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)