GNU.WIKI: The GNU/Linux Knowledge Base

  [HOME] [PHP Manual] [HowTo] [ABS] [MAN1] [MAN2] [MAN3] [MAN4] [MAN5] [MAN6] [MAN7] [MAN8] [MAN9]

  [0-9] [Aa] [Bb] [Cc] [Dd] [Ee] [Ff] [Gg] [Hh] [Ii] [Jj] [Kk] [Ll] [Mm] [Nn] [Oo] [Pp] [Qq] [Rr] [Ss] [Tt] [Uu] [Vv] [Ww] [Xx] [Yy] [Zz]


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>

       int 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
       contents.

   readlinkat()
       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 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().

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
       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.

EXAMPLE

       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>

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

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <pathname>
", 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
");
               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()
");
               exit(EXIT_FAILURE);
           }

           linkname[r] = '';

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

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



  All copyrights belong to their respective owners. Other content (c) 2014-2017, GNU.WIKI. Please report site errors to webmaster@gnu.wiki.
Page load time: 0.087 seconds. Last modified: November 09 2017 18:38:06.