return error when no Metal support

246/mtl/mtl.go

@@ -1,14 +1,18 @@
 // Package mtl is a tiny subset of the Metal API.
 package mtl
 
+import (
+	"errors"
+	"unsafe"
+)
+
 /*
 #cgo darwin CFLAGS: -x objective-c
 #cgo darwin LDFLAGS: -framework Metal
 #include <stdlib.h>
 #include "mtl.h"
 */
 import "C"
-import "unsafe"
 
 // Device is abstract representation of the GPU that
 // serves as the primary interface for a Metal app.
@@ -30,16 +34,20 @@ type Device struct {
 }
 
 // CreateSystemDefaultDevice returns the preferred system default Metal device.
-func CreateSystemDefaultDevice() Device {
+func CreateSystemDefaultDevice() (Device, error) {
 	d := C.CreateSystemDefaultDevice()
+	if d == nil {
+		return Device{}, errors.New("Metal is not supported on this system")
+	}
+	defer C.free(unsafe.Pointer(d))
 
 	return Device{
 		Headless:   d.headless != 0,
 		LowPower:   d.lowPower != 0,
 		Removable:  d.removable != 0,
 		RegistryID: uint64(d.registryID),
 		Name:       C.GoString(d.name),
-	}
+	}, nil
 }
 
 // CopyAllDevices returns all Metal devices in the system.

246/mtl/mtl.h

@@ -14,5 +14,5 @@ struct Devices {
 	int             length;
 };
 
-struct Device CreateSystemDefaultDevice();
+struct Device * CreateSystemDefaultDevice();
 struct Devices CopyAllDevices();

246/mtl/mtl.m

@@ -2,15 +2,19 @@
 #import <Metal/Metal.h>
 #include "mtl.h"
 
-struct Device CreateSystemDefaultDevice() {
+// Caller must call free(d).
+struct Device * CreateSystemDefaultDevice() {
 	id<MTLDevice> device = MTLCreateSystemDefaultDevice();
+	if (!device) {
+		return NULL;
+	}
 
-	struct Device d;
-	d.headless = device.headless;
-	d.lowPower = device.lowPower;
-	d.removable = device.removable;
-	d.registryID = device.registryID;
-	d.name = device.name.UTF8String;
+	struct Device * d = malloc(sizeof(struct Device));
+	d->headless = device.headless;
+	d->lowPower = device.lowPower;
+	d->removable = device.removable;
+	d->registryID = device.registryID;
+	d->name = device.name.UTF8String;
 	return d;
 }