readlinkat man page on Archlinux

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READLINK(2)		   Linux Programmer's Manual		   READLINK(2)

       readlink, readlinkat - read value of a symbolic link

       #include <unistd.h>

       ssize_t readlink(const char *pathname, char *buf, size_t bufsiz);

       #include <fcntl.h>	    /* Definition of AT_* constants */
       #include <unistd.h>

       int readlinkat(int dirfd, const char *pathname,
		      char *buf, size_t bufsiz);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	   _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
	   _POSIX_C_SOURCE >= 200112L

	   Since glibc 2.10:
	       _XOPEN_SOURCE >= 700 || _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  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‐

       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 file referred to by dirfd  (which  may  have  been
       obtained using the open(2) O_PATH flag).	 In this case, dirfd can refer
       to any type of file, not just a directory.

       See openat(2) for an explanation of the need for readlinkat().

       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.

       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

	      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.

       The following additional errors can occur for readlinkat():

       EBADF  dirfd is not a valid file descriptor.

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

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

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

       readlinkat(): POSIX.1-2008.

       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.

       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>

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

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

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

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

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

	   if (r == -1) {

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

	   linkname[r] = '\0';

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


       readlink(1),  lstat(2),	stat(2),  symlink(2), path_resolution(7), sym‐

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-02-21			   READLINK(2)

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