Re: External debug symbols

[Alexander Larsson <alexl@redhat.com>]

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Here comes a new patch that fixes some issues.

On Fri, 20 Sep 2002, Elena Zannoni wrote:

> Does the reread_symbols() function still work?  To see this you need
> to run the inferior, then w/o quitting from gdb, modify the
> executable/recompile and then issue a 'run' command.  That would cause
> the excutable and symbols to be reread.  It walks the list of existing
> objfiles and updates those that need it, but it doesn't call
> symbol_file_add, so the crc check would not be done and the 2 files
> could get out of sync.

I fixed reread_symbols so that it won't ever use an incorrect debug file. 
It doesn't do a full search though, so if you change the file and keep 
using separate debug files you better leave the debug file in the same 
place.
 
reread_symbols is pretty ugly in general though. For instance, what if the 
new file is relocated to a different place? It copies the old 
section_offsets, so that would break i think.

>  > diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/objfiles.h gdb-5.2.1.separate_debug_symbols/gdb/objfiles.h
>  > --- gdb-5.2.1/gdb/objfiles.h	Sat Jun 29 00:05:47 2002
>  > +++ gdb-5.2.1.separate_debug_symbols/gdb/objfiles.h	Mon Sep 16 13:15:15 2002
>  > @@ -409,6 +409,9 @@
>  >      ExportEntry *export_list;
>  >      int export_list_size;
>  >  
>  > +    struct objfile *separate_debug_objfile;
>  > +    struct objfile *separate_debug_objfile_backlink;
>  > +    
> 
> Could you add some comments, here and elsewhere?

I added some.

>  > +  debugfile = find_separate_debug_file (objfile, name);
>  > +  if (debugfile)
>  > +    {
>  > +      printf_filtered ("loading separate debug info from '%s'\n", debugfile);
>  > +
>  > +      objfile->separate_debug_objfile = symbol_file_add (debugfile, from_tty, (addrs != NULL) ? &orig_addrs : NULL, 0, flags);
> 
> 
> Suggest using a simple conditional instead of a conditional expression.

Done.

>  > +    };
>  > +  unsigned char *end;
>  > +
>  > +  crc = ~crc & 0xffffffff;
>  > +  for (end = buf + len; buf < end; ++buf)
>  > +    crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
>  > +  return ~crc & 0xffffffff;;
>  > +}
>  > +
> 
> I don't think this function belongs in this file. It is general enough
> that it could be pushed into utils.c. Maybe even into some directory
> outside of gdb? It could be useful to other tools as well.

Put it in utils.c
 
> For the below, in theory is Ok, but I would suggest looking at the
> function openp() (in source.c) and how it is used in the solib_open()
> function in solib.c.  The tricky part is going to be assuring that the
> filename is portable, i.e. working on windows/dos systems where the
> directory separator is '\'. Look at IS_DIR_SEPARATOR, IS_ABSOLUTE_PATH
> in the libiberty directory.

I don't want to use openp() because i don't want the first availible file. 
I want the first one that matches the checksum. I can't see how i can use 
openp() to accomplish that.
 
> Once this is done, this change would need to be documented in the gdb
> manual, and a testcase added to the testsuite.

I really tried to make a testcase. I promise. I was just completely 
crushed by expect/dejagnu and the testsuite setup. I have a small piece of 
code that can be used with objcopy --add-section to generate a 
.gnu_debuglink section to test this. I tried it manually, and it works 
well. If anyone is willing to do the testsuite work I promise to help as 
much as I can.

--
--------------------------------------------------------------------------
 Alexander Larsson                                            Red Hat, Inc 
                   alexl@redhat.com    alla@lysator.liu.se 
He's an otherworldly drug-addicted firefighter trapped in a world he never 
made. She's a cynical Buddhist museum curator with the power to see death. 
They fight crime! 

