man-pages/man2/readlink.2

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

#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 < 0) {
        perror("lstat");
        exit(EXIT_FAILURE);
    }

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

    linkname[sb.st_size] = \(aq\\0\(aq;

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

    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR readlink (1),
.BR lstat (2),
.BR readlinkat (2),
.BR stat (2),
.BR symlink (2),
.BR path_resolution (7),
.BR symlink (7)