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



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

SYNOPSIS
       #include <elf.h>

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

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

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

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

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

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

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

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

	   #define EI_NIDENT 16

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

       The fields have the following meanings:

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

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

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

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

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

		 EI_CLASS The  fifth byte identifies the architecture for this
			  binary:

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

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

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

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

			  EV_NONE	Invalid version.
			  EV_CURRENT	Current version.

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

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

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

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

		 EI_NIDENT
			  The size of the e_ident array.

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

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

       e_machine This member specifies the required architecture for an	 indi-
		 vidual file.  For example:

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

       e_version This member identifies the file version:

		 EV_NONE	 Invalid version
		 EV_CURRENT	 Current version

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

		 PT_NULL     The array element is unused and  the  other  mem-
			     bers'  values  are undefined.  This lets the pro-
			     gram header have ignored entries.

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

		 PT_DYNAMIC  The  array	 element  specifies  dynamic   linking
			     information.

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

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

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

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

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

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

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

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

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

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

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

       p_flags	 This  member  holds  a bit mask of flags relevant to the seg-
		 ment:

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

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

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

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

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

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

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

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

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

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

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

       The section header has the following structure:

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

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

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

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

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

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

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

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

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

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

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

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

		 SHT_NOTE	This section holds notes (ElfN_Nhdr).

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

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

		 SHT_SHLIB	This  section  is reserved but has unspecified
				semantics.

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

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

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

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

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

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

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

		 SHF_EXECINSTR	This  section  contains	  executable   machine
				instructions.

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

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

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

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

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

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

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

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

       Various sections hold program and control information:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       st_value	 This member gives the value of the associated symbol.

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

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

		 STT_NOTYPE  The symbol's type is not defined.

		 STT_OBJECT  The symbol is associated with a data object.

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

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

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

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

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

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

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

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

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

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

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

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

       st_other	 This member defines the symbol visibility.

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

		 There are macros for extracting the visibility type:

		 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

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

       Relocation structures that do not need an addend:

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

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

       Relocation structures that need an addend:

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

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

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

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

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

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

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

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

       d_tag	 This member may have any of the following values:

		 DT_NULL     Marks end of dynamic section

		 DT_NEEDED   String table offset to name of a needed library

		 DT_PLTRELSZ Size in bytes of PLT relocation entries

		 DT_PLTGOT   Address of PLT and/or GOT

		 DT_HASH     Address of symbol hash table

		 DT_STRTAB   Address of string table

		 DT_SYMTAB   Address of symbol table

		 DT_RELA     Address of Rela relocation table

		 DT_RELASZ   Size in bytes of the Rela relocation table

		 DT_RELAENT  Size in bytes of a Rela relocation table entry

		 DT_STRSZ    Size in bytes of string table

		 DT_SYMENT   Size in bytes of a symbol table entry

		 DT_INIT     Address of the initialization function

		 DT_FINI     Address of the termination function

		 DT_SONAME   String table offset to name of shared object

		 DT_RPATH    String  table offset to library search path (dep-
			     recated)

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

		 DT_REL	     Address of Rel relocation table

		 DT_RELSZ    Size in bytes of Rel relocation table

		 DT_RELENT   Size in bytes of a Rel table entry

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

		 DT_DEBUG    Undefined use for debugging

		 DT_TEXTREL  Absence of this entry indicates that  no  reloca-
			     tion  entries  should apply to a nonwritable seg-
			     ment

		 DT_JMPREL   Address of relocation entries  associated	solely
			     with the PLT

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

		 DT_RUNPATH  String table offset to library search path

		 DT_LOPROC, DT_HIPROC
			     Values   in   the	 inclusive  range  [DT_LOPROC,
			     DT_HIPROC] are  reserved  for  processor-specific
			     semantics

       d_val	 This  member represents integer values with various interpre-
		 tations.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

		 n_name = GNU
		      Extensions used by the GNU tool chain.

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

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

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

			     o word 0: number of entries
			     o word 1: bit mask of enabled entries

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

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

		      NT_GNU_GOLD_VERSION
			     The  desc	contains  the  GNU Gold linker version
			     used.

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

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


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

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

SEE ALSO
       as(1), gdb(1), ld(1), objdump(1), readelf(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 Architec-
       ture Processor Supplement.

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-12-12				ELF(5)