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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
     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 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
     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); }