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ELF(3)			 BSD Library Functions Manual			ELF(3)

NAME
     elf — API for manipulating ELF objects

LIBRARY
     library “libelf”

SYNOPSIS
     #include <libelf.h>

DESCRIPTION
     The library “libelf” 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 objects that use extended sec‐
     tion 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
     machine word size for the architecture the object is associated with.
     Objects 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 and also provides
     higher-level APIs that retrieve and store data from the ELF object in a
     transparent manner.

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

   Descriptors
     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
	       application.

     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 a power of two.
	       uint64_t d_off	       The offset with the containing section
				       where this descriptors data would be
				       placed.	This field will be computed by
				       the library unless the application
				       requests full control of the ELF
				       object's layout.
	       uint64_t d_size	       The number of bytes of data in this
				       descriptor.
	       void *d_buf	       A pointer to data in memory.
	       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 associated with Elf_Scn
	       descriptors.  Existing data descriptors associated with an ELF
	       section may be structures are retrieved using the elf_getdata()
	       function.  The elf_newdata() function may be used to attach new
	       data descriptors to an ELF section.

     Elf_Scn   Elf_Scn descriptors represent a section in an ELF object.

	       They are retrieved using the elf_getscn() function.  An appli‐
	       cation may iterate through the existing sections of an ELF
	       object using the elf_nextscn() function.	 New sections may be
	       allocated using the elf_newscn() function.

	       The Elf_Scn descriptor is opaque and contains no application
	       modifiable fields.

   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
		    data.
     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_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
     library.

     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_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
					   pointers.
     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
					   numbering.
     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_SUNW_move	  ELF_T_MOVE	   ELF move records.
     SHT_SUNW_syminfo	  ELF_T_SYMINFO	   Additional symbol flags.

   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
	     elf_getarsym()    Retrieve the archive symbol table.
	     elf_getarhdr()    Retrieve the archive header for an object.
	     elf_getbase()     Retrieve the offset of a member inside an ar‐
			       chive.
	     elf_next()	       Iterate through an ar(1) archive.
	     elf_rand()	       Random access inside an ar(1) archive.

     Data Structures
	     elf_getdata()     Retrieve translated data for an ELF section.
	     elf_getscn()      Retrieve the section descriptor for a named
			       section.
	     elf_ndxscn()      Retrieve the index for a section.
	     elf_newdata()     Add a new Elf_Data descriptor to an ELF sec‐
			       tion.
	     elf_newscn()      Add a new section descriptor to an ELF descrip‐
			       tor.
	     elf_nextscn()     Iterate through the sections in an ELF object.
	     elf_rawdata()     Retrieve untranslated data for an ELF sectino.
	     elf_rawfile()     Return a pointer to the untranslated file con‐
			       tents 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 object.

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

     Error Reporting
	     elf_errno()       Retrieve the current error.
	     elf_errmsg()      Retrieve a human readable description of the
			       current error.

     Initialization
	     elf_begin()       Opens an ar(1) archive or ELF object given a
			       file descriptor.
	     elf_end()	       Close an ELF descriptor and release all its
			       resources.
	     elf_memory()      Opens an ar(1) archive or ELF object present in
			       a memory arena.
	     elf_version()     Sets the operating version.

     IO Control
	     elf_cntl()		Manage the association between and ELF
				descriptor and its underlying file.
	     elf_flagdata()	Mark an Elf_Data descriptor as dirty.
	     elf_flagehdr()	Mark the ELF Executable Header in an ELF
				descriptor 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.

     Queries
	     elf32_checksum(), elf64_checkum()
				Compute checksum of an ELF object.
	     elf_getident()	Retrieve the identification bytes for an ELF
				object.
	     elf_getshnum()	Retrieve the number of sections in an ELF
				object.
	     elf_getshstrndx()	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
     reclaim the space used by the Elf_Data descriptor itself.

SEE ALSO
     gelf(3), elf(5)

HISTORY
     The original ELF(3) API was developed for Unix System V.  The current
     implementation of the ELF(3) API appeared in FreeBSD 7.0.

AUTHORS
     The ELF library was written by Joseph Koshy ⟨jkoshy@FreeBSD.org⟩.

BSD			       October 21, 2007				   BSD
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