readlink(2) System Calls Manual readlink(2)
NAME
readlink, readlinkat - read value of a symbolic link
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
ssize_t readlink(const char *restrict pathname, char *restrict buf,
size_t bufsiz);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
ssize_t readlinkat(int dirfd, const char *restrict pathname,
char *restrict buf, size_t bufsiz);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
readlink():
_XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
|| /* glibc <= 2.19: */ _BSD_SOURCE
readlinkat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
readlink() places the contents of the symbolic link pathname in the
buffer buf, which has size bufsiz. readlink() does not append a termi-
nating 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.
readlinkat()
The readlinkat() system call operates in exactly the same way as read-
link(), 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().
RETURN VALUE
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.
ERRORS
EACCES Search permission is denied for a component of the path prefix.
(See also path_resolution(7).)
EBADF (readlinkat()) pathname is relative but dirfd is neither AT_FD-
CWD nor a valid file descriptor.
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.
ENOTDIR
(readlinkat()) pathname is relative and dirfd is a file descrip-
tor referring to a file other than a directory.
VERSIONS
readlinkat() was added in Linux 2.6.16; library support was added in
glibc 2.4.
STANDARDS
readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001,
POSIX.1-2008.
readlinkat(): POSIX.1-2008.
NOTES
Up to and including glibc 2.4, the return type of readlink() was de-
clared 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 ob-
tained 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 read-
linkat() 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 porta-
bility problem when using PATH_MAX for the buffer size, as this con-
stant is not guaranteed to be defined per POSIX if the system does not
have such limit.
glibc notes
On older kernels where readlinkat() is unavailable, the glibc wrapper
function falls back to the use of readlink(). When pathname is a rela-
tive pathname, glibc constructs a pathname based on the symbolic link
in /proc/self/fd that corresponds to the dirfd argument.
EXAMPLES
The following program allocates the buffer needed by readlink() dynami-
cally from the information provided by lstat(2), falling back to a buf-
fer of size PATH_MAX in cases where lstat(2) reports a size of zero.
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
char *buf;
ssize_t nbytes, bufsiz;
struct stat sb;
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);
}
/* Print only 'nbytes' of 'buf', as it doesn't contain a terminating
null byte ('\0'). */
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);
}
SEE ALSO
readlink(1), lstat(2), stat(2), symlink(2), realpath(3), path_resolu-
tion(7), symlink(7)
Linux man-pages 6.03 2023-02-05 readlink(2)
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