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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():
	   _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  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 ob-
       tained 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);

	   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 3.74 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				  2014-10-02			   READLINK(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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