// compiled by gcc, into foo.cgo2.o

	void
	_cgo_gcc_Cfunc_sin(void *v)
	{
		struct {
			double p0;
			double r;
		} __attribute__((__packed__)) *a = v;
		a->r = sin(a->p0);
	}

What happens at link time depends on whether the final binary is linked
using the internal or external mode. If other packages are compiled in
"external only" mode, then the final link will be an external one.