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ELF(3)		       FreeBSD Library Functions Manual			ELF(3)

     elf -- API	for manipulating ELF objects

     ELF Parsing Library (libelf, -lelf)

     #include <libelf.h>

     The ELF Parsing Library (libelf, -lelf) provides functions	that allow an
     application to read and manipulate	ELF object files, and to read ar(1)
     archives.	The library allows the manipulation of ELF objects in a	byte
     ordering and word-size independent	way, allowing an application to	read
     and create	ELF objects for	32 and 64 bit architectures and	for little-
     and big-endian machines.  The library is capable of processing ELF	ob-
     jects that	use extended section numbering.

     This manual page serves to	provide	an overview of the functionality in
     the ELF library.  Further information may found in	the manual pages for
     individual	ELF(3) functions that comprise the library.

   ELF Concepts
     As	described in elf(5), ELF files contain several data structures that
     are laid out in a specific	way.  ELF files	begin with an "Executable
     Header", and may contain an optional "Program Header Table", and optional
     data in the form of ELF "sections".  A "Section Header Table" describes
     the content of the	data in	these sections.

     ELF objects have an associated "ELF class"	which denotes the natural ma-
     chine word	size for the architecture the object is	associated with.  Ob-
     jects for 32 bit architectures have an ELF	class of ELFCLASS32.  Objects
     for 64 bit	architectures have an ELF class	of ELFCLASS64.

     ELF objects also have an associated "endianness" which denotes the	endi-
     anness of the machine architecture	associated with	the object.  This may
     be	ELFDATA2LSB for	little-endian architectures and	ELFDATA2MSB for	big-
     endian architectures.

     ELF objects are also associated with an API version number.  This version
     number determines the layout of the individual components of an ELF file
     and the semantics associated with these.

   Data	Representation And Translation
     The ELF(3)	library	distinguishes between "native" representations of ELF
     data structures and their "file" representations.

     An	application would work with ELF	data in	its "native" representation,
     i.e., using the native byteorder and alignment mandated by	the processor
     the application is	running	on.  The "file"	representation of the same
     data could	use a different	byte ordering and follow different constraints
     on	object alignment than these native constraints.

     Accordingly, the ELF(3) library offers translation	facilities
     (elf32_xlatetof(3), elf32_xlatetom(3), elf64_xlatetof(3) and
     elf64_xlatetom(3))	to and from these representations.  It also provides
     higher-level APIs (gelf_xlatetof(3), gelf_xlatetom(3)) that retrieve and
     store data	from the ELF object in a class-agnostic	manner.

   Library Working Version
     Conceptually, there are three version numbers associated with an applica-
     tion using	the ELF	library	to manipulate ELF objects:
	   o   The ELF version that the	application was	compiled against.
	       This version determines the ABI expected	by the application.
	   o   The ELF version of the ELF object being manipulated by the ap-
	       plication through the ELF library.
	   o   The ELF version (or set of versions) supported by the ELF li-
	       brary itself.

     In	order to facilitate working with ELF objects of	differing versions,
     the ELF library requires the application to call the elf_version()	func-
     tion before invoking many of its operations, in order to inform the li-
     brary of the application's	desired	working	version.

     In	the current implementation, all	three versions have to be EV_CURRENT.

   Namespace use
     The ELF library uses the following	prefixes:

     elf_     Used for class-independent functions.

     elf32_   Used for functions working with 32 bit ELF objects.

     elf64_   Used for functions working with 64 bit ELF objects.

     Elf_     Used for class-independent data types.

     ELF_C_   Used for command values used in a	few functions.	These symbols
	      are defined as members of	the Elf_Cmd enumeration.

     ELF_E_   Used for error numbers.

     ELF_F_   Used for flags.

     ELF_K_   These constants define the kind of file associated with an ELF
	      descriptor.  See elf_kind(3).  The symbols are defined by	the
	      Elf_Kind enumeration.

     ELF_T_   These values are defined by the Elf_Type enumeration, and	denote
	      the types	of ELF data structures that can	be present in an ELF

     In	addition, the library uses symbols with	prefixes _ELF and _libelf for
     its internal use.

