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ELF_BEGIN(3)               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
     depending 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)
                  archives 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
                  incremented.  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
                  descriptor 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
                  argument 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
                  arguments 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
                  written 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
                  writing.  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
     descriptor can be passed into to subsequent calls to elf_begin() to
     access individual 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 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
                       unsupported 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
                       version was established with elf_version(3).

     [ELF_E_VERSION]   The ELF object referenced by argument fd was of an
                       unsupported 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)

NetBSD 7.1.2                   December 11, 2011                  NetBSD 7.1.2