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       readlink, readlinkat - read value of a symbolic link


       Standard C library (libc, -lc)


       #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)):

           _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200112L
               || /* glibc <= 2.19: */ _BSD_SOURCE

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:


       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.

       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

       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

       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().


       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

       EBADF  (readlinkat())  pathname is relative but dirfd is neither AT_FDCWD nor a valid file

       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

       EIO    An I/O error occurred while reading from the filesystem.

       ELOOP  Too many symbolic links were encountered in translating the pathname.

              A pathname, or a component of a pathname, was too long.

       ENOENT The named file does not exist.

       ENOMEM Insufficient kernel memory was available.

              A component of the path prefix is not a directory.

              (readlinkat())  pathname  is relative and dirfd is a file descriptor referring to a
              file other than a directory.


       readlinkat() was added in Linux 2.6.16; library support was added in glibc 2.4.


       readlink(): 4.4BSD (readlink() first appeared in 4.2BSD), POSIX.1-2001, POSIX.1-2008.

       readlinkat(): POSIX.1-2008.


       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


       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 <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/stat.h>
       #include <unistd.h>

       main(int argc, char *argv[])
           char         *buf;
           ssize_t      nbytes, bufsiz;
           struct stat  sb;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);

           if (lstat(argv[1], &sb) == -1) {

           /* 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) {

           nbytes = readlink(argv[1], buf, bufsiz);
           if (nbytes == -1) {

           /* 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");



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