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



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 con‐
       tents (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  path‐
       name).

       If  pathname  is relative and dirfd is the special value AT_FDCWD, then pathname is inter‐
       preted 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 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  con‐
       tents.   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 read‐
       link()  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 buf‐
       fer 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  path‐
       name 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);

           exit(EXIT_SUCCESS);
       }

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

COLOPHON
       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.



Linux                                       2014-10-02                                READLINK(2)