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#include <sys/cdefs.h>
20__RCSID("$NetBSD: xreadlink.c,v 1.2 2016/05/17 14:00:09 christos Exp $");
21
22
23/* Written by Jim Meyering <jim@meyering.net>  */
24
25#ifdef HAVE_CONFIG_H
26# include <config.h>
27#endif
28
29#include "xreadlink.h"
30
31#include <stdio.h>
32#include <errno.h>
33#include <limits.h>
34#include <sys/types.h>
35#include <stdlib.h>
36#if HAVE_UNISTD_H
37# include <unistd.h>
38#endif
39
40#ifndef SIZE_MAX
41# define SIZE_MAX ((size_t) -1)
42#endif
43#ifndef SSIZE_MAX
44# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
45#endif
46
47#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
48
49#include "xalloc.h"
50
51/* Call readlink to get the symbolic link value of FILE.
52   SIZE is a hint as to how long the link is expected to be;
53   typically it is taken from st_size.  It need not be correct.
54   Return a pointer to that NUL-terminated string in malloc'd storage.
55   If readlink fails, return NULL (caller may use errno to diagnose).
56   If malloc fails, or if the link value is longer than SSIZE_MAX :-),
57   give a diagnostic and exit.  */
58
59char *
60xreadlink (char const *file, size_t size)
61{
62  /* The initial buffer size for the link value.  A power of 2
63     detects arithmetic overflow earlier, but is not required.  */
64  size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE;
65
66  while (1)
67    {
68      char *buffer = xmalloc (buf_size);
69      ssize_t r = readlink (file, buffer, buf_size);
70      size_t link_length = r;
71
72      /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
73	 with errno == ERANGE if the buffer is too small.  */
74      if (r < 0 && errno != ERANGE)
75	{
76	  int saved_errno = errno;
77	  free (buffer);
78	  errno = saved_errno;
79	  return NULL;
80	}
81
82      if (link_length < buf_size)
83	{
84	  buffer[link_length] = 0;
85	  return buffer;
86	}
87
88      free (buffer);
89      if (buf_size <= MAXSIZE / 2)
90	buf_size *= 2;
91      else if (buf_size < MAXSIZE)
92	buf_size = MAXSIZE;
93      else
94	xalloc_die ();
95    }
96}
97