readlink(2) — Linux manual page


READLINK(2)               Linux Programmer's Manual              READLINK(2)

NAME         top

       readlink, readlinkat - read value of a symbolic link

SYNOPSIS         top

       #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)):

           _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

DESCRIPTION         top

       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().

RETURN VALUE         top

       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

ERRORS         top

       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

              A pathname, or a component of a pathname, was too long.

       ENOENT The named file does not exist.

       ENOMEM Insufficient kernel memory was available.

              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.

              pathname is relative and dirfd is a file descriptor referring
              to a file other than a directory.

VERSIONS         top

       readlinkat() was added to Linux in kernel 2.6.16; library support was
       added to glibc in version 2.4.

CONFORMING TO         top

       readlink(): 4.4BSD (readlink() first appeared in 4.2BSD),
       POSIX.1-2001, POSIX.1-2008.

       readlinkat(): POSIX.1-2008.

NOTES         top

       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.

EXAMPLES         top

       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>

       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]);

           if (lstat(argv[1], &sb) == -1) {

           /* 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) {

           nbytes = readlink(argv[1], buf, bufsiz);
           if (nbytes == -1) {

           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");


SEE ALSO         top

       readlink(1), lstat(2), stat(2), symlink(2), realpath(3),
       path_resolution(7), symlink(7)

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09                      READLINK(2)

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