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.
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);
}
Search permission is denied for a component of the path prefix. (See also path_resolution(7).)
buf
extends outside the process's allocated address
space.
bufsiz
is not positive.
The named file (i.e., the final filename component of
pathname
) is not a symbolic link.
An I/O error occurred while reading from the filesystem.
Too many symbolic links were encountered in translating the pathname.
A pathname, or a component of a pathname, was too long.
The named file does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
The following additional errors can occur for readlinkat():
dirfd
is not a valid file descriptor.
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.
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.