lib/readlinkat.c

   1 /* Read a symlink relative to an open directory.
   2    Copyright (C) 2009-2015 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  16
  17 /* written by Eric Blake */
  18
  19 #include <config.h>
  20
  21 #include <unistd.h>
  22
  23 #if HAVE_READLINKAT
  24
  25 # undef readlinkat
  26
  27 ssize_t
  28 rpl_readlinkat (int fd, char const *file, char *buf, size_t len)
  29 {
  30   return readlinkat (fd, file, buf, len);
  31 }
  32
  33 #else
  34
  35 /* Gnulib provides a readlink stub for mingw; use it for distinction
  36    between EINVAL and ENOENT, rather than always failing with ENOSYS.  */
  37
  38 /* POSIX 2008 says that unlike readlink, readlinkat returns 0 for
  39    success instead of the buffer length.  But this would render
  40    readlinkat worthless since readlink does not guarantee a
  41    NUL-terminated buffer.  Assume this was a bug in POSIX.  */
  42
  43 /* Read the contents of symlink FILE into buffer BUF of size LEN, in the
  44    directory open on descriptor FD.  If possible, do it without changing
  45    the working directory.  Otherwise, resort to using save_cwd/fchdir,
  46    then readlink/restore_cwd.  If either the save_cwd or the restore_cwd
  47    fails, then give a diagnostic and exit nonzero.  */
  48
  49 # define AT_FUNC_NAME readlinkat
  50 # define AT_FUNC_F1 readlink
  51 # define AT_FUNC_POST_FILE_PARAM_DECLS , char *buf, size_t len
  52 # define AT_FUNC_POST_FILE_ARGS        , buf, len
  53 # define AT_FUNC_RESULT ssize_t
  54 # include "at-func.c"
  55 # undef AT_FUNC_NAME
  56 # undef AT_FUNC_F1
  57 # undef AT_FUNC_POST_FILE_PARAM_DECLS
  58 # undef AT_FUNC_POST_FILE_ARGS
  59 # undef AT_FUNC_RESULT
  60
  61 #endif