xreadlink.c revision 1.1 
1/* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
2
3   Copyright (C) 2001, 2003, 2004, 2005 Free Software Foundation, Inc.
4
5   This program is free software; you can redistribute it and/or modify
6   it under the terms of the GNU General Public License as published by
7   the Free Software Foundation; either version 2, or (at your option)
8   any later version.
9
10   This program is distributed in the hope that it will be useful,
11   but WITHOUT ANY WARRANTY; without even the implied warranty of
12   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
13   GNU General Public License for more details.
14
15   You should have received a copy of the GNU General Public License
16   along with this program; see the file COPYING.
17   If not, write to the Free Software Foundation,
18   51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
19
20/* Written by Jim Meyering <jim@meyering.net>  */
21
22#ifdef HAVE_CONFIG_H
23# include <config.h>
24#endif
25
26#include "xreadlink.h"
27
28#include <stdio.h>
29#include <errno.h>
30#include <limits.h>
31#include <sys/types.h>
32#include <stdlib.h>
33#if HAVE_UNISTD_H
34# include <unistd.h>
35#endif
36
37#ifndef SIZE_MAX
38# define SIZE_MAX ((size_t) -1)
39#endif
40#ifndef SSIZE_MAX
41# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
42#endif
43
44#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
45
46#include "xalloc.h"
47
48/* Call readlink to get the symbolic link value of FILE.
49   SIZE is a hint as to how long the link is expected to be;
50   typically it is taken from st_size.  It need not be correct.
51   Return a pointer to that NUL-terminated string in malloc'd storage.
52   If readlink fails, return NULL (caller may use errno to diagnose).
53   If malloc fails, or if the link value is longer than SSIZE_MAX :-),
54   give a diagnostic and exit.  */
55
56char *
57xreadlink (char const *file, size_t size)
58{
59  /* The initial buffer size for the link value.  A power of 2
60     detects arithmetic overflow earlier, but is not required.  */
61  size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE;
62
63  while (1)
64    {
65      char *buffer = xmalloc (buf_size);
66      ssize_t r = readlink (file, buffer, buf_size);
67      size_t link_length = r;
68
69      /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
70	 with errno == ERANGE if the buffer is too small.  */
71      if (r < 0 && errno != ERANGE)
72	{
73	  int saved_errno = errno;
74	  free (buffer);
75	  errno = saved_errno;
76	  return NULL;
77	}
78
79      if (link_length < buf_size)
80	{
81	  buffer[link_length] = 0;
82	  return buffer;
83	}
84
85      free (buffer);
86      if (buf_size <= MAXSIZE / 2)
87	buf_size *= 2;
88      else if (buf_size < MAXSIZE)
89	buf_size = MAXSIZE;
90      else
91	xalloc_die ();
92    }
93}
94