Crash when using QString and va_list under x86 (x64 works!)

Justin Sayne

Does anyone have a moderate idea why the following works perfectly fine and as expected under x64 but crashes on the line indicated under x86? Having a look at argp in the debugger, argp already seems wrong because it contains character -52 like 100 times in a row whereas under x64 it has just around 8 reasonable and positive characters.
Is there a work-around where I possibly don't have to change the method signature?

#include <QString>

static void test(const QString& format, ...)
{
  static char msg[2048];
  
  va_list argp;
  va_start(argp, format);
  // crashes here under x86
  vsnprintf(msg, 2048, format.toStdString().c_str(), argp);
  va_end(argp);
  printf("%s", msg);
}

int main()
{
  test("teststring: %s", "foobar");
  return 0;
}

The funny thing is that this also works perfectly fine under both configurations...

static void test(const char* format, ...)
{
  static char msg[2048];
  
  va_list argp;
  va_start(argp, format);
  vsnprintf(msg, 2048, format, argp);
  va_end(argp);
  printf("%s", msg);
}

int main()
{
  QString qstr = "teststring: %s";
  test(qstr.toUtf8().data(), "foobar");
  return 0;
}

KeithS

You can't pass a reference to the format for va_start(). Use 'const QString format' instead of 'const QString& format'.

Justin Sayne

@KeithS
Wow, thanks. After knowing the keyword I finally found the explanation and it completely makes sense. I'm now just wondering why it worked perfectly fine under x64, and not even by accident because this is a log function of our framework so it's used all over the place.