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

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

     ELF Access	Library	(libelf, -lelf)

     #include <libelf.h>

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

     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 argu-
		  ment fd.  It can be used for both ar(1) archives and for
		  regular ELF files.

		  Argument fd should have been opened for reading.  If argu-
		  ment elf is NULL, the	library	will allocate a	new ELF	de-
		  scriptor for the file	being processed.  If argument elf is
		  not NULL, and	references a regular ELF file previously
		  opened with elf_begin(), then	the activation count for elf
		  is incremented.  If argument elf is not NULL,	and references
		  a descriptor for an ar(1) archive opened earlier with
		  elf_begin(), a descriptor for	an element in the archive is
		  returned as described	in the section Processing ar(1)
		  archives below.

     ELF_C_RDWR	  The command is used to prepare an ELF	file for reading and
		  writing.  It is not supported	for 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
		  are written back to disk using the elf_update(3) function.

		  This command is not valid for	ar(1) archives.

     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).

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

     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);

     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);

     Function elf_begin() can fail with	the following errors:

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

     [ELF_E_ARGUMENT]  An unrecognized value was specified in argument cmd.

     [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_IO]	       Function	elf_begin() was	unable to truncate a file
		       opened for writing using	ELF_C_WRITE.

     [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(3), elf_end(3), elf_errno(3), elf_memory(3), elf_next(3),
     elf_rand(3), elf_update(3), gelf(3)

BSD				 June 20, 2010				   BSD


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