/* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
Copyright (C) 2001, 2003, 2005 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; see the file COPYING.
If not, write to the Free Software Foundation,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
/* Written by Jim Meyering <jim@meyering.net>
and Bruno Haible <bruno@clisp.org>. */
#ifdef HAVE_CONFIG_H
# include <config.h>
#endif
/* Specification. */
#include "xreadlink.h"
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <limits.h>
#include <sys/types.h>
#if HAVE_STDLIB_H
# include <stdlib.h>
#endif
#if HAVE_UNISTD_H
# include <unistd.h>
#endif
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif
#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif
#ifdef NO_XMALLOC
# define xmalloc malloc
#else
# include "xalloc.h"
#endif
/* Call readlink to get the symbolic link value of FILENAME.
Return a pointer to that NUL-terminated string in malloc'd storage.
If readlink fails, return NULL (caller may use errno to diagnose).
If realloc fails, or if the link value is longer than SIZE_MAX :-),
give a diagnostic and exit. */
char *
xreadlink (char const *filename)
{
/* The initial buffer size for the link value. A power of 2
detects arithmetic overflow earlier, but is not required. */
#define INITIAL_BUF_SIZE 1024
/* Allocate the initial buffer on the stack. This way, in the common
case of a symlink of small size, we get away with a single small malloc()
instead of a big malloc() followed by a shrinking realloc(). */
char initial_buf[INITIAL_BUF_SIZE];
char *buffer = initial_buf;
size_t buf_size = sizeof (initial_buf);
while (1)
{
/* Attempt to read the link into the current buffer. */
ssize_t link_length = readlink (filename, buffer, buf_size);
if (link_length < 0)
{
if (buffer != initial_buf)
{
int saved_errno = errno;
free (buffer);
errno = saved_errno;
}
return NULL;
}
if ((size_t) link_length < buf_size)
{
buffer[link_length++] = '\0';
/* Return it in a chunk of memory as small as possible. */
if (buffer == initial_buf)
{
buffer = (char *) xmalloc (link_length);
#ifdef NO_XMALLOC
if (buffer == NULL)
return NULL;
#endif
memcpy (buffer, initial_buf, link_length);
}
else
{
/* Shrink buffer before returning it. */
if ((size_t) link_length < buf_size)
{
char *smaller_buffer = (char *) realloc (buffer, link_length);
if (smaller_buffer != NULL)
buffer = smaller_buffer;
}
}
return buffer;
}
if (buffer != initial_buf)
free (buffer);
buf_size *= 2;
if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
#ifdef NO_XMALLOC
return NULL;
#else
xalloc_die ();
#endif
buffer = (char *) xmalloc (buf_size);
#ifdef NO_XMALLOC
if (buffer == NULL)
return NULL;
#endif
}
}
