switch fname {
		case "goCallback":
			// TODO(qmuntal): investigate why this function doesn't appear
			// when using the external linker.
			continue
		}
		// In module mode, this package has a fully-qualified import path.
		// Remove it if present.
		fname = strings.TrimPrefix(fname, "cmd/cgo/internal/")
		if !strings.HasPrefix(fname, "test.") {
			continue
		}
		got = append(got, fname)
	}

additional context for the C file.  Here it is just a single #include
but it could contain arbitrary C definitions to be imported and used.

Cgo recognizes any use of a qualified identifier C.xxx and uses gcc to
find the definition of xxx.  If xxx is a type, cgo replaces C.xxx with
a Go translation.  C arithmetic types translate to precisely-sized Go