1 /* areadlinkat.c -- readlinkat wrapper to return malloc'd link name
2 Unlike xreadlinkat, only call exit on failure to change directory.
4 Copyright (C) 2001, 2003-2007, 2009 Free Software Foundation, Inc.
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.
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.
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/>. */
19 /* Written by Jim Meyering <jim@meyering.net>,
20 and Bruno Haible <bruno@clisp.org>,
21 and Eric Blake <ebb9@byu.net>. */
26 #include "areadlink.h"
31 #include <sys/types.h>
37 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
42 /* Call readlinkat to get the symbolic link value of FILENAME relative to FD.
43 Return a pointer to that NUL-terminated string in malloc'd storage.
44 If readlinkat fails, return NULL and set errno (although failure to
45 change directory will issue a diagnostic and exit).
46 If realloc fails, or if the link value is longer than SIZE_MAX :-),
47 return NULL and set errno to ENOMEM. */
50 areadlinkat (int fd, char const *filename)
52 /* The initial buffer size for the link value. A power of 2
53 detects arithmetic overflow earlier, but is not required. */
54 # define INITIAL_BUF_SIZE 1024
56 /* Allocate the initial buffer on the stack. This way, in the common
57 case of a symlink of small size, we get away with a single small malloc()
58 instead of a big malloc() followed by a shrinking realloc(). */
59 char initial_buf[INITIAL_BUF_SIZE];
61 char *buffer = initial_buf;
62 size_t buf_size = sizeof (initial_buf);
66 /* Attempt to read the link into the current buffer. */
67 ssize_t link_length = readlinkat (fd, filename, buffer, buf_size);
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 (link_length < 0 && errno != ERANGE)
73 if (buffer != initial_buf)
75 int saved_errno = errno;
82 if ((size_t) link_length < buf_size)
84 buffer[link_length++] = '\0';
86 /* Return it in a chunk of memory as small as possible. */
87 if (buffer == initial_buf)
89 buffer = (char *) malloc (link_length);
91 /* errno is ENOMEM. */
93 memcpy (buffer, initial_buf, link_length);
97 /* Shrink buffer before returning it. */
98 if ((size_t) link_length < buf_size)
100 char *smaller_buffer = (char *) realloc (buffer, link_length);
102 if (smaller_buffer != NULL)
103 buffer = smaller_buffer;
109 if (buffer != initial_buf)
112 if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
117 buffer = (char *) malloc (buf_size);
119 /* errno is ENOMEM. */
124 #else /* !HAVE_READLINKAT */
126 /* It is more efficient to change directories only once and call
127 areadlink, rather than repeatedly call the replacement
130 # define AT_FUNC_NAME areadlinkat
131 # define AT_FUNC_F1 areadlink
132 # define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */
133 # define AT_FUNC_POST_FILE_ARGS /* empty */
134 # define AT_FUNC_RESULT char *
135 # define AT_FUNC_FAIL NULL
136 # include "at-func.c"
139 # undef AT_FUNC_POST_FILE_PARAM_DECLS
140 # undef AT_FUNC_POST_FILE_ARGS
141 # undef AT_FUNC_RESULT
144 #endif /* !HAVE_READLINKAT */