2004-11-03 13:51:07 +00:00
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.\" Copyright (c) 1983, 1991 The Regents of the University of California.
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2011-09-25 04:54:33 +00:00
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.\" And Copyright (C) 2011 Guillem Jover <guillem@hadrons.org>
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2004-11-03 13:51:07 +00:00
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)readlink.2 6.8 (Berkeley) 3/10/91
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.\"
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.\" Modified Sat Jul 24 00:10:21 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified Tue Jul 9 23:55:17 1996 by aeb
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.\" Modified Fri Jan 24 00:26:00 1997 by aeb
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2011-09-20 06:50:20 +00:00
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.\" 2011-09-20, Guillem Jover <guillem@hadrons.org>:
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.\" Added text on dynamically allocating buffer + example program
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2004-11-03 13:51:07 +00:00
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.\"
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2011-09-20 06:39:57 +00:00
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.TH READLINK 2 2011-09-20 "Linux" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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readlink \- read value of a symbolic link
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.SH SYNOPSIS
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.B #include <unistd.h>
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.sp
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2006-07-04 11:13:13 +00:00
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.BI "ssize_t readlink(const char *" path ", char *" buf ", size_t " bufsiz );
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2007-07-08 12:11:40 +00:00
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.ad l
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.BR readlink ():
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2010-09-18 09:02:19 +00:00
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.RS 4
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_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
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_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED || _POSIX_C_SOURCE\ >=\ 200112L
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.RE
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2007-07-08 12:11:40 +00:00
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.ad b
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2004-11-03 13:51:07 +00:00
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.SH DESCRIPTION
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2005-10-19 06:54:38 +00:00
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.BR readlink ()
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2004-11-03 13:51:07 +00:00
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places the contents of the symbolic link
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.I path
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in the buffer
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.IR buf ,
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which has size
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.IR bufsiz .
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2005-10-19 06:54:38 +00:00
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.BR readlink ()
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2006-01-13 02:09:44 +00:00
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does not append a null byte to
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2004-11-03 13:51:07 +00:00
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.IR buf .
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It will truncate the contents (to a length of
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.I bufsiz
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characters), in case the buffer is too small to hold all of the contents.
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.SH "RETURN VALUE"
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2008-07-12 11:11:58 +00:00
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On success,
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.BR readlink ()
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returns the number of bytes placed in
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.IR buf .
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On error, \-1 is returned and
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.I errno
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is set to indicate the error.
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2004-11-03 13:51:07 +00:00
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.SH ERRORS
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.TP
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.B EACCES
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Search permission is denied for a component of the path prefix.
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(See also
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2007-05-26 12:41:39 +00:00
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.BR path_resolution (7).)
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2004-11-03 13:51:07 +00:00
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.TP
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.B EFAULT
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.I buf
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extends outside the process's allocated address space.
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.TP
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.B EINVAL
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.I bufsiz
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is not positive.
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2008-11-04 16:21:18 +00:00
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.\" At the glibc level, bufsiz is unsigned, so this error can only occur
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.\" if bufsiz==0. However, the in the kernel syscall, bufsiz is signed,
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.\" and this error can also occur if bufsiz < 0.
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.\" See: http://thread.gmane.org/gmane.linux.man/380
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.\" Subject: [patch 0/3] [RFC] kernel/glibc mismatch of "readlink" syscall?
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2004-11-03 13:51:07 +00:00
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.TP
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.B EINVAL
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The named file is not a symbolic link.
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.TP
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.B EIO
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An I/O error occurred while reading from the file system.
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.TP
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.B ELOOP
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Too many symbolic links were encountered in translating the pathname.
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.TP
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.B ENAMETOOLONG
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A pathname, or a component of a pathname, was too long.
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.TP
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.B ENOENT
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The named file does not exist.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.TP
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.B ENOTDIR
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A component of the path prefix is not a directory.
