Yolinux.com

readlinkat manpage

Search topic Section


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():
	   _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
	   _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED ||
	   _POSIX_C_SOURCE >= 200112L

       readlinkat():
	   Since glibc 2.10:
	       _XOPEN_SOURCE >= 700 || _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  truncate the contents (to a length of bufsiz
       characters), in case the buffer is too small to hold all	 of  the  con-
       tents.

   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.  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 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(), making sure there's  no
       race condition between the calls.

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

       int
       main(int argc, char *argv[])
       {
	   struct stat sb;
	   char *linkname;
	   ssize_t r;

	   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);
	   }

	   linkname = malloc(sb.st_size + 1);
	   if (linkname == NULL) {
	       fprintf(stderr, "insufficient memory\n");
	       exit(EXIT_FAILURE);
	   }

	   r = readlink(argv[1], linkname, sb.st_size + 1);

	   if (r == -1) {
	       perror("readlink");
	       exit(EXIT_FAILURE);
	   }

	   if (r > sb.st_size) {
	       fprintf(stderr, "symlink increased in size "
			       "between lstat() and readlink()\n");
	       exit(EXIT_FAILURE);
	   }

	   linkname[r] = '\0';

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

	   free(linkname);

	   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 4.04 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
       http://www.kernel.org/doc/man-pages/.



Linux				  2014-10-15			   READLINK(2)