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diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/acconfig.h gdb-5.2.1.separate_debug_symbols/gdb/acconfig.h
--- gdb-5.2.1/gdb/acconfig.h	Sat Jan  5 19:36:32 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/acconfig.h	Mon Sep 16 15:28:37 2002
@@ -173,3 +173,7 @@
 
 /* nativefile */
 #undef GDB_NM_FILE
+
+/* debugdir */
+#undef DEBUGDIR
+
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/acinclude.m4 gdb-5.2.1.separate_debug_symbols/gdb/acinclude.m4
--- gdb-5.2.1/gdb/acinclude.m4	Mon Apr 15 06:28:49 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/acinclude.m4	Mon Aug 26 11:16:40 2002
@@ -976,3 +976,18 @@
   *) CC="$CC $am_cv_prog_cc_stdc" ;;
 esac
 ])
+
+dnl written by Guido Draheim <guidod@gmx.de>, original by Alexandre Oliva 
+dnl Version 1.3 (2001/03/02)
+dnl source http://www.gnu.org/software/ac-archive/Miscellaneous/ac_define_dir.html
+
+AC_DEFUN([AC_DEFINE_DIR], [
+  test "x$prefix" = xNONE && prefix="$ac_default_prefix"
+  test "x$exec_prefix" = xNONE && exec_prefix='${prefix}'
+  ac_define_dir=`eval echo [$]$2`
+  ac_define_dir=`eval echo [$]ac_define_dir`
+  ifelse($3, ,
+    AC_DEFINE_UNQUOTED($1, "$ac_define_dir"),
+    AC_DEFINE_UNQUOTED($1, "$ac_define_dir", $3))
+])
+
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/configure.in gdb-5.2.1.separate_debug_symbols/gdb/configure.in
--- gdb-5.2.1/gdb/configure.in	Thu Mar 28 05:28:00 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/configure.in	Mon Aug 26 17:02:48 2002
@@ -234,6 +234,14 @@
 AC_CHECK_LIB(socket, socketpair)
 AC_CHECK_FUNCS(socketpair)
 
+debugdir=${libdir}/debug
+	 
+AC_ARG_WITH(separate-debug-dir,
+[  --with-separate-debug-dir=path   Look for global separate debug info in this path [LIBDIR/debug]],
+[debugdir="${withval}"])
+	
+AC_DEFINE_DIR(DEBUGDIR, debugdir)
+#AC_DEFINE_UNQUOTED(DEBUGDIR, "$debugdir"),
 
 BFD_NEED_DECLARATION(malloc)
 BFD_NEED_DECLARATION(realloc)
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/defs.h gdb-5.2.1.separate_debug_symbols/gdb/defs.h
--- gdb-5.2.1/gdb/defs.h	Mon Mar 25 17:50:20 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/defs.h	Mon Sep 23 12:37:23 2002
@@ -577,6 +577,8 @@
 
 extern char *gdb_realpath (const char *);
 
+extern unsigned long calc_crc32 (unsigned long crc, unsigned char *buf, size_t len);
+
 /* From demangle.c */
 
 extern void set_demangling_style (char *);
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/objfiles.c gdb-5.2.1.separate_debug_symbols/gdb/objfiles.c
--- gdb-5.2.1/gdb/objfiles.c	Thu Dec  6 21:59:11 2001
+++ gdb-5.2.1.separate_debug_symbols/gdb/objfiles.c	Mon Sep 23 13:07:24 2002
@@ -325,6 +325,30 @@
   return (objfile);
 }
 
+/* Put one object file before a specified on in the global list.
+   This can be used to make sure an object file is destroyed before
+   another when using ALL_OBJFILES_SAFE to free all objfiles. */
+void
+put_objfile_before (struct objfile *objfile, struct objfile *before_this)
+{
+  struct objfile **objp;
+
+  unlink_objfile (objfile);
+  
+  for (objp = &object_files; *objp != NULL; objp = &((*objp)->next))
+    {
+      if (*objp == before_this)
+	{
+	  objfile->next = *objp;
+	  *objp = objfile;
+	  return;
+	}
+    }
+  
+  internal_error (__FILE__, __LINE__,
+		  "put_objfile_before: before objfile not in list");
+}
+
 /* Put OBJFILE at the front of the list.  */
 
