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ELF(5)

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NAME    [Toc]    [Back]

     elf -- format of ELF executable binary files

SYNOPSIS    [Toc]    [Back]

     #include <elf.h>

DESCRIPTION    [Toc]    [Back]

     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.

     Applications which wish to process ELF binary files for their native
     architecture only should include <elf.h> in their source code.  These
     applications should need to refer to all the types and structures by
     their generic names ``Elf_xxx'' and to the macros by ``ELF_xxx''.	Applications
 written this way can be compiled on any architecture, regardless
     whether the host is 32-bit or 64-bit.

     Should an application need to process ELF files of an unknown architecture
 then the application needs to include both <sys/elf32.h> and
     <sys/elf64.h> instead of <elf.h>.	Furthermore, all types and structures
     need to be identified by either ``Elf32_xxx'' or ``Elf64_xxx''.  The
     macros need to be identified by ``ELF32_xxx'' or ``ELF64_xxx''.

     Whatever the system's architecture is, it will always include
     <sys/elf_common.h> as well as <sys/elf_generic.h>.

     These header files describe the above mentioned headers as C structures
     and also include structures for dynamic sections, relocation sections and
     symbol tables.

     The following types are being used for 32-bit architectures:

	   Elf32_Addr	   Unsigned program address
	   Elf32_Half	   Unsigned halfword field
	   Elf32_Off	   Unsigned file offset
	   Elf32_Sword	   Signed large integer
	   Elf32_Word	   Field or unsigned large integer
	   Elf32_Size	   Unsigned object size

     For 64-bit architectures we have the following types:

	   Elf64_Addr	   Unsigned program address
	   Elf64_Half	   Unsigned halfword field
	   Elf64_Off	   Unsigned file offset
	   Elf64_Sword	   Signed large integer
	   Elf64_Word	   Field or unsigned large integer
	   Elf64_Size	   Unsigned object size
	   Elf64_Quarter   Unsigned quarterword field

     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:

	   typedef struct {
		   unsigned char   e_ident[EI_NIDENT];
		   Elf32_Half	   e_type;
		   Elf32_Half	   e_machine;
		   Elf32_Word	   e_version;
		   Elf32_Addr	   e_entry;
		   Elf32_Off	   e_phoff;
		   Elf32_Off	   e_shoff;
		   Elf32_Word	   e_flags;
		   Elf32_Half	   e_ehsize;
		   Elf32_Half	   e_phentsize;
		   Elf32_Half	   e_phnum;
		   Elf32_Half	   e_shentsize;
		   Elf32_Half	   e_shnum;
		   Elf32_Half	   e_shstrndx;
	   } Elf32_Ehdr;

	   typedef struct {
		   unsigned char   e_ident[EI_NIDENT];
		   Elf64_Quarter   e_type;
		   Elf64_Quarter   e_machine;
		   Elf64_Half	   e_version;
		   Elf64_Addr	   e_entry;
		   Elf64_Off	   e_phoff;
		   Elf64_Off	   e_shoff;
		   Elf64_Half	   e_flags;
		   Elf64_Quarter   e_ehsize;
		   Elf64_Quarter   e_phentsize;
		   Elf64_Quarter   e_phnum;
		   Elf64_Quarter   e_shentsize;
		   Elf64_Quarter   e_shnum;
		   Elf64_Quarter   e_shstrndx;
	   } Elf64_Ehdr;

     The fields have the following meanings:

	   e_ident	This array of bytes specifies 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.
			EI_MAG1        The second byte of the magic number.
				       It must be filled with ELFMAG1.
			EI_MAG2        The third byte of the magic number.  It
				       must be filled with ELFMAG2.
			EI_MAG3        The fourth byte of the magic number.
				       It must be filled with ELFMAG3.
			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, littleendian.

				       ELFDATA2MSB  Two's complement, bigendian.

			EI_VERSION     The version number of the ELF specification:


				       EV_NONE	   Invalid version.
				       EV_CURRENT  Current version.
			EI_OSABI       This 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.  The following
 values are currently defined:

				       ELFOSABI_SYSV	    UNIX System V ABI.
				       ELFOSABI_HPUX	    HP-UX operating
							    system ABI.
				       ELFOSABI_NETBSD	    NetBSD operating
							    system ABI.
				       ELFOSABI_LINUX	    GNU/Linux operating
 system ABI.
				       ELFOSABI_HURD	    GNU/Hurd operating
							    system ABI.
				       ELFOSABI_86OPEN	    86Open Common IA32
							    ABI.
				       ELFOSABI_SOLARIS     Solaris operating
							    system ABI.
				       ELFOSABI_MONTEREY    Monterey project
							    ABI.
				       ELFOSABI_IRIX	    IRIX operating
							    system ABI.
				       ELFOSABI_FREEBSD     FreeBSD operating
							    system ABI.
				       ELFOSABI_TRU64	    TRU64 UNIX operating
 system ABI.
				       ELFOSABI_ARM	    ARM architecture
							    ABI.
				       ELFOSABI_STANDALONE  Standalone (embedded)
 ABI.
			EI_ABIVERSION  This 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_BRAND       Start of architecture identification.
			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:

