readlinkat

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

       readlinkat():
           Since glibc 2.10:
               _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 (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 read-
       link(), 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.  (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
       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
              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  ob-
       tained  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 read-
       linkat()  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 porta-
       bility problem when using PATH_MAX for the buffer size,  as  this  con-
       stant  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 rela-
       tive 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() dynami-
       cally from the information provided by lstat(2), falling back to a buf-
       fer 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);
           }

           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
       readlink(1),  lstat(2),  stat(2), symlink(2), realpath(3), path_resolu-
       tion(7), symlink(7)

COLOPHON
       This page is part of release 5.05 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                             2019-03-06                       READLINK(2)