     Applications communicate with the library using descriptors.  These are:

     Elf       An Elf descriptor represents an ELF object or an	ar(1) archive.
	       It is allocated using one of the	elf_begin() or elf_memory()
	       functions.  An Elf descriptor can be used to read and write
	       data to an ELF file.  An	Elf descriptor can be associated with
	       zero or more Elf_Scn section descriptors.

	       Given an	ELF descriptor,	the application	may retrieve the ELF
	       object's	class-dependent	"Executable Header" structures using
	       the elf32_getehdr() or elf64_getehdr() functions.  A new	Ehdr
	       structure may be	allocated using	the elf64_newehdr() or
	       elf64_newehdr() functions.

	       The "Program Header Table" associated with an ELF descriptor
	       may be allocated	using the elf32_getphdr() or elf64_getphdr()
	       functions.  A new program header	table may be allocated or an
	       existing	table resized using the	elf32_newphdr()	or
	       elf64_newphdr() functions.

	       The Elf structure is opaque and has no members visible to the

     Elf_Data  An Elf_Data data	structure describes an individual chunk	of a
	       ELF file	as represented in memory.  It has the following	appli-
	       cation-visible members:
	       uint64_t	d_align	       The in-file alignment of	the data buf-
				       fer within its containing ELF section.
				       This value must be non-zero and a power
				       of two.
	       void *d_buf	       A pointer to data in memory.
	       uint64_t	d_off	       The offset within the containing	sec-
				       tion where this descriptor's data would
				       be placed.  This	field will be computed
				       by the library unless the application
				       requests	full control of	the ELF	ob-
				       ject's layout.
	       uint64_t	d_size	       The number of bytes of data in this de-
	       Elf_Type	d_type	       The ELF type (see below)	of the data in
				       this descriptor.
	       unsigned	int d_version  The operating version for the data in
				       this buffer.

	       Elf_Data	descriptors are	usually	used in	conjunction with
	       Elf_Scn descriptors.

     Elf_Scn   Elf_Scn descriptors represent sections in an ELF	object.	 These
	       descriptors are opaque and contain no application modifiable

	       The Elf_Scn descriptor for a specific section in	an ELF object
	       can be retrieved	using the elf_getscn() function.  The sections
	       contained in an ELF object can be traversed using the
	       elf_nextscn() function.	New sections are allocated using the
	       elf_newscn() function.

	       The Elf_Data descriptors	associated with	a given	section	can be
	       retrieved using the elf_getdata() function.  New	data descrip-
	       tors can	be added to a section descriptor using the
	       elf_newdata() function.	The untranslated "file"	representation
	       of data in a section can	be retrieved using the elf_rawdata()

   Supported Elf Types
     The following ELF datatypes are supported by the library.

     ELF_T_ADDR	    Machine addresses.
     ELF_T_BYTE	    Byte data.	The library will not attempt to	translate byte
     ELF_T_CAP	    Software and hardware capability records.
     ELF_T_DYN	    Records used in a section of type SHT_DYNAMIC.
     ELF_T_EHDR	    ELF	executable header.
     ELF_T_GNUHASH  GNU-style hash tables.
     ELF_T_HALF	    16-bit unsigned words.
     ELF_T_LWORD    64 bit unsigned words.
     ELF_T_MOVE	    ELF	Move records.
     ELF_T_NOTE	    ELF	Note structures.
     ELF_T_OFF	    File offsets.
     ELF_T_PHDR	    ELF	program	header table entries.
     ELF_T_REL	    ELF	relocation entries.
     ELF_T_RELA	    ELF	relocation entries with	addends.
     ELF_T_SHDR	    ELF	section	header entries.
     ELF_T_SWORD    Signed 32-bit words.
     ELF_T_SXWORD   Signed 64-bit words.
     ELF_T_SYMINFO  ELF	symbol information.
     ELF_T_SYM	    ELF	symbol table entries.
     ELF_T_VDEF	    Symbol version definition records.
     ELF_T_VNEED    Symbol version requirement records.
     ELF_T_WORD	    Unsigned 32-bit words.
     ELF_T_XWORD    Unsigned 64-bit words.

