header
       table  or  a  section  header  table, or both.  The ELF header is always at offset zero of the file.  The
       program header table and the section header table's offset in the file are defined  in  the  ELF  header.
       The two tables describe the rest of the particularities of the file.

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

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

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

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

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

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

           #define EI_NIDENT 16

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

       The fields have the following meanings:

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

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

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

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

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

              EI_CLASS
                     The fifth byte identifies the architecture for this binary:

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

              EI_DATA
                     The  sixth  byte  specifies  the  data encoding of the processor-specific data in the file.
                     Currently, these encodings are supported:

                       ELFDATANONE   Unknown data format.
                       ELFDATA2LSB   Two's complement, little-endian.
                       ELFDATA2MSB   Two's complement, big-endian.

              EI_VERSION
                     The seventh byte is the version number of the ELF specification:

                     EV_NONE       Invalid version.
                     EV_CURRENT    Current version.

              EI_OSABI
                     The eighth byte identifies the operating system and ABI to which the  object  is  targeted.
                     Some  fields  in  other  ELF  structures  have flags and values that have platform-specific
                     meanings; the interpretation of those fields is determined by the value of this byte.   For
                     example:

                     ELFOSABI_NONE        Same as ELFOSABI_SYSV
                     ELFOSABI_SYSV        UNIX System V ABI
                     ELFOSABI_HPUX        HP-UX ABI
                     ELFOSABI_NETBSD      NetBSD ABI
                     ELFOSABI_LINUX       Linux ABI
                     ELFOSABI_SOLARIS     Solaris ABI
                     ELFOSABI_IRIX        IRIX ABI
                     ELFOSABI_FREEBSD     FreeBSD ABI
                     ELFOSABI_TRU64       TRU64 UNIX ABI
                     ELFOSABI_ARM         ARM architecture ABI
                     ELFOSABI_STANDALONE  Stand-alone (embedded) ABI

              EI_ABIVERSION
                     The  ninth  byte  identifies  the version of the ABI to which the object is targeted.  This
                     field is used to distinguish among incompatible versions of an ABI.  The interpretation  of
                     this version number is dependent on the ABI identified by the EI_OSABI field.  Applications
                     conforming to this specification use the value 0.

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

              EI_NIDENT
                     The size of the e_ident array.

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

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

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

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

       e_version
              This member identifies the file version:

              EV_NONE         Invalid version
              EV_CURRENT      Current version

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

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

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

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

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

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

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

              If  the number of entries in the program header table is larger than or equal to PN_XNUM (0xffff),
              this member holds PN_XNUM (0xffff) and the real number of entries in the program header  table  is
              held  in  the sh_info member of the initial entry in section header table.  Otherwise, the sh_info
              member of the initial entry contains the value zero.

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

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

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

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

       e_shstrndx
              This  member  holds  the  section header table index of the entry associated with the section name
              string table.  If the file has  no  section  name  string  table,  this  member  holds  the  value
              SHN_UNDEF.

              If  the  index  of  section  name  string  table  section is larger than or equal to SHN_LORESERVE
              (0xff00), this member holds SHN_XINDEX (0xffff) and the real index  of  the  section  name  string
              table  section  is  held  in  the  sh_link  member  of  the initial entry in section header table.
              Otherwise, the sh_link member of the initial entry in section  header  table  contains  the  value
              zero.

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

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

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

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

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

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

                 PT_LOAD
                        The  array element specifies a loadable segment, described by p_filesz and p_memsz.  The
                        bytes from the file are mapped to the beginning of the memory segment.  If the segment's
                        memory size p_memsz is larger than the file size p_filesz, the "extra" bytes are defined
                        to hold the value 0 and to follow the segment's initialized area.  The file size may not
                        be  larger  than  the memory size.  Loadable segment entries in the program header table
                        appear in ascending order, sorted on the p_vaddr member.

                 PT_DYNAMIC
                        The array element specifies dynamic linking information.

                 PT_INTERP
                        The array element specifies the location and  size  of  a  null-terminated  pathname  to
                        invoke  as  an  interpreter.   This segment type is meaningful only for executable files
                        (though it may occur for shared objects).  However it may not occur more than once in  a
                        file.  If it is present, it must precede any loadable segment entry.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The section header has the following structure:

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

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

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

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

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

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

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

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

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

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

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

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

              SHT_NOTE
                     This section holds notes (ElfN_Nhdr).

