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, 2009-2010 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"
36 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
41 /* The initial buffer size for the link value. A power of 2
42 detects arithmetic overflow earlier, but is not required. */
44 INITIAL_BUF_SIZE = 1024
47 /* Call readlinkat to get the symbolic link value of FILENAME relative to FD.
48 Return a pointer to that NUL-terminated string in malloc'd storage.
49 If readlinkat fails, return NULL and set errno (although failure to
50 change directory will issue a diagnostic and exit).
51 If realloc fails, or if the link value is longer than SIZE_MAX :-),
52 return NULL and set errno to ENOMEM. */
55 areadlinkat (int fd, char const *filename)
57 /* Allocate the initial buffer on the stack. This way, in the common
58 case of a symlink of small size, we get away with a single small malloc()
59 instead of a big malloc() followed by a shrinking realloc(). */
60 char initial_buf[INITIAL_BUF_SIZE];
62 char *buffer = initial_buf;
63 size_t buf_size = sizeof initial_buf;
67 /* Attempt to read the link into the current buffer. */
68 ssize_t link_length = readlinkat (fd, filename, buffer, buf_size);
70 /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
71 with errno == ERANGE if the buffer is too small. */
72 if (link_length < 0 && errno != ERANGE)
74 if (buffer != initial_buf)
76 int saved_errno = errno;
83 if ((size_t) link_length < buf_size)
85 buffer[link_length++] = '\0';
87 /* Return it in a chunk of memory as small as possible. */
88 if (buffer == initial_buf)
90 buffer = (char *) malloc (link_length);
92 /* errno is ENOMEM. */
94 memcpy (buffer, initial_buf, link_length);
98 /* Shrink buffer before returning it. */
99 if ((size_t) link_length < buf_size)
101 char *smaller_buffer = (char *) realloc (buffer, link_length);
103 if (smaller_buffer != NULL)
104 buffer = smaller_buffer;
110 if (buffer != initial_buf)
113 if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
118 buffer = (char *) malloc (buf_size);
120 /* errno is ENOMEM. */
125 #else /* !HAVE_READLINKAT */
127 /* It is more efficient to change directories only once and call
128 areadlink, rather than repeatedly call the replacement
131 # define AT_FUNC_NAME areadlinkat
132 # define AT_FUNC_F1 areadlink
133 # define AT_FUNC_POST_FILE_PARAM_DECLS /* empty */
134 # define AT_FUNC_POST_FILE_ARGS /* empty */
135 # define AT_FUNC_RESULT char *
136 # define AT_FUNC_FAIL NULL
137 # include "at-func.c"
140 # undef AT_FUNC_POST_FILE_PARAM_DECLS
141 # undef AT_FUNC_POST_FILE_ARGS
142 # undef AT_FUNC_RESULT
145 #endif /* !HAVE_READLINKAT */