ELF_BEGIN(3) BSD Library Functions Manual ELF_BEGIN(3)NAMEelf_begin — open an ELF file or ar(1) archive
LIBRARY
library “libelf”
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 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
descriptor 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(). 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_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] Argument elf was not a descriptor for an ar(1) archive.
[ELF_E_ARGUMENT] An ar(1) archive was opened with 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).
SEE ALSOelf(3), elf_end(3), elf_errno(3), elf_memory(3), elf_next(3),
elf_rand(3), elf_update(3), gelf(3)BSD June 21, 2006 BSD
