misc/cgo: Add test for C union read/write

For #39537

Author: howjmay

misc/cgo/test/cgo_test.go

@@ -79,6 +79,7 @@ func TestCthread(t *testing.T)               { testCthread(t) }
 func TestEnum(t *testing.T)                  { testEnum(t) }
 func TestNamedEnum(t *testing.T)             { testNamedEnum(t) }
 func TestCastToEnum(t *testing.T)            { testCastToEnum(t) }
+func TestUnion(t *testing.T)                 { testUnion(t) }
 func TestErrno(t *testing.T)                 { testErrno(t) }
 func TestFpVar(t *testing.T)                 { testFpVar(t) }
 func TestHandle(t *testing.T)                { testHandle(t) }

misc/cgo/test/test.go

@@ -95,6 +95,11 @@ enum E {
 	Enum2 = 2,
 };
 
+union U {
+	float f32;
+	uint64_t u64;
+};
+
 typedef unsigned char cgo_uuid_t[20];
 
 void uuid_generate(cgo_uuid_t x) {
@@ -1034,6 +1039,30 @@ func testCastToEnum(t *testing.T) {
 	}
 }
 
+func testUnion(t *testing.T) {
+	var u C.union_U
+	byteU := (*byte)(unsafe.Pointer(&u))
+
+	// Union U has a field whose variable type is uint64_t, so the size of this union is 8 bytes
+	if unsafe.Sizeof(byteU) != 8 {
+		t.Error("wrong union size", unsafe.Sizeof(byteU))
+	}
+
+	goU64 := (*uint64)(unsafe.Pointer(&u))
+	*goU64 = math.MaxUint64
+
+	if *goU64 != math.MaxUint64 {
+		t.Error("failed to assign value to union", *goU64)
+	}
+
+	goF32 := (*float32)(unsafe.Pointer(&u))
+	*goF32 = 1.0
+
+	if *goF32 != 1.0 {
+		t.Error("failed to assign value to union", *goF32)
+	}
+}
+
 func testAtol(t *testing.T) {
 	l := Atol("123")
 	if l != 123 {