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NAME
readlink, readlinkat - read value of a symbolic link
SYNOPSIS
#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():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
_POSIX_C_SOURCE >= 200112L
readlinkat():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _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 null byte to buf. It will 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 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().
RETURN VALUE
On success, these calls return the number of bytes placed in buf. 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).)
EFAULT buf extends outside the process's allocated address space.
EINVAL bufsiz is not positive.
EINVAL The named file 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.
VERSIONS
readlinkat() was added to Linux in kernel 2.6.16; library support was added to glibc in
version 2.4.
CONFORMING TO
readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001, POSIX.1-2008.
readlinkat(): POSIX.1-2008.
NOTES
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.
Glibc notes
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.
EXAMPLE
The following program allocates the buffer needed by readlink() dynamically from the
information provided by lstat(), making sure there's no race condition between the calls.
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
struct stat sb;
char *linkname;
ssize_t r;
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);
}
linkname = malloc(sb.st_size + 1);
if (linkname == NULL) {
fprintf(stderr, "insufficient memory\n");
exit(EXIT_FAILURE);
}
r = readlink(argv[1], linkname, sb.st_size + 1);
if (r == -1) {
perror("readlink");
exit(EXIT_FAILURE);
}
if (r > sb.st_size) {
fprintf(stderr, "symlink increased in size "
"between lstat() and readlink()\n");
exit(EXIT_FAILURE);
}
linkname[r] = '\0';
printf("'%s' points to '%s'\n", argv[1], linkname);
free(linkname);
exit(EXIT_SUCCESS);
}
SEE ALSO
readlink(1), lstat(2), stat(2), symlink(2), realpath(3), path_resolution(7), symlink(7)
COLOPHON
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