Replaced the horribly memory-inefficient NSIndexSet operation to get the list of selected indices in the Mac OS X Listbox with a far simpler option that avoids deferring some of the work (apart from objc_msgSend() calls) to cgo and the use of reflect.SliceHeader at all! It just grabs the indices from the NSIndexSet one at a time using the previous index as an anchor.

bleh_darwin.m

@@ -30,7 +30,6 @@ id *_NSObservedObjectKey = (id *) (&NSObservedObjectKey);
 These are all the selectors and class IDs used by the functions below.
 */
 
-static SEL s_getIndexes;			/* NSIndexSetEntries() */
 static id c_NSEvent;				/* makeDummyEvent() */
 static SEL s_newEvent;
 static id c_NSBitmapImageRep;	/* drawImage() */
@@ -50,7 +49,6 @@ static SEL s_initTrackingArea;
 
 void initBleh()
 {
-	s_getIndexes = sel_getUid("getIndexes:maxCount:inIndexRange:");
 	c_NSEvent = objc_getClass("NSEvent");
 	s_newEvent = sel_getUid("otherEventWithType:location:modifierFlags:timestamp:windowNumber:context:subtype:data1:data2:");
 	c_NSBitmapImageRep = objc_getClass("NSBitmapImageRep");
@@ -78,6 +76,11 @@ uintptr_t objc_msgSend_uintret_noargs(id obj, SEL sel)
 	return (uintptr_t) ((NSUInteger) objc_msgSend(obj, sel));
 }
 
+uintptr_t objc_msgSend_uintret_uint(id obj, SEL sel, uintptr_t a)
+{
+	return (uintptr_t) ((NSUInteger) objc_msgSend(obj, sel, (NSUInteger) a));
+}
+
 id objc_msgSend_uint(id obj, SEL sel, uintptr_t a)
 {
 	return objc_msgSend(obj, sel, (NSUInteger) a);
@@ -193,30 +196,6 @@ id objc_msgSend_point(id obj, SEL sel, int64_t x, int64_t y)
 	return objc_msgSend(obj, sel, NSMakePoint((CGFloat) x, (CGFloat) y));
 }
 
-/*
-This is a doozy: it deals with a NSUInteger array needed for this one selector, and converts them all into a uintptr_t array so we can use it from Go. The two arrays are created at runtime with malloc(); only the NSUInteger one is freed here, while Go frees the returned one. It's not optimal.
-*/
-
-uintptr_t *NSIndexSetEntries(id indexset, uintptr_t count)
-{
-	NSUInteger *nsuints;
-	uintptr_t *ret;
-	uintptr_t i;
-	size_t countsize;
-
-	countsize = (size_t) count;
-	nsuints = (NSUInteger *) malloc(countsize * sizeof (NSUInteger));
-	/* TODO check return value */
-	objc_msgSend(indexset, s_getIndexes,
-		nsuints, (NSUInteger) count, nil);
-	ret = (uintptr_t *) malloc(countsize * sizeof (uintptr_t));
-	for (i = 0; i < count; i++) {
-		ret[i] = (uintptr_t) nsuints[i];
-	}
-	free(nsuints);
-	return ret;
-}
-
 /*
 See uitask_darwin.go: we need to synthesize a NSEvent so -[NSApplication stop:] will work. We cannot simply init the default NSEvent though (it throws an exception) so we must do it "the right way". This involves a very convoluted initializer; we'll just do it here to keep things clean on the Go side (this will only be run once anyway, on program exit).
 */

listbox_darwin.go

@@ -3,8 +3,7 @@
 package ui
 
 import (
-	"reflect"
+	// ...
-	"unsafe"
 )
 
 /*
@@ -230,6 +229,8 @@ var (
 	_setHeaderView = sel_getUid("setHeaderView:")
 	_selectedRowIndexes = sel_getUid("selectedRowIndexes")
 	_count = sel_getUid("count")
+	_firstIndex = sel_getUid("firstIndex")
+	_indexGreaterThanIndex = sel_getUid("indexGreaterThanIndex:")
 	_numberOfRows = sel_getUid("numberOfRows")
 	_deselectAll = sel_getUid("deselectAll:")
 )
@@ -261,24 +262,19 @@ func insertListboxBefore(listbox C.id, what string, before int, alternate bool)
 	insertListboxArrayBefore(array, what, before)
 }
 
-// TODO change to use [indices firstIndex] and [indices indexGreaterThanIndex:], as suggested http://stackoverflow.com/questions/3773180/how-to-get-indexes-from-nsindexset-into-an-nsarray-in-cocoa
+// technique from http://stackoverflow.com/questions/3773180/how-to-get-indexes-from-nsindexset-into-an-nsarray-in-cocoa
+// we don't care that the indices were originally NSUInteger since by this point we have a problem anyway; Go programs generally use int indices anyway
+// we also don't care about NSNotFound because we check the count first AND because NSIndexSet is always sorted (and NSNotFound can be a valid index if the list is large enough... since it's NSIntegerMax, not NSUIntegerMax)
 func selectedListboxIndices(listbox C.id) (list []int) {
-	var cindices []C.uintptr_t
-
 	indices := C.objc_msgSend_noargs(listboxInScrollView(listbox), _selectedRowIndexes)
 	count := int(C.objc_msgSend_uintret_noargs(indices, _count))
 	if count == 0 {
 		return nil
 	}
 	list = make([]int, count)
-	cidx := C.NSIndexSetEntries(indices, C.uintptr_t(count))
+	list[0] = int(C.objc_msgSend_uintret_noargs(indices, _firstIndex))
-	defer C.free(unsafe.Pointer(cidx))
+	for i := 1; i < count; i++ {
-	pcindices := (*reflect.SliceHeader)(unsafe.Pointer(&cindices))
+		list[i] = int(C.objc_msgSend_uintret_uint(indices, _indexGreaterThanIndex, C.uintptr_t(list[i - 1])))
-	pcindices.Cap = count
-	pcindices.Len = count
-	pcindices.Data = uintptr(unsafe.Pointer(cidx))
-	for i := 0; i < count; i++ {
-		list[i] = int(cindices[i])
 	}
 	return list
 }

objc_darwin.h

@@ -49,7 +49,8 @@ struct xpoint {
 	int64_t y;
 };
 
-extern uintptr_t objc_msgSend_uintret_noargs(id objc, SEL sel);
+extern uintptr_t objc_msgSend_uintret_noargs(id obj, SEL sel);
+extern uintptr_t objc_msgSend_uintret_uint(id obj, SEL sel, uintptr_t a);
 
 extern intptr_t objc_msgSend_intret_noargs(id obj, SEL sel);