 void
@@ -397,6 +421,18 @@
 void
 free_objfile (struct objfile *objfile)
 {
+  if (objfile->separate_debug_objfile)
+    {
+      free_objfile (objfile->separate_debug_objfile);
+    }
+  
+  if (objfile->separate_debug_objfile_backlink)
+    {
+      /* We freed the separate debug file, make sure the base objfile
+	 doesn't reference it.  */
+      objfile->separate_debug_objfile_backlink->separate_debug_objfile = NULL;
+    }
+  
   /* First do any symbol file specific actions required when we are
      finished with a particular symbol file.  Note that if the objfile
      is using reusable symbol information (via mmalloc) then each of
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/objfiles.h gdb-5.2.1.separate_debug_symbols/gdb/objfiles.h
--- gdb-5.2.1/gdb/objfiles.h	Sat Jun 29 00:05:47 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/objfiles.h	Mon Sep 23 13:02:53 2002
@@ -409,6 +409,15 @@
     ExportEntry *export_list;
     int export_list_size;
 
+    /* Link to objfile that contains the debug symbols for this one.
+       One is loaded if this file has an debug link to an existing
+       debug file with the right checksum */
+    struct objfile *separate_debug_objfile;
+
+    /* If this is a separate debug object, this is used as a link to the
+       actual executable objfile. */
+    struct objfile *separate_debug_objfile_backlink;
+    
     /* Place to stash various statistics about this objfile */
       OBJSTATS;
   };
@@ -500,6 +509,8 @@
 
 extern int build_objfile_section_table (struct objfile *);
 
+extern void put_objfile_before (struct objfile *, struct objfile *);
+
 extern void objfile_to_front (struct objfile *);
 
 extern void unlink_objfile (struct objfile *);
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/symfile.c gdb-5.2.1.separate_debug_symbols/gdb/symfile.c
--- gdb-5.2.1/gdb/symfile.c	Sat Jun 22 18:49:33 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/symfile.c	Mon Sep 23 14:49:05 2002
@@ -160,6 +160,8 @@
 
 static void info_ext_lang_command (char *args, int from_tty);
 
+static char *find_separate_debug_file (struct objfile *objfile, const char *name);
+
 static void init_filename_language_table (void);
 
 void _initialize_symfile (void);
@@ -836,7 +838,12 @@
 {
   struct objfile *objfile;
   struct partial_symtab *psymtab;
+  char *debugfile;
   bfd *abfd;
+  struct section_addr_info orig_addrs;
+  
+  if (addrs)
+    orig_addrs = *addrs;
 
   /* Open a bfd for the file, and give user a chance to burp if we'd be
      interactively wiping out any existing symbols.  */
@@ -926,6 +933,28 @@
   if (target_new_objfile_hook)
     target_new_objfile_hook (objfile);
 
+  debugfile = find_separate_debug_file (objfile, name);
+  if (debugfile)
+    {
+      printf_filtered ("loading separate debug info from '%s'\n", debugfile);
+
+      if (addrs != NULL)
+	{
+	  objfile->separate_debug_objfile = symbol_file_add (debugfile, from_tty, &orig_addrs, 0, flags);
+	}
+      else
+	{
+	  objfile->separate_debug_objfile = symbol_file_add (debugfile, from_tty, NULL, 0, flags);
+	}
+      objfile->separate_debug_objfile->separate_debug_objfile_backlink = objfile;
+      
+      /* Put the separate debug object before the normal one, this is so that
+         usage of the ALL_OBJFILES_SAFE macro will stay safe. */
+      put_objfile_before (objfile->separate_debug_objfile, objfile);
+      
+      xfree (debugfile);
+    }
+  
   return (objfile);
 }
 