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.SH "CONFORMING TO"
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2010-08-29 05:53:20 +00:00
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4.4BSD
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.RB ( readlink ()
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first appeared in 4.2BSD),
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2006-08-03 13:57:17 +00:00
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POSIX.1-2001.
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2007-05-18 11:59:14 +00:00
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.SH NOTES
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2006-07-04 11:13:13 +00:00
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In versions of glibc up to and including glibc 2.4, the return type of
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.BR readlink ()
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was declared as
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.IR int .
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Nowadays, the return type is declared as
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.IR ssize_t ,
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as (newly) required in POSIX.1-2001.
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2011-09-20 06:39:57 +00:00
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Using a statically sized buffer might not provide enough room for the
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symbolic link contents.
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The required size for the buffer can be obtained from the
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.I stat.st_size
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value returned by a call to
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.BR lstat (2)
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on the link.
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However, the number of bytes written by
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.BR readlink ()
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should be checked to make sure that the size of the
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symbolic link did not increase between the calls.
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Dynamically allocating the buffer for
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.BR readlink ()
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also addresses a common portability problem when using
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.I PATH_MAX
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for the buffer size,
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as this constant is not guaranteed to be defined per POSIX
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if the system does not have such limit.
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2011-09-20 06:19:52 +00:00
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.SH EXAMPLE
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2011-09-20 06:39:57 +00:00
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The following program allocates the buffer needed by
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2011-09-20 06:19:52 +00:00
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.BR readlink ()
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dynamically from the information provided by
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.BR lstat (),
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2011-09-20 06:39:57 +00:00
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making sure there's no race condition between the calls.
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2011-09-20 06:19:52 +00:00
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.nf
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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int
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main(int argc, char *argv[])
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{
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struct stat sb;
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char *linkname;
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ssize_t r;
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if (argc != 2) {
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fprintf(stderr, "Usage: %s <pathname>\\n", argv[0]);
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exit(EXIT_FAILURE);
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}
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if (lstat(argv[1], &sb) == \-1) {
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perror("lstat");
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exit(EXIT_FAILURE);
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}
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linkname = malloc(sb.st_size + 1);
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if (linkname == NULL) {
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fprintf(stderr, "insufficient memory\\n");
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exit(EXIT_FAILURE);
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}
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r = readlink(argv[1], linkname, sb.st_size + 1);
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2011-09-23 06:01:18 +00:00
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2011-09-20 06:19:52 +00:00
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if (r < 0) {
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perror("lstat");
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exit(EXIT_FAILURE);
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2011-09-23 06:01:18 +00:00
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}
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2011-09-20 06:39:57 +00:00
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2011-09-25 04:54:33 +00:00
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if (r > sb.st_size) {
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2011-09-20 06:39:57 +00:00
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fprintf(stderr, "symlink increased in size "
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"between lstat() and readlink()\\n");
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2011-09-20 06:19:52 +00:00
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exit(EXIT_FAILURE);
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}
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2011-09-20 06:39:57 +00:00
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2011-09-28 02:52:42 +00:00
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linkname[sb.st_size] = \(aq\\0\(aq;
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2011-09-20 06:19:52 +00:00
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2011-09-28 02:52:42 +00:00
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printf("\(aq%s\(aq points to \(aq%s\(aq\\n", argv[1], linkname);
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2011-09-20 06:19:52 +00:00
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exit(EXIT_SUCCESS);
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}
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.fi
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2004-11-03 13:51:07 +00:00
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.SH "SEE ALSO"
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2011-09-08 23:50:13 +00:00
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.BR readlink (1),
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2004-11-03 13:51:07 +00:00
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.BR lstat (2),
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2006-07-21 14:38:06 +00:00
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.BR readlinkat (2),
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2004-11-03 13:51:07 +00:00
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.BR stat (2),
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2007-05-26 12:41:39 +00:00
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.BR symlink (2),
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2008-06-11 21:50:53 +00:00
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.BR path_resolution (7),
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2008-06-11 22:19:03 +00:00
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.BR symlink (7)
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