1/* areadlinkat.c -- readlinkat wrapper to return malloc'd link name
2   Unlike xreadlinkat, only call exit on failure to change directory.
3
4   Copyright (C) 2001, 2003-2007, 2009-2010 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 <errno.h>
29#include <limits.h>
30#include <stdint.h>
31#include <stdlib.h>
32#include <string.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/* The initial buffer size for the link value.  A power of 2
42   detects arithmetic overflow earlier, but is not required.  */
43enum {
44  INITIAL_BUF_SIZE = 1024
45};
46
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.  */
53
54char *
55areadlinkat (int fd, char const *filename)
56{
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];
61
62  char *buffer = initial_buf;
63  size_t buf_size = sizeof initial_buf;
64
65  while (1)
66    {
67      /* Attempt to read the link into the current buffer.  */
68       ssize_t link_length = readlinkat (fd, filename, buffer, buf_size);
69
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)
73        {
74          if (buffer != initial_buf)
75            {
76              int saved_errno = errno;
77              free (buffer);
78              errno = saved_errno;
79            }
80          return NULL;
81        }
82
83      if ((size_t) link_length < buf_size)
84        {
85          buffer[link_length++] = '\0';
86
87          /* Return it in a chunk of memory as small as possible.  */
88          if (buffer == initial_buf)
89            {
90              buffer = (char *) malloc (link_length);
91              if (buffer == NULL)
92                /* errno is ENOMEM.  */
93                return NULL;
94              memcpy (buffer, initial_buf, link_length);
95            }
96          else
97            {
98              /* Shrink buffer before returning it.  */
99              if ((size_t) link_length < buf_size)
100                {
101                  char *smaller_buffer = (char *) realloc (buffer, link_length);
102
103                  if (smaller_buffer != NULL)
104                    buffer = smaller_buffer;
105                }
106            }
107          return buffer;
108        }
109
110      if (buffer != initial_buf)
111        free (buffer);
112      buf_size *= 2;
113      if (SSIZE_MAX < buf_size || (SIZE_MAX / 2 < SSIZE_MAX && buf_size == 0))
114        {
115          errno = ENOMEM;
116          return NULL;
117        }
118      buffer = (char *) malloc (buf_size);
119      if (buffer == NULL)
120        /* errno is ENOMEM.  */
121        return NULL;
122    }
123}
124
125#else /* !HAVE_READLINKAT */
126
127/* It is more efficient to change directories only once and call
128   areadlink, rather than repeatedly call the replacement
129   readlinkat.  */
130
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"
138# undef AT_FUNC_NAME
139# undef AT_FUNC_F1
140# undef AT_FUNC_POST_FILE_PARAM_DECLS
141# undef AT_FUNC_POST_FILE_ARGS
142# undef AT_FUNC_RESULT
143# undef AT_FUNC_FAIL
144
145#endif /* !HAVE_READLINKAT */
146