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comparison lib/areadlinkat.c @ 12136:e81e96b1161c

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areadlinkat: new module * lib/at-func.c (FUNC_FAIL): New define. (AT_FUNC_NAME, VALIDATE_FLAG): Use it rather than raw -1. * modules/areadlinkat: New module. * lib/linkat.c (areadlinkat): Move... * lib/areadlinkat.c (areadlinkat): ...to new file. * lib/areadlink.h (areadlinkat): Declare it. * modules/linkat (Depends-on): Add areadlinkat. * MODULES.html.sh (File system functions): Mention it. * modules/areadlinkat-tests: New test. * tests/test-areadlinkat.c: New file. Signed-off-by: Eric Blake <ebb9@byu.net>
author Eric Blake <ebb9@byu.net>
date Wed, 07 Oct 2009 10:15:33 -0600
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children 66ce99c59da1
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12135:1e701cd8a52d 12136:e81e96b1161c
1 /* areadlink.c -- readlink wrapper to return the link name in malloc'd storage
2 Unlike xreadlinkat, only call exit on failure to change directory.
3
4 Copyright (C) 2001, 2003-2007, 2009 Free Software Foundation, Inc.
5
6 This program is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
10
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
15
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>. */
18
19 /* Written by Jim Meyering <jim@meyering.net>,
20 and Bruno Haible <bruno@clisp.org>,
21 and Eric Blake <ebb9@byu.net>. */
22
23 #include <config.h>
24
25 /* Specification. */
26 #include "areadlink.h"
27
28 #include <string.h>
29 #include <errno.h>
30 #include <limits.h>
31 #include <sys/types.h>
32 #include <stdlib.h>
33 #include <unistd.h>
34
35 #ifndef SSIZE_MAX
36 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
37 #endif
38
39 #if HAVE_READLINKAT
40
41 /* Call readlinkat to get the symbolic link value of FILENAME relative to FD.
42 Return a pointer to that NUL-terminated string in malloc'd storage.
43 If readlinkat fails, return NULL and set errno (although failure to
44 change directory will issue a diagnostic and exit).
45 If realloc fails, or if the link value is longer than SIZE_MAX :-),
46 return NULL and set errno to ENOMEM. */
47
48 char *
49 areadlinkat (int fd, char const *filename)
50 {
51 /* The initial buffer size for the link value. A power of 2
52 detects arithmetic overflow earlier, but is not required. */
53 # define INITIAL_BUF_SIZE 1024
54
55 /* Allocate the initial buffer on the stack. This way, in the common
56 case of a symlink of small size, we get away with a single small malloc()
57 instead of a big malloc() followed by a shrinking realloc(). */
58 char initial_buf[INITIAL_BUF_SIZE];
59
60 char *buffer = initial_buf;
61 size_t buf_size = sizeof (initial_buf);
62
63 while (1)
64 {
65 /* Attempt to read the link into the current buffer. */
66 ssize_t link_length = readlinkat (fd, filename, buffer, buf_size);
67
68 /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
69 with errno == ERANGE if the buffer is too small. */
70 if (link_length < 0 && errno != ERANGE)
71 {
72 if (buffer != initial_buf)
73 {
74 int saved_errno = errno;
75 free (buffer);
76 errno = saved_errno;
77 }
78 return NULL;
79 }
80
81 if ((size_t) link_length < buf_size)
82 {
83 buffer[link_length++] = '\0';
84
85 /* Return it in a chunk of memory as small as possible. */
86 if (buffer == initial_buf)
87 {
88 buffer = (char *) malloc (link_length);
89 if (buffer == NULL)
90 /* errno is ENOMEM. */
91 return NULL;
92 memcpy (buffer, initial_buf, link_length);
93 }
94 else
95 {
96 /* Shrink buffer before returning it. */
97 if ((size_t) link_length < buf_size)
98 {
99 char *smaller_buffer = (char *) realloc (buffer, link_length);
100
101 if (smaller_buffer != NULL)
102 buffer = smaller_buffer;
103 }
104 }
105 return buffer;
106 }
107
108 if (buffer != initial_buf)
109 free (buffer);
110 buf_size *= 2;
111 if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
112 {
113 errno = ENOMEM;
114 return NULL;
115 }
116 buffer = (char *) malloc (buf_size);
117 if (buffer == NULL)
118 /* errno is ENOMEM. */
119 return NULL;
120 }
121 }
122
123 #else /* !HAVE_READLINKAT */
124
125 /* It is more efficient to change directories only once and call
126 areadlink, rather than repeatedly call the replacement
127 readlinkat. */
128
129 # define AT_FUNC_NAME areadlinkat
130 # define AT_FUNC_F1 areadlink
131 # define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */
132 # define AT_FUNC_POST_FILE_ARGS /* empty */
133 # define AT_FUNC_RESULT char *
134 # define AT_FUNC_FAIL NULL
135 # include "at-func.c"
136 # undef AT_FUNC_NAME
137 # undef AT_FUNC_F1
138 # undef AT_FUNC_POST_FILE_PARAM_DECLS
139 # undef AT_FUNC_POST_FILE_ARGS
140 # undef AT_FUNC_RESULT
141 # undef AT_FUNC_FAIL
142
143 #endif /* !HAVE_READLINKAT */