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

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

              SHT_SHLIB
                     This section is reserved but has unspecified semantics.

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

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

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

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

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

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

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

              SHF_EXECINSTR
                     This section contains executable machine instructions.

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

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

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

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

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

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

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

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

       Various sections hold program and control information:

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

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

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

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

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

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

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

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

       .dynstr
              This section holds strings needed for dynamic linking, most commonly the  strings  that  represent
              the  names  associated  with  symbol  table  entries.   This  section  is of type SHT_STRTAB.  The
              attribute type used is SHF_ALLOC.

       .dynsym
              This section holds the dynamic linking symbol table.  This section is  of  type  SHT_DYNSYM.   The
              attribute used is SHF_ALLOC.

       .fini  This  section holds executable instructions that contribute to the process termination code.  When
              a program exits normally the system arranges to execute the code in this section.  This section is
              of type SHT_PROGBITS.  The attributes used are SHF_ALLOC and SHF_EXECINSTR.

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

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

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

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

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

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

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

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

       .note  This section holds various notes.  This section is of type SHT_NOTE.  No attribute types are used.

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

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

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

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

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

       .relNAME
              This section holds relocation information as described below.  If the file has a loadable  segment
              that includes relocation, the section's attributes will include the SHF_ALLOC bit.  Otherwise, the
              bit will be off.  By convention, "NAME" is supplied by the section to which the relocations apply.
              Thus  a  relocation  section for .text normally would have the name .rel.text.  This section is of
              type SHT_REL.

       .relaNAME
              This section holds relocation information as described below.  If the file has a loadable  segment
              that includes relocation, the section's attributes will include the SHF_ALLOC bit.  Otherwise, the
              bit will be off.  By convention, "NAME" is supplied by the section to which the relocations apply.
              Thus  a  relocation section for .text normally would have the name .rela.text.  This section is of
              type SHT_RELA.

       .rodata
              This section holds read-only data that typically contributes  to  a  nonwritable  segment  in  the
              process image.  This section is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

       .rodata1
              This  section  holds  read-only  data  that  typically contributes to a nonwritable segment in the
              process image.  This section is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

       .shstrtab
              This section holds section names.  This section is of type SHT_STRTAB.   No  attribute  types  are
              used.

       .strtab
              This  section  holds  strings,  most commonly the strings that represent the names associated with
              symbol table entries.  If the file has a loadable segment that includes the symbol  string  table,
              the  section's  attributes  will include the SHF_ALLOC bit.  Otherwise, the bit will be off.  This
              section is of type SHT_STRTAB.

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

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

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

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

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

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

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

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

       st_value
              This member gives the value of the associated symbol.

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

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

              STT_NOTYPE
                     The symbol's type is not defined.

              STT_OBJECT
                     The symbol is associated with a data object.

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

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

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

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

              STB_LOCAL
                     Local  symbols  are not visible outside the object file containing their definition.  Local
                     symbols of the same name may exist in multiple files without interfering with each other.

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

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

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

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

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

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

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

       st_other
              This member defines the symbol visibility.

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

              There are macros for extracting the visibility type:

              ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

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

       Relocation structures that do not need an addend:

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

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

       Relocation structures that need an addend:

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

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

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

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

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

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

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

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

       d_tag  This member may have any of the following values:

              DT_NULL     Marks end of dynamic section

              DT_NEEDED   String table offset to name of a needed library

              DT_PLTRELSZ Size in bytes of PLT relocation entries

              DT_PLTGOT   Address of PLT and/or GOT

              DT_HASH     Address of symbol hash table

              DT_STRTAB   Address of string table

              DT_SYMTAB   Address of symbol table

              DT_RELA     Address of Rela relocation table

              DT_RELASZ   Size in bytes of the Rela relocation table

              DT_RELAENT  Size in bytes of a Rela relocation table entry

              DT_STRSZ    Size in bytes of string table

              DT_SYMENT   Size in bytes of a symbol table entry

              DT_INIT     Address of the initialization function

              DT_FINI     Address of the termination function

              DT_SONAME   String table offset to name of shared object

              DT_RPATH    String table offset to library search path (deprecated)

              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 interpretations.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              n_name = GNU
                   Extensions used by the GNU tool chain.

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

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

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

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

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

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

                   NT_GNU_GOLD_VERSION
                          The desc contains the GNU Gold linker version used.

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

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

NOTES

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

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

SEE ALSO

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

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

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

       Sun Microsystems, Linker and Libraries Guide.

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