			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_486		Intel 80486.
			EM_860		Intel 80860.
			EM_MIPS 	MIPS RS3000 (big-endian only).
			EM_MIPS_RS4_BE	MIPS RS4000 (big-endian only).
			EM_SPARC64	SPARC v9 64-bit unofficial.
			EM_PARISC	HPPA.
			EM_PPC		PowerPC.
			EM_ALPHA	Compaq [DEC] Alpha.

	   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.
	   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.
	   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.

			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 indexes.
			SHN_LOPROC     This value up to and including
				       SHN_HIPROC are reserved for processorspecific
 semantics.
			SHN_HIPROC     This value down to and including
				       SHN_LOPROC are reserved for processorspecific
 semantics.
			SHN_ABS        This value specifies absolute values
				       for the corresponding reference.  For
				       example, symbols defined relative to
				       section number SHN_ABS have absolute
				       values and are not affected by relocation.

			SHN_COMMON     Symbols defined relative to this section
 are common symbols, such as Fortran
 COMMON or unallocated C external
				       variables.
			SHN_HIRESERVE  This value specifies the upper bound of
				       the range of the range of reserved
				       indices between SHN_LORESERVE and
				       SHN_HIRESERVE, inclusive; the values do
				       not reference the section header table.
				       That is, the section header table does
				       not contain entries for the reserved
				       indices.

     An executable or shared object file's program header table is an array of
     structures, each describing a segment or other information the system
     needs to prepare the program for 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.  As with the
     Elf executable header, the program header also has different versions
     depending on the architecture:

	   typedef struct {
		   Elf32_Word	   p_type;
		   Elf32_Off	   p_offset;
		   Elf32_Addr	   p_vaddr;
		   Elf32_Addr	   p_paddr;
		   Elf32_Size	   p_filesz;
		   Elf32_Size	   p_memsz;
		   Elf32_Word	   p_flags;
		   Elf32_Size	   p_align;
	   } Elf32_Phdr;

	   typedef struct {
		   Elf64_Half	   p_type;
		   Elf64_Half	   p_flags;
		   Elf64_Off	   p_offset;
		   Elf64_Addr	   p_vaddr;
		   Elf64_Addr	   p_paddr;
		   Elf64_Size	   p_filesz;
		   Elf64_Size	   p_memsz;
		   Elf64_Size	   p_align;
	   } Elf64_Phdr;

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

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

		     PT_NULL	 The array element is unused and the other
				 members' values are undefined.  This lets the
				 program header have ignored entries.
		     PT_LOAD	 The array element specifies a loadable segment,
 described by p_filesz and p_memsz.  The
				 bytes from the file are mapped to the beginning
 of the memory segment.  If the segment's
				 memory size (p_memsz) is larger than the file
				 size (p_filesz), the ``extra'' bytes are
				 defined to hold the value 0 and to follow the
				 segment's 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 path name to invoke
				 as an interpreter.  This segment type is
				 meaningful only for executable files (though
				 it may occur for shared objects). However it
				 may not occur more than once in a file.  If
				 it is present it must precede any loadable
				 segment entry.
		     PT_NOTE	 The array element specifies the location and
				 size for auxiliary information.
		     PT_SHLIB	 This segment type is reserved but has unspecified
 semantics.  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 only occur 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	 This value up to and including PT_HIPROC are
				 reserved for processor-specific semantics.
		     PT_HIPROC	 This value down to and including PT_LOPROC
				 are reserved for processor-specific semantics.


	   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 flags relevant to the segment:

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

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

     An 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.  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     This value up to and including SHN_HIPROC are reserved for
		    processor-specific semantics.
     SHN_HIPROC     This value down to and including SHN_LOPROC are reserved
		    for processor-specific semantics.
     SHN_ABS	    This value specifies absolute values for the corresponding
		    reference.	For example, symbols defined relative to section
 number SHN_ABS have absolute values and are not
		    affected by relocation.
     SHN_COMMON     Symbols defined relative to this section are common symbols,
 such as FORTRAN COMMON or unallocated C external
		    variables.
     SHN_HIRESERVE  This value specifies the upper bound of the range of
		    reserved indices.  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 {
		   Elf32_Word	   sh_name;
		   Elf32_Word	   sh_type;
		   Elf32_Word	   sh_flags;
		   Elf32_Addr	   sh_addr;
		   Elf32_Off	   sh_offset;
		   Elf32_Size	   sh_size;
		   Elf32_Word	   sh_link;
		   Elf32_Word	   sh_info;
		   Elf32_Size	   sh_addralign;
		   Elf32_Size	   sh_entsize;
	   } Elf32_Shdr;

