Fix OS::VSNPrint to make it behave similar to linux and macos versions.

runtime/vm/os_win.cc

Index: runtime/vm/os_win.cc
===================================================================
--- runtime/vm/os_win.cc	(revision 6377)
+++ runtime/vm/os_win.cc	(working copy)
@@ -161,13 +161,20 @@
 
 
 int OS::VSNPrint(char* str, size_t size, const char* format, va_list args) {
+  if (str == NULL || size == 0) {
+    return _vscprintf(format, args);
+  }
   int written =_vsnprintf(str, size, format, args);
+  if (written < 0) {
+    // _vsnprintf returns -1 if the number of characters to be written is
+    // larger than 'size', so we call _vscprintf which returns the number
+    // of characters that would have been written.
+    written = _vscprintf(format, args);

[Ivan Posva, 2012/04/11 07:34:39]

You are reusing the args here which is a big NO-NO. It tends to work on ia32,
but starts breaking for other architectures such as x64.

You need to use va_copy, which unfortunately is not available in older versions
of MSVC.

Other suggestions: http://stackoverflow.com/questions/2288680/reuse-of-va-list
or http://stackoverflow.com/questions/1259280/c-varargs-va-copy-issues

[siva, 2012/04/11 23:12:51]

Good point. I have uploaded a CL doing the va_copy stuff.

On 2012/04/11 07:34:39, Ivan Posva wrote:

+  }
   // Make sure to zero-terminate the string if the output was
   // truncated or if there was an error.
-  if (written < 0 || written >= size) {
-    if (size > 0) {
-      str[size - 1] = '\0';
-    }
+  if (written >= size) {
+    str[size - 1] = '\0';
   }
   return written;
 }