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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 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 10.99 December 11, 2011 NetBSD 10.99