Question

I have a variable-argument function in C that looks roughly like this:

void log(const char * format, ...) {

  va_list args;
  va_start(args, format);
  vfprintf( stderr, format, args );
  va_end(args);
  exit(1);
}

I was able crash my app by callilng it like this,

log("%s %d", 1);

because the function was missing an argument. Is there a way to determine an argument is missing at runtime?

Answer 1

I don’t believe there would be any standard mechanism for determining that at runtime. The parameters after the format specifier are simply values on the stack. For example, if a format specifier indicated a 4-byte integer was next, there would be no way of knowing if the next 4 bytes on the stack were an integer or just whatever happened to be on the stack from a previous call.