     The symbol	ELF_T_NUM denotes the number of	Elf types known	to the li-

     The following table shows the mapping between ELF section types defined
     in	elf(5) and the types supported by the library.

     Section Type	  Library Type	   Description
     SHT_DYNAMIC	  ELF_T_DYN	   `.dynamic' section entries.
     SHT_DYNSYM		  ELF_T_SYM	   Symbols for dynamic linking.
     SHT_FINI_ARRAY	  ELF_T_ADDR	   Termination function	pointers.
     SHT_GNU_HASH	  ELF_T_GNUHASH	   GNU hash sections.
     SHT_GNU_LIBLIST	  ELF_T_WORD	   List	of libraries to	be pre-linked.
     SHT_GNU_verdef	  ELF_T_VDEF	   Symbol version definitions.
     SHT_GNU_verneed	  ELF_T_VNEED	   Symbol versioning requirements.
     SHT_GNU_versym	  ELF_T_HALF	   Version symbols.
     SHT_GROUP		  ELF_T_WORD	   Section group marker.
     SHT_HASH		  ELF_T_HASH	   Symbol hashes.
     SHT_INIT_ARRAY	  ELF_T_ADDR	   Initialization function pointers.
     SHT_NOBITS		  ELF_T_BYTE	   Empty sections.  See	elf(5).
     SHT_NOTE		  ELF_T_NOTE	   ELF note records.
     SHT_PREINIT_ARRAY	  ELF_T_ADDR	   Pre-initialization function
     SHT_PROGBITS	  ELF_T_BYTE	   Machine code.
     SHT_REL		  ELF_T_REL	   ELF relocation records.
     SHT_RELA		  ELF_T_RELA	   Relocation records with addends.
     SHT_STRTAB		  ELF_T_BYTE	   String tables.
     SHT_SYMTAB		  ELF_T_SYM	   Symbol tables.
     SHT_SYMTAB_SHNDX	  ELF_T_WORD	   Used	with extended section
     SHT_SUNW_dof	  ELF_T_BYTE	   Used	by dtrace(1).
     SHT_SUNW_move	  ELF_T_MOVE	   ELF move records.
     SHT_SUNW_syminfo	  ELF_T_SYMINFO	   Additional symbol flags.
     SHT_SUNW_verdef	  ELF_T_VDEF	   Same	as SHT_GNU_verdef.
     SHT_SUNW_verneed	  ELF_T_VNEED	   Same	as SHT_GNU_verneed.
     SHT_SUNW_versym	  ELF_T_HALF	   Same	as SHT_GNU_versym.

     Section types in the range	[SHT_LOOS, SHT_HIUSER] are otherwise consid-
     ered to be	of type	ELF_T_BYTE.

   Functional Grouping
     This section contains a brief overview of the available functionality in
     the ELF library.  Each function listed here is described further in its
     own manual	page.

     Archive Access
		     Retrieve the archive symbol table.
		     Retrieve the archive header for an	object.
		     Retrieve the offset of a member inside an archive.
		     Iterate through an	ar(1) archive.
		     Random access inside an ar(1) archive.

     Data Structures
		     Retrieve translated data for an ELF section.
		     Retrieve the section descriptor for a named section.
		     Retrieve the index	for a section.
		     Add a new Elf_Data	descriptor to an ELF section.
		     Add a new section descriptor to an	ELF descriptor.
		     Iterate through the sections in an	ELF object.
		     Retrieve untranslated data	for an ELF section.
		     Return a pointer to the untranslated file contents	for an
		     ELF object.
	     elf32_getehdr(), elf64_getehdr()
		     Retrieve the Executable Header in an ELF object.
	     elf32_getphdr(), elf64_getphdr()
		     Retrieve the Program Header Table in an ELF object.
	     elf32_getshdr(), elf64_getshdr()
		     Retrieve the ELF section header associated	with an
		     Elf_Scn descriptor.
	     elf32_newehdr(), elf64_newehdr()
		     Allocate an Executable Header in an ELF object.
	     elf32_newphdr(), elf64_newphdr()
		     Allocate or resize	the Program Header Table in an ELF ob-

     Data Translation
	     elf32_xlatetof(), elf64_xlatetof()
		     Translate an ELF data structure from its native represen-
		     tation to its file	representation.
	     elf32_xlatetom(), elf64_xlatetom()
		     Translate an ELF data structure from its file representa-
		     tion to a native representation.

