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

Fix OS::VSNPrint to make it behave similar to linux and macos versions.
Mathias found this bug will using it in the JSON parser.
Committed: https://code.google.com/p/dart/source/detail?r=6380

[hausner, 2012-04-10 20:40:04]

lgtm w/comment.

Thank you for fixing this.

https://chromiumcodereview.appspot.com/10037018/diff/1/runtime/vm/os_win.cc
File runtime/vm/os_win.cc (right):

https://chromiumcodereview.appspot.com/10037018/diff/1/runtime/vm/os_win.cc#n...
runtime/vm/os_win.cc:177: if (size > 0) {
The size > 0 check is no longer necessary since we handle size == 0 at the
beginning.

[siva, 2012-04-10 20:42:25]

https://chromiumcodereview.appspot.com/10037018/diff/1/runtime/vm/os_win.cc
File runtime/vm/os_win.cc (right):

https://chromiumcodereview.appspot.com/10037018/diff/1/runtime/vm/os_win.cc#n...
runtime/vm/os_win.cc:177: if (size > 0) {
On 2012/04/10 20:40:04, hausner wrote:
> The size > 0 check is no longer necessary since we handle size == 0 at the
> beginning.

Done.

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

https://chromiumcodereview.appspot.com/10037018/diff/2001/runtime/vm/os_win.cc
File runtime/vm/os_win.cc (right):

https://chromiumcodereview.appspot.com/10037018/diff/2001/runtime/vm/os_win.c...
runtime/vm/os_win.cc:172: written = _vscprintf(format, args);
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]

https://chromiumcodereview.appspot.com/10037018/diff/2001/runtime/vm/os_win.cc
File runtime/vm/os_win.cc (right):

https://chromiumcodereview.appspot.com/10037018/diff/2001/runtime/vm/os_win.c...
runtime/vm/os_win.cc:172: written = _vscprintf(format, args);
Good point. I have uploaded a CL doing the va_copy stuff.

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