ELF_BEGIN(3) | Library Functions Manual | ELF_BEGIN(3)

# NAME

`elf_begin` — open an ELF file or ar(1) archive

# SYNOPSIS

ELF Access Library (libelf, -lelf)
`#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)) == -1) {
    	... 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 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)

December 11, 2011 | Debian