readlink, readlinkat - read value of a symbolic link
#include <unistd.h>
ssize_t readlink(const char *pathname, char *buf, size_t bufsiz);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
ssize_t readlinkat(int dirfd, const char *pathname,
char *buf, size_t bufsiz);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
readlink():
_XOPEN_SOURCE >= 500 ||
_POSIX_C_SOURCE >= 200112L
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
readlinkat():
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
- _ATFILE_SOURCE
readlink() places the contents of the symbolic link
pathname in
the buffer
buf, which has size
bufsiz.
readlink() does
not append a null byte to
buf. It will (silently) truncate the contents
(to a length of
bufsiz characters), in case the buffer is too small to
hold all of the contents.
The
readlinkat() system call operates in exactly the same way as
readlink(), except for the differences described here.
If the pathname given in
pathname is relative, then it is interpreted
relative to the directory referred to by the file descriptor
dirfd
(rather than relative to the current working directory of the calling process,
as is done by
readlink() for a relative pathname).
If
pathname is relative and
dirfd is the special value
AT_FDCWD, then
pathname is interpreted relative to the current
working directory of the calling process (like
readlink()).
If
pathname is absolute, then
dirfd is ignored.
Since Linux 2.6.39,
pathname can be an empty string, in which case the
call operates on the symbolic link referred to by
dirfd (which should
have been obtained using
open(2) with the
O_PATH and
O_NOFOLLOW flags).
See
openat(2) for an explanation of the need for
readlinkat().
On success, these calls return the number of bytes placed in
buf. (If the
returned value equals
bufsiz, then truncation may have occurred.) On
error, -1 is returned and
errno is set to indicate the error.
- EACCES
- Search permission is denied for a component of the path prefix. (See also
path_resolution(7).)
- EFAULT
- buf extends outside the process's allocated address space.
- EINVAL
- bufsiz is not positive.
- EINVAL
- The named file (i.e., the final filename component of pathname) is
not a symbolic link.
- EIO
- An I/O error occurred while reading from the filesystem.
- ELOOP
- Too many symbolic links were encountered in translating the pathname.
- ENAMETOOLONG
- A pathname, or a component of a pathname, was too long.
- ENOENT
- The named file does not exist.
- ENOMEM
- Insufficient kernel memory was available.
- ENOTDIR
- A component of the path prefix is not a directory.
The following additional errors can occur for
readlinkat():
- EBADF
- dirfd is not a valid file descriptor.
- ENOTDIR
- pathname is relative and dirfd is a file descriptor
referring to a file other than a directory.
readlinkat() was added to Linux in kernel 2.6.16; library support was
added to glibc in version 2.4.
readlink(): 4.4BSD (
readlink() first appeared in 4.2BSD),
POSIX.1-2001, POSIX.1-2008.
readlinkat(): POSIX.1-2008.
In versions of glibc up to and including glibc 2.4, the return type of
readlink() was declared as
int. Nowadays, the return type is
declared as
ssize_t, as (newly) required in POSIX.1-2001.
Using a statically sized buffer might not provide enough room for the symbolic
link contents. The required size for the buffer can be obtained from the
stat.st_size value returned by a call to
lstat(2) on the link.
However, the number of bytes written by
readlink() and
readlinkat() should be checked to make sure that the size of the
symbolic link did not increase between the calls. Dynamically allocating the
buffer for
readlink() and
readlinkat() also addresses a common
portability problem when using
PATH_MAX for the buffer size, as this
constant is not guaranteed to be defined per POSIX if the system does not have
such limit.
On older kernels where
readlinkat() is unavailable, the glibc wrapper
function falls back to the use of
readlink(). When
pathname is a
relative pathname, glibc constructs a pathname based on the symbolic link in
/proc/self/fd that corresponds to the
dirfd argument.
The following program allocates the buffer needed by
readlink()
dynamically from the information provided by
lstat(2), falling back to
a buffer of size
PATH_MAX in cases where
lstat(2) reports a size
of zero.
#include <sys/types.h>
#include <sys/stat.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
struct stat sb;
char *buf;
ssize_t nbytes, bufsiz;
if (argc != 2) {
fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
exit(EXIT_FAILURE);
}
if (lstat(argv[1], &sb) == -1) {
perror("lstat");
exit(EXIT_FAILURE);
}
/* Add one to the link size, so that we can determine whether
the buffer returned by readlink() was truncated. */
bufsiz = sb.st_size + 1;
/* Some magic symlinks under (for example) /proc and /sys
report 'st_size' as zero. In that case, take PATH_MAX as
a "good enough" estimate. */
if (sb.st_size == 0)
bufsiz = PATH_MAX;
buf = malloc(bufsiz);
if (buf == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
nbytes = readlink(argv[1], buf, bufsiz);
if (nbytes == -1) {
perror("readlink");
exit(EXIT_FAILURE);
}
printf("'%s' points to '%.*s'\n", argv[1], (int) nbytes, buf);
/* If the return value was equal to the buffer size, then the
the link target was larger than expected (perhaps because the
target was changed between the call to lstat() and the call to
readlink()). Warn the user that the returned target may have
been truncated. */
if (nbytes == bufsiz)
printf("(Returned buffer may have been truncated)\n");
free(buf);
exit(EXIT_SUCCESS);
}
readlink(1),
lstat(2),
stat(2),
symlink(2),
realpath(3),
path_resolution(7),
symlink(7)