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
56 char *
xreadlink(char const * file,size_t size)57 xreadlink (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