cgo: implement C unions

Unions are somewhat hard to implement in Go because they are not a
native type. But it is actually possible with some compiler magic.

This commit inserts a special "C union" field at the start of a struct
to indicate that it is a union. As such a field cannot be written
directly in Go, this is a useful to distinguish structs and unions.

testdata/cgo/main.c

@@ -8,8 +8,11 @@ double globalDouble = 3.2;
 _Complex float globalComplexFloat = 4.1+3.3i;
 _Complex double globalComplexDouble = 4.2+3.4i;
 _Complex double globalComplexLongDouble = 4.3+3.5i;
-collection_t globalStruct = {256, -123456, 3.14};
+collection_t globalStruct = {256, -123456, 3.14, 88};
+int globalStructSize = sizeof(globalStruct);
 short globalArray[3] = {5, 6, 7};
+joined_t globalUnion;
+int globalUnionSize = sizeof(globalUnion);
 
 int fortytwo() {
 	return 42;
@@ -26,3 +29,17 @@ int doCallback(int a, int b, binop_t callback) {
 void store(int value, int *ptr) {
 	*ptr = value;
 }
+
+void unionSetShort(short s) {
+	globalUnion.s = s;
+}
+
+void unionSetFloat(float f) {
+	globalUnion.f = f;
+}
+
+void unionSetData(short f0, short f1, short f2) {
+	globalUnion.data[0] = 5;
+	globalUnion.data[1] = 8;
+	globalUnion.data[2] = 1;
+}

testdata/cgo/main.go

@@ -9,6 +9,10 @@ import "C"
 
 import "unsafe"
 
+func (s C.myint) Int() int {
+	return int(s)
+}
+
 func main() {
 	println("fortytwo:", C.fortytwo())
 	println("add:", C.add(C.int(3), 5))
@@ -36,9 +40,28 @@ func main() {
 	println("complex float:", C.globalComplexFloat)
 	println("complex double:", C.globalComplexDouble)
 	println("complex long double:", C.globalComplexLongDouble)
-	println("struct:", C.globalStruct.s, C.globalStruct.l, C.globalStruct.f)
+
+	// complex types
+	println("struct:", C.int(unsafe.Sizeof(C.globalStruct)) == C.globalStructSize, C.globalStruct.s, C.globalStruct.l, C.globalStruct.f)
 	var _ [3]C.short = C.globalArray
 	println("array:", C.globalArray[0], C.globalArray[1], C.globalArray[2])
+	println("union:", C.int(unsafe.Sizeof(C.globalUnion)) == C.globalUnionSize)
+	C.unionSetShort(22)
+	println("union s:", C.globalUnion.s)
+	C.unionSetFloat(3.14)
+	println("union f:", C.globalUnion.f)
+	C.unionSetData(5, 8, 1)
+	println("union global data:", C.globalUnion.data[0], C.globalUnion.data[1], C.globalUnion.data[2])
+	println("union field:", printUnion(C.globalUnion).f)
+}
+
+func printUnion(union C.joined_t) C.joined_t {
+	println("union local data: ", union.data[0], union.data[1], union.data[2])
+	union.s = -33
+	println("union s method:", union.s.Int(), union.data[0] == 5)
+	union.f = 6.28
+	println("union f:", union.f)
+	return union
 }
 
 //export mul

testdata/cgo/main.h

@@ -6,11 +6,21 @@ typedef int * intPointer;
 void store(int value, int *ptr);
 
 typedef struct collection {
-	short s;
-	long  l;
-	float f;
+	short         s;
+	long          l;
+	float         f;
+	unsigned char c;
 } collection_t;
 
+typedef union joined {
+	myint s;
+	float f;
+	short data[3];
+} joined_t;
+void unionSetShort(short s);
+void unionSetFloat(float f);
+void unionSetData(short f0, short f1, short f2);
+
 // test globals
 extern int global;
 extern _Bool globalBool;
@@ -21,7 +31,10 @@ extern _Complex float globalComplexFloat;
 extern _Complex double globalComplexDouble;
 extern _Complex double globalComplexLongDouble;
 extern collection_t globalStruct;
+extern int globalStructSize;
 extern short globalArray[3];
+extern joined_t globalUnion;
+extern int globalUnionSize;
 
 // test duplicate definitions
 int add(int a, int b);

testdata/cgo/out.txt

@@ -14,3 +14,13 @@ double: +3.200000e+000
 complex float: (+4.100000e+000+3.300000e+000i)
 complex double: (+4.200000e+000+3.400000e+000i)
 complex long double: (+4.300000e+000+3.500000e+000i)
+struct: true 256 -123456 +3.140000e+000
+array: 5 6 7
+union: true
+union s: 22
+union f: +3.140000e+000
+union global data: 5 8 1
+union local data:  5 8 1
+union s method: -33 false
+union f: +6.280000e+000
+union field: +6.280000e+000