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ELF_BEGIN(3)		 BSD Library Functions Manual		  ELF_BEGIN(3)

NAME
     elf_begin -- open an ELF file or ar(1) archive

LIBRARY
     ELF Access	Library	(libelf, -lelf)

SYNOPSIS
     #include <libelf.h>

     Elf *
     elf_begin(int fd, Elf_Cmd cmd, Elf	*elf);

DESCRIPTION
     Function elf_begin() is used to open ELF files and	ar(1) archives for
     further processing	by other APIs in the elf(3) library.  It is also used
     to	access individual ELF members of an ar(1) archive in combination with
     the elf_next(3) and elf_rand(3) APIs.

     Argument fd is an open file descriptor returned from an open(2) system
     call.  Function elf_begin() uses argument fd for reading or writing de-
     pending on	the value of argument cmd.  Argument elf is primarily used for
     iterating through archives.

     The argument cmd can have the following values:

     ELF_C_NULL	  Causes elf_begin() to	return NULL.  Arguments	fd and elf are
		  ignored, and no additional error is signalled.

     ELF_C_READ	  This value is	to be when the application wishes to examine
		  (but not modify) the contents	of the file specified by the
		  arguments fd and elf.	 It can	be used	for both ar(1) ar-
		  chives and for ELF objects.

		  If argument elf is NULL, the library will allocate a new ELF
		  descriptor for the file being	processed.  The	argument fd
		  should have been opened for reading.

		  If argument elf is not NULL, and references a	regular	ELF
		  file previously opened with elf_begin(), then	the activation
		  count	for the	descriptor referenced by argument elf is in-
		  cremented.  The value	in argument fd should match that used
		  to open the descriptor argument elf.

		  If argument elf is not NULL, and references a	descriptor for
		  an ar(1) archive opened earlier with elf_begin(), a descrip-
		  tor for an element in	the archive is returned	as described
		  in the section Processing ar(1) archives below.  The value
		  for argument fd should match that used to open the archive
		  earlier.

		  If argument elf is not NULL, and references an ar(1) archive
		  opened earlier with elf_memory(), then the value of the ar-
		  gument fd is ignored.

     ELF_C_RDWR	  This command is used to prepare an ELF file for reading and
		  writing.  This command is not	supported for ar(1) archives.

		  Argument fd should have been opened for reading and writing.
		  If argument elf is NULL, the library will allocate a new ELF
		  descriptor for the file being	processed.  If the argument
		  elf is non-null, it should point to a	descriptor previously
		  allocated with elf_begin() with the same values for argu-
		  ments	fd and cmd; in this case the library will increment
		  the activation count for descriptor elf and return the same
		  descriptor.

		  Changes to the in-memory image of the	ELF file may be	writ-
		  ten back to disk using the elf_update(3) function.

     ELF_C_WRITE  This command is used when the	application wishes to create a
		  new ELF file.	 Argument fd should have been opened for writ-
		  ing.	Argument elf is	ignored, and the previous contents of
		  file referenced by argument fd are overwritten.

   Processing ar(1) archives
     An	ar(1) archive may be opened in read mode (with argument	cmd set	to
     ELF_C_READ) using elf_begin() or elf_memory().  The returned ELF descrip-
     tor can be	passed into to subsequent calls	to elf_begin() to access indi-
     vidual members of the archive.

     Random access within an opened archive is possible	using the elf_next(3)
     and elf_rand(3) functions.

     The symbol	table of the archive may be retrieved using elf_getarsym(3).

RETURN VALUES
     The function returns a pointer to a ELF descriptor	if successful, or NULL
     if	an error occurred.

EXAMPLES
     To	iterate	through	the members of an ar(1)	archive, use:

	   Elf_Cmd c;
	   Elf *ar_e, *elf_e;
	   ...
	   c = ELF_C_READ;
	   if ((ar_e = elf_begin(fd, c,	(Elf *)	0)) == 0) {
		   ... handle error in opening the archive ...
	   }
	   while ((elf_e = elf_begin(fd, c, ar_e)) != 0) {
		   ... process member referenced by elf_e here ...
		   c = elf_next(elf_e);
		   elf_end(elf_e);
	   }

     To	create a new ELF file, use:

	   int fd;
	   Elf *e;
	   ...
	   if ((fd = open("filename", O_RDWR|O_TRUNC|O_CREAT, 0666)) < 0) {
		   ... handle the error	from open(2) ...
	   }
	   if ((e = elf_begin(fd, ELF_C_WRITE, (Elf *) 0)) == 0) {
		   ... handle the error	from elf_begin() ...
	   }
	   ... create the ELF image using other	elf(3) APIs ...
	   elf_update(e, ELF_C_WRITE);
	   elf_end(e);

ERRORS
     Function elf_begin() can fail with	the following errors:

     [ELF_E_ARCHIVE]   The archive denoted by argument elf could not be
		       parsed.

     [ELF_E_ARGUMENT]  The value in argument cmd was unrecognized.

     [ELF_E_ARGUMENT]  A non-null value	for argument elf was specified when
		       cmd was set to ELF_C_RDWR.

     [ELF_E_ARGUMENT]  The value of argument fd	differs	from the one the ELF
		       descriptor elf was created with.

     [ELF_E_ARGUMENT]  Argument	cmd differs from the value specified when ELF
		       descriptor elf was created.

     [ELF_E_ARGUMENT]  An ar(1)	archive	was opened with	cmd set	to ELF_C_RDWR.

     [ELF_E_ARGUMENT]  The file	referenced by argument fd was empty.

     [ELF_E_ARGUMENT]  The underlying file for argument	fd was of an unsup-
		       ported type.

     [ELF_E_IO]	       The file	descriptor in argument fd was invalid.

     [ELF_E_IO]	       The file	descriptor in argument fd could	not be read or
		       written to.

     [ELF_E_RESOURCE]  An out of memory	condition was encountered.

     [ELF_E_SEQUENCE]  Function	elf_begin() was	called before a	working	ver-
		       sion was	established with elf_version(3).

     [ELF_E_VERSION]   The ELF object referenced by argument fd	was of an un-
		       supported ELF version.

SEE ALSO
     elf(3), elf_end(3), elf_errno(3), elf_memory(3), elf_next(3),
     elf_rand(3), elf_update(3), gelf(3)

BSD			       December	11, 2011			   BSD

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO

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