Make GUID generation on POSIX compliant with GUID v4 format.

base/guid_posix.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/guid.h"
 
 #include "base/rand_util.h"
 #include "base/strings/stringprintf.h"
 
 namespace base {
 
 std::string GenerateGUID() {
   uint64 sixteen_bytes[2] = { base::RandUint64(), base::RandUint64() };
+
+  // Set the GUID to version 4 as described in RFC 4122, section 4.4.
+  // The format of GUID version 4 must be xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx,
+  // where y is one of [8, 9, A, B].
+
+  // Clear the version bits and set the version to 4:
+  sixteen_bytes[0] &= 0xffffffffffff0fffULL;
+  sixteen_bytes[0] |= 0x0000000000004000ULL;
+
+  // Set the two most significant bits (bits 6 and 7) of the
+  // clock_seq_hi_and_reserved to zero and one, respectively:
+  sixteen_bytes[1] &= 0x3fffffffffffffffULL;
+  sixteen_bytes[1] |= 0x8000000000000000ULL;
+
   return RandomDataToGUIDString(sixteen_bytes);
 }
 
 // TODO(cmasone): Once we're comfortable this works, migrate Windows code to
 // use this as well.
 std::string RandomDataToGUIDString(const uint64 bytes[2]) {
   return StringPrintf("%08X-%04X-%04X-%04X-%012llX",
                       static_cast<unsigned int>(bytes[0] >> 32),
                       static_cast<unsigned int>((bytes[0] >> 16) & 0x0000ffff),
                       static_cast<unsigned int>(bytes[0] & 0x0000ffff),
                       static_cast<unsigned int>(bytes[1] >> 48),
                       bytes[1] & 0x0000ffffffffffffULL);
 }
 
 }  // namespace guid