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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 *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_resolution(7), symlink(7)

COLOPHON

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