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.

Change-Id: I6d813815228efda95a7a6a9bbf9c9a787ff4f420

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

@@ -880,3 +880,52 @@ 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, err := os.MkdirTemp("", "cshared-TestIssue68411")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	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", "extern void exportFuncWithNoParams(void)"},
+		{"exportFuncWithParams", "extern void exportFuncWithParams(GoInt a, GoInt b)"},
+	}
+
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	var found int
+	for scanner.Scan() {
+		b := scanner.Bytes()
+		for _, fn := range funcs {
+			if bytes.Contains(b, []byte(fn.name)) {
+				found++
+				if !bytes.Contains(b, []byte(fn.signature)) {
+					t.Errorf("function signature mismatch; got %q, want %q", b, fn.signature)
+				}
+			}
+		}
+	}
+	if err = scanner.Err(); err != nil {
+		t.Errorf("scanner encountered error: %v", err)
+	}
+	if found != len(funcs) {
+		t.Error("missing functions")
+	}
+}

src/cmd/cgo/internal/testcshared/testdata/issue68411/issue68411.go

@@ -0,0 +1,15 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "C"
+
+//export exportFuncWithNoParams
+func exportFuncWithNoParams() {}
+
+//export exportFuncWithParams
+func exportFuncWithParams(a, b int) {}
+
+func main() {}

src/cmd/cgo/out.go

@@ -1012,13 +1012,18 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 			s += p.cgoType(fn.Recv.List[0].Type).C.String()
 			s += " recv"
 		}
-		forFieldList(fntype.Params,
-			func(i int, aname string, atype ast.Expr) {
-				if i > 0 || fn.Recv != nil {
-					s += ", "
-				}
-				s += fmt.Sprintf("%s %s", p.cgoType(atype).C, exportParamName(aname, i))
-			})
+
+		if len(fntype.Params.List) > 0 {
+			forFieldList(fntype.Params,
+				func(i int, aname string, atype ast.Expr) {
+					if i > 0 || fn.Recv != nil {
+						s += ", "
+					}
+					s += fmt.Sprintf("%s %s", p.cgoType(atype).C, exportParamName(aname, i))
+				})
+		} else {
+			s += "void"
+		}
 		s += ")"
 
 		if len(exp.Doc) > 0 {