srclib/xreadlink.c

/* 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
    }
}