Manpages - readlink.2

readlink, readlinkat - read value of a symbolic link

  #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

*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.

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.

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.

*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.

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);
  }

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

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