contrib/cvs-1.12/lib/xreadlink.c

   1 /* xreadlink.c -- readlink wrapper to return the link name in malloc'd storage
   2
   3    Copyright (C) 2001, 2003, 2004, 2005 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program; see the file COPYING.
  17    If not, write to the Free Software Foundation,
  18    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  19
  20 /* Written by Jim Meyering <jim@meyering.net>  */
  21
  22 #ifdef HAVE_CONFIG_H
  23 # include <config.h>
  24 #endif
  25
  26 #include "xreadlink.h"
  27
  28 #include <stdio.h>
  29 #include <errno.h>
  30 #include <limits.h>
  31 #include <sys/types.h>
  32 #include <stdlib.h>
  33 #if HAVE_UNISTD_H
  34 # include <unistd.h>
  35 #endif
  36
  37 #ifndef SIZE_MAX
  38 # define SIZE_MAX ((size_t) -1)
  39 #endif
  40 #ifndef SSIZE_MAX
  41 # define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
  42 #endif
  43
  44 #define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
  45
  46 #include "xalloc.h"
  47
  48 /* Call readlink to get the symbolic link value of FILE.
  49    SIZE is a hint as to how long the link is expected to be;
  50    typically it is taken from st_size.  It need not be correct.
  51    Return a pointer to that NUL-terminated string in malloc'd storage.
  52    If readlink fails, return NULL (caller may use errno to diagnose).
  53    If malloc fails, or if the link value is longer than SSIZE_MAX :-),
  54    give a diagnostic and exit.  */
  55
  56 char *
  57 xreadlink (char const *file, size_t size)
  58 {
  59   /* The initial buffer size for the link value.  A power of 2
  60      detects arithmetic overflow earlier, but is not required.  */
  61   size_t buf_size = size < MAXSIZE ? size + 1 : MAXSIZE;
  62
  63   while (1)
  64     {
  65       char *buffer = xmalloc (buf_size);
  66       ssize_t r = readlink (file, buffer, buf_size);
  67       size_t link_length = r;
  68
  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 (r < 0 && errno != ERANGE)
  72         {
  73           int saved_errno = errno;
  74           free (buffer);
  75           errno = saved_errno;
  76           return NULL;
  77         }
  78
  79       if (link_length < buf_size)
  80         {
  81           buffer[link_length] = 0;
  82           return buffer;
  83         }
  84
  85       free (buffer);
  86       if (buf_size <= MAXSIZE / 2)
  87         buf_size *= 2;
  88       else if (buf_size < MAXSIZE)
  89         buf_size = MAXSIZE;
  90       else
  91         xalloc_die ();
  92     }
  93 }