Add compiler-specific "examine printf format" attributes to printfs.

base/format_macros.h

 // Copyright (c) 2009 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.
 
 #ifndef BASE_FORMAT_MACROS_H_
 #define BASE_FORMAT_MACROS_H_
 
-// This file defines the C99 format macros for 64-bit values. If you wish to
-// print a 64-bit value in a portable way do:
+// This file defines the format macros for some integer types.
+
+// To print a 64-bit value in a portable way:
 //   int64_t value;
 //   printf("xyz:%" PRId64, value);
+// The "d" in the macro corresponds to %d; you can also use PRIu64 etc.
 //
 // For wide strings, prepend "Wide" to the macro:
 //   int64_t value;
 //   StringPrintf(L"xyz: %" WidePRId64, value);
+//
+// To print a size_t value in a portable way:
+//   size_t size;
+//   printf("xyz: %" PRIuS, size);
+// The "u" in the macro corresponds to %u, and S is for "size".
 
 #include "build/build_config.h"
 
 #if defined(OS_POSIX)
 
 #if (defined(_INTTYPES_H) || defined(_INTTYPES_H_)) && !defined(PRId64)
 #error "inttypes.h has already been included before this header file, but "
 #error "without __STDC_FORMAT_MACROS defined."
 #endif
 
 #if !defined(__STDC_FORMAT_MACROS)
 #define __STDC_FORMAT_MACROS
 #endif
 
 #include <inttypes.h>
 
 // GCC will concatenate wide and narrow strings correctly, so nothing needs to
 // be done here.
 #define WidePRId64 PRId64
 #define WidePRIu64 PRIu64
 #define WidePRIx64 PRIx64
 
+#define PRIuS "zu"
+
 #else  // OS_WIN
 
 #if !defined(PRId64)
 #define PRId64 "I64d"
 #endif
 
 #if !defined(PRIu64)
 #define PRIu64 "I64u"
 #endif
 
 #if !defined(PRIx64)
 #define PRIx64 "I64x"
 #endif
 
 #define WidePRId64 L"I64d"
 #define WidePRIu64 L"I64u"
 #define WidePRIx64 L"I64x"
 
+#define PRIuS "Iu"
+
 #endif
 
 #endif  // !BASE_FORMAT_MACROS_H_