readlink(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

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 *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         top

       readlink() places the contents of the symbolic link pathname in
       the buffer buf, which has size bufsiz.  readlink() does not
       append a terminating 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
       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 error.

ERRORS         top

       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_FDCWD 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
              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>

       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);
           }

           /* 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         top

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

COLOPHON         top

       This page is part of release 5.13 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/.

Linux                          2021-08-27                    READLINK(2)

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