	   typedef struct {
		   Elf64_Half	   sh_name;
		   Elf64_Half	   sh_type;
		   Elf64_Size	   sh_flags;
		   Elf64_Addr	   sh_addr;
		   Elf64_Off	   sh_offset;
		   Elf64_Size	   sh_size;
		   Elf64_Half	   sh_link;
		   Elf64_Half	   sh_info;
		   Elf64_Size	   sh_addralign;
		   Elf64_Size	   sh_entsize;
	   } Elf64_Shdr;

     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  The 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 SHN_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.  All
				 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
				 SHN_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 SHN_SYMTAB section.
		   SHT_LOPROC	 This value up to and including SHT_HIPROC are
				 reserved for processor-specific semantics.
		   SHT_HIPROC	 This value down to and including SHT_LOPROC
				 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	  The 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  The section contains executable machine
				  instructions.
		   SHF_MASKPROC   All bits included in this mask are reserved
				  for processor-specific semantics.

     sh_addr	   If the section will appear 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
		   non-zero 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.  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 initializes
		the data with zeros when the program begins to run.  This section
 is of type SHT_NOBITS.  The attributes 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.
     .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.
     .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.
     .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
		described below.  This section is of type SHT_NOTE.  No
		attribute types are used.
     .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 non-writable segment in the process image.  This section
		is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.
     .rodata1	This section hold read-only data that typically contributes to
		a non-writable 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.
     .jcr	This section holds information about Java classes that must be
		registered.
     .eh_frame	This section holds information used for C++ exception-handling.


     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 character.  Similarly, a string table's last byte is defined to hold
     a null character, 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 {
		   Elf32_Word	   st_name;
		   Elf32_Addr	   st_value;
		   Elf32_Size	   st_size;
		   unsigned char   st_info;
		   unsigned char   st_other;
		   Elf32_Half	   st_shndx;
	   } Elf32_Sym;

	   typedef struct {
		   Elf64_Half	   st_name;
		   unsigned char   st_info;
		   unsigned char   st_other;
		   Elf64_Quarter   st_shndx;
		   Elf64_Addr	   st_value;
		   Elf64_Size	   st_size;
	   } Elf64_Sym;

     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 non-zero, 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 are
			    reserved for processor-specific semantics.
	       STT_HIPROC   This value down to and including STT_LOPROC 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 file without
			   interfering with each other.
	       STB_GLOBAL  Global symbols are visible to all object files
			   being combined.  One file's definition of a global
			   symbol will satisfy another file's undefined reference
 to the same symbol.
	       STB_WEAK    Weak symbols resemble global symbols, but their
			   definitions have lower precedence.
	       STB_LOPROC  This value up to and including STB_HIPROC are
			   reserved for processor-specific semantics.
	       STB_HIPROC  This value down to and including STB_LOPROC are
			   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.
			   ELF64_ST_TYPE(info)	      or ELF32_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 currently holds zero and has no defined meaning.
     st_shndx  Every symbol table entry is ``defined'' in relation to some
	       section.  This member holds the relevant section header table
	       index.

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

     Relocation structures that do not need an addend:

	   typedef struct {
		   Elf32_Addr	   r_offset;
		   Elf32_Word	   r_info;
	   } Elf32_Rel;

	   typedef struct {
		   Elf64_Addr	   r_offset;
		   Elf64_Size	   r_info;
	   } Elf64_Rel;

     Relocation structures that need an addend:

	   typedef struct {
		   Elf32_Addr	   r_offset;
		   Elf32_Word	   r_info;
		   Elf32_Sword	   r_addend;
	   } Elf32_Rela;

	   typedef struct {
		   Elf64_Addr	   r_offset;
		   Elf64_Size	   r_info;
		   Elf64_Off	   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.

SEE ALSO    [Toc]    [Back]

      
      
     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).

HISTORY    [Toc]    [Back]

     The ELF header files made their appearance in FreeBSD 2.2.6.  ELF in
     itself first appeared in AT&T System V UNIX.  The ELF format is an
     adopted standard.

AUTHORS    [Toc]    [Back]

     This manual page was written by Jeroen Ruigrok van der Werven
     <asmodai@FreeBSD.org> with inspiration from BSDi's BSD/OS elf(5) manpage.


FreeBSD 5.2.1			 July 31, 1999			 FreeBSD 5.2.1
[ Back ]
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