@@ -985,6 +1014,146 @@
 #endif
 }
 
+static char *
+get_debug_link_info (struct objfile *objfile, unsigned long *crc32_out)
+{
+  asection *sect;
+  bfd_size_type debuglink_size;
+  unsigned long crc32;
+  char *contents;
+  int crc_offset;
+  unsigned char *crc_data;
+  unsigned char *p;
+  
+  sect = bfd_get_section_by_name (objfile->obfd, ".gnu_debuglink");
+
+  if (sect == NULL)
+    return NULL;
+
+  debuglink_size = bfd_section_size (objfile->obfd, sect);
+  
+  contents = xmalloc (debuglink_size);
+  bfd_get_section_contents (objfile->obfd, sect, contents,
+			    (file_ptr)0, (bfd_size_type)debuglink_size);
+
+  /* Crc value is stored after the filename, aligned up to 4 bytes. */
+  crc_offset = strlen (contents) + 1;
+  if (crc_offset & 3)
+    crc_offset += 4 - (crc_offset & 3);
+  crc_data = contents + crc_offset;
+
+  crc32 = 0;
+  if (TARGET_BYTE_ORDER == BFD_ENDIAN_BIG)
+    {
+      for (p = crc_data; p < crc_data + 4; ++p)
+	crc32 = (crc32 << 8) | *p;
+    }
+  else
+    {
+      for (p = crc_data + 4 - 1; p >= crc_data; --p)
+	crc32 = (crc32 << 8) | *p;
+    }
+  
+  *crc32_out = crc32;
+  return contents;
+}
+
+static int
+separate_debug_file_exists (const char *name, unsigned long crc)
+{
+  unsigned long file_crc = 0;
+  int fd;
+  char buffer[8*1024];
+  int count;
+
+  fd = open (name, O_RDONLY);
+  if (fd < 0)
+    return 0;
+
+  while ((count = read (fd, buffer, sizeof (buffer))) > 0)
+    file_crc = calc_crc32 (file_crc, buffer, count);
+
+  close (fd);
+
+  return crc == file_crc;
+}
+
+static char *debug_file_search_path = NULL;
+
+static char *
+find_separate_debug_file (struct objfile *objfile, const char *name)
+{
+  asection *sect;
+  char *basename;
+  char *dir;
+  char *debugfile;
+  char *name_copy;
+  bfd_size_type debuglink_size;
+  unsigned long crc32;
+  unsigned char *p;
+
+  basename = get_debug_link_info (objfile, &crc32);
+
+  if (basename == NULL)
+    return NULL;
+  
+  dir = xstrdup (name);
+  p = strrchr (dir, '/');
+  if (p != NULL)
+    {
+      *(p+1) = 0;
+    }
+  else
+    {
+      *dir = 0;
+    }
+
+  debugfile = alloca (strlen (debug_file_search_path) + 1 +
+		      strlen (dir) + strlen (".debug/") + strlen (basename) + 1);
+
+  /* First try in the same directory as the original file: */
+  strcpy (debugfile, dir);
+  strcat (debugfile, basename);
+
+  if (separate_debug_file_exists (debugfile, crc32))
+    {
+      xfree (basename);
+      xfree (dir);
+      return xstrdup (debugfile);
+    }
+  
+  /* Then try in a subdirectory called .debug */
+  strcpy (debugfile, dir);
+  strcat (debugfile, ".debug/");
+  strcat (debugfile, basename);
+
+  if (separate_debug_file_exists (debugfile, crc32))
+    {
+      xfree (basename);
+      xfree (dir);
+      return xstrdup (debugfile);
+    }
+  
+  /* Then try in the global debugfile directory */
+  strcpy (debugfile, debug_file_search_path);
+  strcat (debugfile, "/");
+  strcat (debugfile, dir);
+  strcat (debugfile, "/");
+  strcat (debugfile, basename);
+
+  if (separate_debug_file_exists (debugfile, crc32))
+    {
+      xfree (basename);
+      xfree (dir);
+      return xstrdup (debugfile);
+    }
+  
+  xfree (basename);
+  xfree (dir);
+  return NULL;
+}
+
+
 /* This is the symbol-file command.  Read the file, analyze its
    symbols, and add a struct symtab to a symtab list.  The syntax of
    the command is rather bizarre--(1) buildargv implements various
@@ -1639,6 +1808,8 @@
   long new_modtime;
   int reread_one = 0;
   struct stat new_statbuf;
+  char *debug_file;
+  unsigned long crc32;
   int res;
 
