func withVaList(_:_:)(_ args: [CVarArg], _ body: (CVaListPointer) -> R) -> R Invokes the given closure with a C `va_list` argument derived from the given array of arguments. The pointer passed as an argument to body is valid only during the execution of withVaList(_:_:). Do not store or return the pointer for later use. If you need to pass an optional pointer as a CVarArg argument, use the Int(bitPattern:) initializer to interpret the optional pointer as an Int value, which has the same C variadic calling conventions as a pointer on all supported platforms.