1 /* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
2
3 Copyright (C) 2001, 2003, 2005-2006 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 and Bruno Haible <bruno@clisp.org>. */
22
23 #include <config.h>
24
25 /* Specification. */
26 #include "xreadlink.h"
27
28 #include <stdio.h>
29 #include <string.h>
30 #include <errno.h>
31 #include <limits.h>
32 #include <sys/types.h>
33 #include <stdlib.h>
34 #if HAVE_UNISTD_H
35 # include <unistd.h>
36 #endif
37
38 #ifndef SIZE_MAX
39 # define SIZE_MAX ((size_t) -1)
40 #endif
41 #ifndef SSIZE_MAX
42 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
43 #endif
44
45 #ifdef NO_XMALLOC
46 # define xmalloc malloc
47 #else
48 # include "xalloc.h"
49 #endif
50
51 /* Call readlink to get the symbolic link value of FILENAME.
52 Return a pointer to that NUL-terminated string in malloc'd storage.
53 If readlink fails, return NULL (caller may use errno to diagnose).
54 If realloc fails, or if the link value is longer than SIZE_MAX :-),
55 give a diagnostic and exit. */
56
57 char *
xreadlink(char const * filename)58 xreadlink (char const *filename)
59 {
60 /* The initial buffer size for the link value. A power of 2
61 detects arithmetic overflow earlier, but is not required. */
62 #define INITIAL_BUF_SIZE 1024
63
64 /* Allocate the initial buffer on the stack. This way, in the common
65 case of a symlink of small size, we get away with a single small malloc()
66 instead of a big malloc() followed by a shrinking realloc(). */
67 char initial_buf[INITIAL_BUF_SIZE];
68
69 char *buffer = initial_buf;
70 size_t buf_size = sizeof (initial_buf);
71
72 while (1)
73 {
74 /* Attempt to read the link into the current buffer. */
75 ssize_t link_length = readlink (filename, buffer, buf_size);
76
77 if (link_length < 0)
78 {
79 if (buffer != initial_buf)
80 {
81 int saved_errno = errno;
82 free (buffer);
83 errno = saved_errno;
84 }
85 return NULL;
86 }
87
88 if ((size_t) link_length < buf_size)
89 {
90 buffer[link_length++] = '\0';
91
92 /* Return it in a chunk of memory as small as possible. */
93 if (buffer == initial_buf)
94 {
95 buffer = (char *) xmalloc (link_length);
96 #ifdef NO_XMALLOC
97 if (buffer == NULL)
98 return NULL;
99 #endif
100 memcpy (buffer, initial_buf, link_length);
101 }
102 else
103 {
104 /* Shrink buffer before returning it. */
105 if ((size_t) link_length < buf_size)
106 {
107 char *smaller_buffer = (char *) realloc (buffer, link_length);
108
109 if (smaller_buffer != NULL)
110 buffer = smaller_buffer;
111 }
112 }
113 return buffer;
114 }
115
116 if (buffer != initial_buf)
117 free (buffer);
118 buf_size *= 2;
119 if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
120 #ifdef NO_XMALLOC
121 return NULL;
122 #else
123 xalloc_die ();
124 #endif
125 buffer = (char *) xmalloc (buf_size);
126 #ifdef NO_XMALLOC
127 if (buffer == NULL)
128 return NULL;
129 #endif
130 }
131 }
132