purego: fix returning a C function pointer as a Go func (#500)

The RegisterFunc type table has func <=> C function in it, but that
only holds in the argument direction. Declare a C function as returning
a func, and calling it panics:

panic: reflect.MakeFunc: value of type *func(int32, int32) int32 is not
assignable to type func(int32, int32) int32

Registering is fine, it's the call that goes, and it goes for every
signature. Every other case in that return switch leaves the result as
the declared type. The reflect.Func case swapped in a *func, registered
into that, and handed the pointer back to MakeFunc, which wont take it.

If the C function returns NULL the inner RegisterFunc still panics
purego: cfn is nil. Left that alone, since whether it should hand you a
nil Go func instead looked like your call rather than a bug.

Test is a block in TestABI plus a return_func_ptr helper in abi_test.c,
and it fails on main with teh panic above. Been broken since #49 back
in 2022, which is probably why nobody's hit it: you'd normally take the
pointer back as a uintptr and register it youself, and nothing in the
repo declares a func return.

Closes #501

commit: a66c0b0d41190bc8a18e87f30da7d18453d7d40a
author: Iain · 59d ago · 2026-08-24 12:41 UTC
committer: GitHub · 59d ago · 2026-08-24 12:41 UTC
parent: f2f4202
signature: GPG UnverifiedB5690EEEBB952194 The key is unknown

3 files · +35 -2

M func.go +3 -2


@@ -409,8 +409,9 @@ func RegisterFunc(fptr any, cfn uintptr) {
409	409		v = reflect.NewAt(outType, unsafe.Pointer(&a1)).Elem()
410	410		case reflect.Func:
411	411		// wrap this C function in a nicely typed Go function
412		-	v = reflect.New(outType)
413		-	RegisterFunc(v.Interface(), syscall.a1)
	412	+	if syscall.a1 != 0 {
	413	+	RegisterFunc(v.Addr().Interface(), syscall.a1)
	414	+	}
414	415		case reflect.String:
415	416		v.SetString(strings.GoString(syscall.a1))
416	417		case reflect.Float32:

 			v = reflect.NewAt(outType, unsafe.Pointer(&a1)).Elem()
 		case reflect.Func:
 			// wrap this C function in a nicely typed Go function
-			v = reflect.New(outType)
+			if syscall.a1 != 0 {
-			RegisterFunc(v.Interface(), syscall.a1)
+				RegisterFunc(v.Addr().Interface(), syscall.a1)
+			}
 		case reflect.String:
 			v.SetString(strings.GoString(syscall.a1))
 		case reflect.Float32:

M func_test.go +18 -0


@@ -239,6 +239,24 @@ func TestABI(t *testing.T) {
239	239		t.Fatalf("%s: got %q, want %q", cName, res, want)
240	240		}
241	241		}
	242	+	{
	243	+	const cName = "return_func_ptr"
	244	+	var fn func() func(a, b int32) int32
	245	+	purego.RegisterLibFunc(&fn, lib, cName)
	246	+	add := fn()
	247	+	const expect = 5
	248	+	if res := add(2, 3); res != expect {
	249	+	t.Fatalf("%s: got %d, want %d", cName, res, expect)
	250	+	}
	251	+	}
	252	+	{
	253	+	const cName = "return_null_func_ptr"
	254	+	var fn func() func(a, b int32) int32
	255	+	purego.RegisterLibFunc(&fn, lib, cName)
	256	+	if fn() != nil {
	257	+	t.Fatalf("%s: got a non-nil func, want nil", cName)
	258	+	}
	259	+	}
242	260		}
243	261
244	262		func TestABI_ArgumentPassing(t *testing.T) {


@@ -239,6 +239,24 @@ func TestABI(t *testing.T) {
239	t.Fatalf("%s: got %q, want %q", cName, res, want)	239		t.Fatalf("%s: got %q, want %q", cName, res, want)
240	}	240		}
241	}	241		}
		242	+	{
		243	+	const cName = "return_func_ptr"
		244	+	var fn func() func(a, b int32) int32
		245	+	purego.RegisterLibFunc(&fn, lib, cName)
		246	+	add := fn()
		247	+	const expect = 5
		248	+	if res := add(2, 3); res != expect {
		249	+	t.Fatalf("%s: got %d, want %d", cName, res, expect)
		250	+	}
		251	+	}
		252	+	{
		253	+	const cName = "return_null_func_ptr"
		254	+	var fn func() func(a, b int32) int32
		255	+	purego.RegisterLibFunc(&fn, lib, cName)
		256	+	if fn() != nil {
		257	+	t.Fatalf("%s: got a non-nil func, want nil", cName)
		258	+	}
		259	+	}
242	}	260		}
243		261
244	func TestABI_ArgumentPassing(t *testing.T) {	262		func TestABI_ArgumentPassing(t *testing.T) {

M testdata/abitest/abi_test.c +14 -0


@@ -197,3 +197,17 @@ double arm_float64_unaligned_on_stack(uintptr_t a1, uintptr_t a2, uintptr_t a3,
197	197		return (double)a1 * 1 + (double)a2 * 2 + (double)a3 * 3 + (double)a4 * 4 +
198	198		(double)a5 * 5 + a6;
199	199		}
	200	+
	201	+	typedef int32_t (*AddFunc)(int32_t, int32_t);
	202	+
	203	+	int32_t returned_add(int32_t a, int32_t b) {
	204	+	return a + b;
	205	+	}
	206	+
	207	+	AddFunc return_func_ptr(void) {
	208	+	return returned_add;
	209	+	}
	210	+
	211	+	AddFunc return_null_func_ptr(void) {
	212	+	return NULL;
	213	+	}


@@ -197,3 +197,17 @@ double arm_float64_unaligned_on_stack(uintptr_t a1, uintptr_t a2, uintptr_t a3,
197	return (double)a1 * 1 + (double)a2 * 2 + (double)a3 * 3 + (double)a4 * 4 +	197		return (double)a1 * 1 + (double)a2 * 2 + (double)a3 * 3 + (double)a4 * 4 +
198	(double)a5 * 5 + a6;	198		(double)a5 * 5 + a6;
199	}	199		}
		200	+
		201	+	typedef int32_t (*AddFunc)(int32_t, int32_t);
		202	+
		203	+	int32_t returned_add(int32_t a, int32_t b) {
		204	+	return a + b;
		205	+	}
		206	+
		207	+	AddFunc return_func_ptr(void) {
		208	+	return returned_add;
		209	+	}
		210	+
		211	+	AddFunc return_null_func_ptr(void) {
		212	+	return NULL;
		213	+	}