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       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>

       ssize_t 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

       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 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().


       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
       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 argument.


       The  following  program  allocates  the  buffer  needed  by  readlink()
       dynamically 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),    realpath(3),
       path_resolution(7), symlink(7)


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