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
59 char *
xreadlink(char const * file,size_t size)60 xreadlink (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