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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 libraries.

       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.

       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, etc.

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

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

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

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

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

		   EI_CLASS    The fifth byte identifies the architecture  for
			       this binary:

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

		   EI_DATA     The  sixth  byte specifies the data encoding of
			       the processor-specific data in the file.	  Cur‐
			       rently 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	deter‐
			       mined by the value of this byte.	 E.g.:

			       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 identi‐
			       fied by the EI_OSABI field.  Applications  con‐
			       forming 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.  E.g.:

		   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  mem‐
		   ber 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 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 num‐
			    ber 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.

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

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

		   SHN_LOPROC	 Values greater than or	 equal	to  SHN_HIPROC
				 are  reserved	for  processor-specific seman‐
				 tics.

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

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

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

		   SHN_HIRESERVE This  value  specifies the upper bound of the
				 range of reserved indices  between  SHN_LORE‐
				 SERVE	and SHN_HIRESERVE, inclusive; the val‐
				 ues do not reference the section  header  ta‐
				 ble.	That is, the section header table does
				 not contain entries for the reserved indices.

       An executable or shared object file's program header table is an	 array
       of  structures, each describing a segment or other information the 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 Phdr struct tells what kind  of  segment
		   this	 array element describes or how to interpret the array
		   element's information.

		   PT_NULL     The array element is unused and the other  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  ini‐
			       tialized area.  The file size may not be larger
			       than the memory size.  Loadable segment entries
			       in the program header table appear in ascending
			       order, sorted on the p_vaddr member.

		   PT_DYNAMIC  The array  element  specifies  dynamic  linking
			       information.

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

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

		   PT_SHLIB    This  segment type is reserved but has unspeci‐
			       fied 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   Values  greater	than or equal to PT_HIPROC are
			       reserved for processor-specific semantics.

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

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

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

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

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

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

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

       p_flags	   This	 member holds a bit mask of flags relevant to the 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  seg‐
		   ments  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 pos‐
		   itive, integral power of  two,  and	p_vaddr	 should	 equal
		   p_offset, modulo p_align.

       A  file's section header table lets one locate all the file's sections.
       The section header table is an array of Elf32_Shdr or Elf64_Shdr 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    Values  greater  than  or  equal to SHN_HIPROC are
			    reserved for processor-specific semantics.

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

	      SHN_ABS	    This  value	 specifies  the absolute value for the
			    corresponding 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  com‐
			    mon 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,	inclu‐
			    sive.   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 information that marks the
				file in some way.

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

		 SHT_HIPROC	This value down to and including SHT_LOPROC is
				reserved for processor-specific 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.  Values of zero or one mean the section  has  no
		 alignment constraints.

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

       Various sections hold program and control information:

       .bss	 This section holds uninitialized data that contributes to the
		 program's  memory  image.  By definition, the system 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 information in the "Note Section"  format.
		 This  section	is  of	type SHT_NOTE.	No attribute types are
		 used.	 OpenBSD  native   executables	 usually   contain   a
		 .note.openbsd.ident  section  to identify themselves, for the
		 kernel to bypass any compatibility ELF binary emulation tests
		 when loading the file.

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

       .plt	 This section holds the procedure linkage table.  This section
		 is  of	 type SHT_PROGBITS.  The attributes are processor 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 sup‐
		 plied 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 sup‐
		 plied 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  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 table has no name.

       st_value	 This member gives the value of the associated symbol.

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

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

		 STT_NOTYPE  The symbol's type is not defined.

		 STT_OBJECT  The symbol is associated with a data object.

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

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

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

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

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

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

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

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

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

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

       st_other	 This member defines the symbol visibility.

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

		 There are macros for extracting the visibility type:

		 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

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

       Relocation is the process of connecting symbolic references  with  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	 relo‐
		   cation  action.   For  a relocatable file, the value is the
		   byte offset from the beginning of the section to the	 stor‐
		   age	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	 relo‐
		   cation  to apply.  Relocation types are processor specific.
		   When the text refers to  a  relocation  entry's  relocation
		   type or symbol table index, it means the result of applying
		   ELF[32|64]_R_TYPE or ELF[32|64]_R_SYM, respectively, to the
		   entry's r_info member.

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

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

		 DT_PLTGOT   Address of PLT and/or GOT

		 DT_HASH     Address of symbol hash table

		 DT_STRTAB   Address of string table

		 DT_SYMTAB   Address of symbol table

		 DT_RELA     Address of Rela relocs table

		 DT_RELASZ   Size in bytes of Rela table

		 DT_RELAENT  Size in bytes of a Rela table entry

		 DT_STRSZ    Size in bytes of string table

		 DT_SYMENT   Size in bytes of a symbol table entry

		 DT_INIT     Address of the initialization function

		 DT_FINI     Address of the termination function

		 DT_SONAME   String table offset to name of shared object

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

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

		 DT_REL	     Address of Rel relocs table

		 DT_RELSZ    Size in bytes of Rel table

		 DT_RELENT   Size in bytes of a Rel table entry

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

		 DT_DEBUG    Undefined use for debugging

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

		 DT_JMPREL   Address of reloc entries solely for the PLT

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

		 DT_RUNPATH  String table offset to library search path

		 DT_LOPROC   Start of processor-specific semantics

		 DT_HIPROC   End of processor-specific semantics

       d_val	 This member represents integer values with various  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
       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), 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 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-04-17				ELF(5)
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