   /* With the addition of shared libraries, this should be modified,
@@ -1822,6 +1993,30 @@
 	         needs to keep track of (such as _sigtramp, or whatever).  */
 
 	      TARGET_SYMFILE_POSTREAD (objfile);
+
+	      /* Verify the checksum of a previously loaded separate debug file. */
+	      if (objfile->separate_debug_objfile)
+		{
+		  printf ("reread sep debug objfile\n");
+		  debug_file = get_debug_link_info (objfile, &crc32);
+		  if (debug_file)
+		    {
+		      if (!separate_debug_file_exists (objfile->separate_debug_objfile->name, crc32))
+			{
+			  printf ("removing invalid separate debug symbols\n");
+			  /* The old separate debug symbols file is invalid */
+			  free_objfile (objfile->separate_debug_objfile);
+			}
+		      xfree (debug_file);
+		    }
+		  else
+		    {
+		      printf ("removing debug symbols, since i found no debug link\n");
+		      /* No more debug link, kill old separate debug symbols */
+		      free_objfile (objfile->separate_debug_objfile);
+		    }
+		}
+	      
 	    }
 	}
     }
@@ -3336,4 +3531,17 @@
 		  "cache.\n",
 		  &setlist),
      &showlist);
+
+  debug_file_search_path = xstrdup (DEBUGDIR);
+  c = add_set_cmd ("debug-file-search-path", class_support, var_string,
+		   (char *) &debug_file_search_path,
+		   "Set the search path for loading separate debug symbols.\n"
+		   "Separate debug symbols are first searched for in the same\n"
+		   "directory as the binary, then in the .debug subdirectory,\n"
+		   "and lastly at the path of the directory of the binary with\n"
+		   "the global debug-file search path prepended\n",
+		   &setlist);
+  add_show_from_set (c, &showlist);
+  c->completer = filename_completer;
+
 }
diff --exclude '*~' --exclude '*.orig' --exclude '*.rej' --exclude '.#*' --exclude '*.o' --exclude CVS --exclude SCCS --exclude RCS --exclude '.*.flags' --exclude .cvsignore --exclude .depend -ur gdb-5.2.1/gdb/utils.c gdb-5.2.1.separate_debug_symbols/gdb/utils.c
--- gdb-5.2.1/gdb/utils.c	Thu Mar 28 05:28:01 2002
+++ gdb-5.2.1.separate_debug_symbols/gdb/utils.c	Mon Sep 23 12:37:05 2002
@@ -2563,3 +2563,69 @@
   return xstrdup (filename);
 #endif
 }
+
+unsigned long
+calc_crc32 (unsigned long crc, unsigned char *buf, size_t len)
+{
+  static const unsigned long crc32_table[256] =
+    {
+      0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419,
+      0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4,
+      0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07,
+      0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
+      0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856,
+      0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
+      0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4,
+      0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
+      0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3,
+      0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a,
+      0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599,
+      0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
+      0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190,
+      0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f,
+      0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e,
+      0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
+      0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed,
+      0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
+      0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3,
+      0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
+      0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a,
+      0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5,
+      0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010,
+      0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
+      0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17,
+      0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6,
+      0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615,
+      0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
+      0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344,
+      0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
+      0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a,
+      0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
+      0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1,
+      0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c,
+      0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef,
+      0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
+      0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe,
+      0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31,
+      0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c,
+      0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
+      0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b,
+      0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
+      0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1,
+      0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
+      0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278,
+      0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7,
+      0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66,
+      0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
+      0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605,
+      0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8,
+      0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b,
+      0x2d02ef8d
+    };
+  unsigned char *end;
+
+  crc = ~crc & 0xffffffff;
+  for (end = buf + len; buf < end; ++buf)
+    crc = crc32_table[(crc ^ *buf) & 0xff] ^ (crc >> 8);
+  return ~crc & 0xffffffff;;
+}