gl/xreadlink.c

   1 /* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
   2
   3    Copyright (C) 2001, 2003, 2004, 2005, 2006 Free Software
   4    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 2, or (at your option)
   9    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; see the file COPYING.
  18    If not, write to the Free Software Foundation,
  19    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  20
  21 /* Written by Jim Meyering <jim@meyering.net>  */
  22
  23 #include <config.h>
  24
  25 #include "xreadlink.h"
  26
  27 #include <stdio.h>
  28 #include <errno.h>
  29 #include <limits.h>
  30 #include <sys/types.h>
  31 #include <stdlib.h>
  32 #include <unistd.h>
  33
  34 #ifndef SIZE_MAX
  35 # define SIZE_MAX ((size_t) -1)
  36 #endif
  37 #ifndef SSIZE_MAX
  38 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  39 #endif
  40
  41 /* SYMLINK_MAX is used only for an initial memory-allocation sanity
  42    check, so it's OK to guess too small on hosts where there is no
  43    arbitrary limit to symbolic link length.  */
  44 #ifndef SYMLINK_MAX
  45 # define SYMLINK_MAX 1024
  46 #endif
  47
  48 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  49
  50 #include "xalloc.h"
  51
  52 /* Call readlink to get the symbolic link value of FILE.
  53    SIZE is a hint as to how long the link is expected to be;
  54    typically it is taken from st_size.  It need not be correct.
  55    Return a pointer to that NUL-terminated string in malloc'd storage.
  56    If readlink fails, return NULL (caller may use errno to diagnose).
  57    If malloc fails, or if the link value is longer than SSIZE_MAX :-),
  58    give a diagnostic and exit.  */
  59
  60 char *
  61 xreadlink (char const *file, size_t size)
  62 {
  63   /* Some buggy file systems report garbage in st_size.  Defend
  64      against them by ignoring outlandish st_size values in the initial
  65      memory allocation.  */
  66   size_t symlink_max = SYMLINK_MAX;
  67   size_t INITIAL_LIMIT_BOUND = 8 * 1024;
  68   size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
  69                           ? symlink_max + 1
  70                           : INITIAL_LIMIT_BOUND);
  71
  72   /* The initial buffer size for the link value.  */
  73   size_t buf_size = size < initial_limit ? size + 1 : initial_limit;
  74
  75   while (1)
  76     {
  77       char *buffer = xmalloc (buf_size);
  78       ssize_t r = readlink (file, buffer, buf_size);
  79       size_t link_length = r;
  80
  81       /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
  82          with errno == ERANGE if the buffer is too small.  */
  83       if (r < 0 && errno != ERANGE)
  84         {
  85           int saved_errno = errno;
  86           free (buffer);
  87           errno = saved_errno;
  88           return NULL;
  89         }
  90
  91       if (link_length < buf_size)
  92         {
  93           buffer[link_length] = 0;
  94           return buffer;
  95         }
  96
  97       free (buffer);
  98       if (buf_size <= MAXSIZE / 2)
  99         buf_size *= 2;
 100       else if (buf_size < MAXSIZE)
 101         buf_size = MAXSIZE;
 102       else
 103         xalloc_die ();
 104     }
 105 }