     Error Reporting
		     Retrieve the current error.
		     Retrieve a	human readable description of the current er-

		     Opens an ar(1) archive or ELF object given	a file de-
		     Close an ELF descriptor and release all its resources.
		     Opens an ar(1) archive or ELF object present in a memory
		     Sets the operating	version.

     IO	Control
	     elf_cntl()		Manage the association between and ELF de-
				scriptor and its underlying file.
	     elf_flagdata()	Mark an	Elf_Data descriptor as dirty.
	     elf_flagehdr()	Mark the ELF Executable	Header in an ELF de-
				scriptor as dirty.
	     elf_flagphdr()	Mark the ELF Program Header Table in an	ELF
				descriptor as dirty.
	     elf_flagscn()	Mark an	Elf_Scn	descriptor as dirty.
	     elf_flagshdr()	Mark an	ELF Section Header as dirty.
	     elf_setshstrndx()	Set the	index of the section name string table
				for the	ELF object.
	     elf_update()	Recompute ELF object layout and	optionally
				write the modified object back to the underly-
				ing file.

	     elf32_checksum(), elf64_checkum()
				Compute	checksum of an ELF object.
	     elf_getident()	Retrieve the identification bytes for an ELF
	     elf_getphdrnum()	Retrieve the number of program headers in an
				ELF object.
	     elf_getshdrnum()	Retrieve the number of sections	in an ELF ob-
				Retrieve the section index of the section name
				string table in	an ELF object.
	     elf_hash()		Compute	the ELF	hash value of a	string.
	     elf_kind()		Query the kind of object associated with an
				ELF descriptor.
	     elf32_fsize(), elf64_fsize()
				Return the size	of the file representation of
				an ELF type.

   Controlling ELF Object Layout
     In	the usual mode of operation, library will compute section offsets and
     alignments	based on the contents of an ELF	descriptor's sections without
     need for further intervention by the application.

     However, if the application wishes	to take	complete charge	of the layout
     of	the ELF	file, it may set the ELF_F_LAYOUT flag on an ELF descriptor
     using elf_flagelf(3), following which the library will use	the data off-
     sets and alignments specified by the application when laying out the
     file.  Application	control	of file	layout is described further in the
     elf_update(3) manual page.

     Gaps in between sections will be filled with the fill character set by
     function elf_fill().

   Error Handling
     In	case an	error is encountered, these library functions set an internal
     error number and signal the presence of the error by returning an special
     return value.  The	application can	check the current error	number by
     calling elf_errno(3).  A human readable description of the	recorded error
     is	available by calling elf_errmsg(3).

   Memory Management Rules
     The library keeps track of	all Elf_Scn and	Elf_Data descriptors associ-
     ated with an ELF descriptor and recovers them when	the descriptor is
     closed using elf_end(3).  Thus the	application must not call free(3) on
     data structures allocated by the ELF library.

     Conversely	the library will not free data that it has not allocated.  As
     an	example, an application	may call elf_newdata(3)	to allocate a new
     Elf_Data descriptor and can set the d_off member of the descriptor	to
     point to a	region of memory allocated using malloc(3).  It	is the appli-
     cations responsibility to free this arena,	though the library will	re-
     claim the space used by the Elf_Data descriptor itself.

     gelf(3), ar(5), elf(5)

     The original elf API was developed	for AT&T System	V UNIX.	 The current
     implementation of the API appeared	in FreeBSD 7.0.

     The ELF library was written by Joseph Koshy <>.

FreeBSD	13.0			 June 12, 2019			  FreeBSD 13.0


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