cmd/cgo: explicitly use void for functions with no parameters

Currently, exported Go functions with no parameters generate C functions
with an empty parameter list. In C, a function with an empty parameter
list can accept any number of arguments, whereas a function with a single
void parameter explicitly declares that it takes no arguments.

To align the generated C functions with their Go prototypes, update the
code generation to explicitly include a void parameter for functions
with no parameters.

Fixes #68411

Change-Id: Iab9456aa0236200bf21d1181a2e18e82869df63f
GitHub-Last-Rev: 6ff21a98df
GitHub-Pull-Request: golang/go#70981
Reviewed-on: https://go-review.googlesource.com/c/go/+/638635
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/cgo/internal/testcshared/cshared_test.go

@@ -880,3 +880,44 @@ func TestIssue36233(t *testing.T) {
 		t.Error("missing functions")
 	}
 }
+
+func TestIssue68411(t *testing.T) {
+	globalSkip(t)
+	testenv.MustHaveCGO(t)
+
+	t.Parallel()
+
+	// Test that the export header uses a void function parameter for
+	// exported Go functions with no parameters.
+
+	tmpdir := t.TempDir()
+
+	const exportHeader = "issue68411.h"
+
+	run(t, nil, "go", "tool", "cgo", "-exportheader", exportHeader, "-objdir", tmpdir, "./issue68411/issue68411.go")
+	data, err := os.ReadFile(exportHeader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	funcs := []struct{ name, signature string }{
+		{"exportFuncWithNoParams", "void exportFuncWithNoParams(void)"},
+		{"exportFuncWithParams", "exportFuncWithParams(GoInt a, GoInt b)"},
+	}
+
+	var found int
+	for line := range bytes.Lines(data) {
+		for _, fn := range funcs {
+			if bytes.Contains(line, []byte(fn.name)) {
+				found++
+				if !bytes.Contains(line, []byte(fn.signature)) {
+					t.Errorf("function signature mismatch; got %q, want %q", line, fn.signature)
+				}
+			}
+		}
+	}
+
+	if found != len(funcs) {
+		t.Error("missing functions")
+	}
+}