A C function may be declared in the Go file with a parameter type of
the special name _GoString_. This function may be called with an
ordinary Go string value. The string length, and a pointer to the
string contents, may be accessed by calling the C functions

	size_t _GoStringLen(_GoString_ s);
	const char *_GoStringPtr(_GoString_ s);

These functions are only available in the preamble, not in other C