readlinkat(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
   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

   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
   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.

EXAMPLE

   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 *linkname;
       ssize_t r, 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);
       }

       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;

       printf("%zd\n", bufsiz);

       linkname = malloc(bufsiz);
       if (linkname == NULL) {
           perror("malloc");
           exit(EXIT_FAILURE);
       }

       r = readlink(argv[1], linkname, bufsiz);
       if (r == -1) {
           perror("readlink");
           exit(EXIT_FAILURE);
       }

       linkname[r] = '\0';

       printf("'%s' points to '%s'\n", argv[1], linkname);

       if (r == bufsiz)
           printf("(Returned buffer may have been truncated)\n");

       free(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 4.09 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/.

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