[jckarter]

The language in the standard may have changed in C99 due to unprototyped functions being deprecated. In C89, however, I'm pretty sure that unprototyped and prototyped functions need to have compatible calling conventions. In practice, there's no platform I know of where a prototyped function cannot be called as a variadic or unprototyped function (barring special platform-specific ABIs like stdcall that weren't designed for C). Some RISC-style ABIs with pass-by-register and shadow space will fail in the reverse case (a variadic function called as a fully prototyped function) since the shadow space doesn't get populated by the caller for a fully prototyped function, but for a variadic call site, both the shadow space and parameter registers will be populated, specifically to allow prototyped functions to be called as unprototyped.