Understanding objc_assign_strongCast
-
Hi,
I am having a lot of trouble understanding exactly what the compiler-
generated objc_assign_strongCast is supposed to do. The only
documentation on this is one sentence in the Objective-C release notes.
From http://developer.apple.com/releasenotes/Cocoa/RN-ObjectiveC/
> The compiler uses three "helper" functions for assignments of strong
> pointers to garbage collected memory into global memory
> (objc_assign_global), garbage collected heap memory
> (objc_assign_ivar), or into unknown memory (objc_assign_strongCast).
> For assignments of weak pointers it uses objc_assign_weak and for
> reads it uses objc_read_weak.
>
From what I can tell the compiler (Objective-C++ in my case)
generates calls to objc_assign_strongCast whenever assigning an
Objective-C pointer that is an i-var of a C++ class. I assume it
would do something similar when assigning a field in a plain-old C
struct.
But I have yet to find a single case where this keeps the object from
being finalized! I initially thought it wasn't working when the C++
struct was located in the global data area. No problem, I followed
Chris Hanson's advice and introduced CFRetain/CFRelease calls.
But now I've come across a case where I am certain the C++ object is
allocated on the C++ heap (via a regular new operator, not
overloaded). Even in this case, the objc_assign_strongCast which I am
reasonably certain the compiler is generating (it shows in the output
of gcc -S around where I expect it to) completely fails to keep the
Objective-C object from being finalized. If that is not the purpose
of objc_assign_strongCast then may I ask what is its purpose?
Note that the i-var in this case is a plain-old NSColor*. There is
nothing fancy going on here. The only solution I have found is to
again apply CFRetain/CFRelease appropriately.
So what gives? What am I misunderstanding?
-Dave -
Dave,
objc_assign_strongCast() will issue a write barrier, informing GC
that the destination value has changed. But if the only references
to this pointer are in unscanned (not GC) memory, than the GC system
will think it's dead as no references to that pointer exist in
scanned (GC live) memory.
The C++ new operator allocates from malloc(), just as before.
malloc() memory is not GC scanned. It's probably easiest to instead
use CFRetain and balance it with CFRelease in delete/etc.
In fact, I've written some pretty hairy hybrid memory model code, and
have yet to find a use for calling objc_assign_strongCast() myself.
The GC programming guide talks about using CFRetain/CFRelease in the
section on Core Foundation objects. The primary difference with C++
objects is that C++'s new is from unscanned malloc memory where as CF
types (with the default allocator) are allocated from scanned memory.
--
-Ben -
Hi Ben,
Thank you for your response. It is most helpful to know that CFRetain/
CFRelease are the most appropriate option for maintaining strong
references from non-GC objects and that objc_assign_strongCast has
zero effect unless the pointer itself is located in a GC-managed
heap. I've observed first-hand that it has zero effect in global
memory and zero effect in non-GC heaps and from my understanding ought
not to be needed on the stack. Did I miss any other notable types of
memory?
Now that you mention CF objects it occurs to me that
objc_assign_strongCast is intended for i-vars of CF objects located in
GC-managed heap. Or to put it another way, it is less related to
objc_assign_global and more like objc_assign_ivar for i-vars that are
in POD structs allocated in GC-managed heap. Some time in the future
I may endeavor to push the limits of this theory as it sounds quite
tempting to apply the GC to other code.
The only remaining question I have pertains to compatibility with
prior OS X versions. Clearly CFRetain/CFRelease work on Leopard. And
as far as I know code with GC-write barriers can only work on Leopard,
even when it is compiled to support both RR and GC, due to the simple
fact that the objc_assign_* series of functions do not exist on older
versions of OS X.
Keep in mind I am writing a library (wxCocoa) that must work in both
RR and GC mode. Therefore my usage of CFRetain/CFRelease will be
balanced appropriately so as to function at runtime correctly
regardless of mode. However, if compiling for pure-RR mode is it
still reasonable to use CFRetain/CFRelease in lieu of retain/release
or will this cause me problems when targeting older versions of OS X?
Do I need to condition CFRetain/CFRelease on garbage collection being
potentially supported or is it safe to just judiciously replace retain/
release with CFRetain/CFRelease and expect the same code to run on
prior versions of OS X?
Hopefully I understand enough now and hopefully your reply will prove
useful in the archives to other people. Again, thanks for the response.
-Dave
On Feb 6, 2008, at 8:23 PM, Ben Trumbull wrote:
> Dave,
>
> objc_assign_strongCast() will issue a write barrier, informing GC
> that the destination value has changed. But if the only references
> to this pointer are in unscanned (not GC) memory, than the GC system
> will think it's dead as no references to that pointer exist in
> scanned (GC live) memory.
>
> The C++ new operator allocates from malloc(), just as before.
> malloc() memory is not GC scanned. It's probably easiest to instead
> use CFRetain and balance it with CFRelease in delete/etc.
>
> In fact, I've written some pretty hairy hybrid memory model code,
> and have yet to find a use for calling objc_assign_strongCast()
> myself.
>
> The GC programming guide talks about using CFRetain/CFRelease in the
> section on Core Foundation objects. The primary difference with C++
> objects is that C++'s new is from unscanned malloc memory where as
> CF types (with the default allocator) are allocated from scanned
> memory.
> --
>
> -Ben
-
On Feb 6, 2008, at 5:23 PM, Ben Trumbull wrote:
> objc_assign_strongCast() will issue a write barrier, informing GC
> that the destination value has changed. But if the only references
> to this pointer are in unscanned (not GC) memory, than the GC system
> will think it's dead as no references to that pointer exist in
> scanned (GC live) memory.
>
> The C++ new operator allocates from malloc(), just as before.
> malloc() memory is not GC scanned. It's probably easiest to instead
> use CFRetain and balance it with CFRelease in delete/etc.
You could also define a custom "new" operator for the class in
question that allocates its memory using NSAllocateCollectable(...,
NSScannedOption), but that might be more trouble than it's worth.
--Chris N. -
Hi Chris,
On Feb 7, 2008, at 12:50 PM, Christopher Nebel wrote:
> On Feb 6, 2008, at 5:23 PM, Ben Trumbull wrote:
>
>> objc_assign_strongCast() will issue a write barrier, informing GC
>> that the destination value has changed. But if the only references
>> to this pointer are in unscanned (not GC) memory, than the GC
>> system will think it's dead as no references to that pointer exist
>> in scanned (GC live) memory.
>>
>> The C++ new operator allocates from malloc(), just as before.
>> malloc() memory is not GC scanned. It's probably easiest to
>> instead use CFRetain and balance it with CFRelease in delete/etc.
>
> You could also define a custom "new" operator for the class in
> question that allocates its memory using NSAllocateCollectable(...,
> NSScannedOption), but that might be more trouble than it's worth.
>
The problem then becomes: what references that object. Because
obviously if that object doesn't have a strong reference, it will be
considered finalizable and so will the other objects.
But yes, the thought crossed my mind and I'm sure I will find such a
technique useful in the future.
-Dave -
On Feb 6, 2008, at 11:50 PM, David Elliott wrote:
> Thank you for your response. It is most helpful to know that
> CFRetain/CFRelease are the most appropriate option for maintaining
> strong references from non-GC objects and that
> objc_assign_strongCast has zero effect unless the pointer itself is
> located in a GC-managed heap. I've observed first-hand that it has
> zero effect in global memory and zero effect in non-GC heaps and
> from my understanding ought not to be needed on the stack. Did I
> miss any other notable types of memory?
There is a separate objc_assign_global(). You *can* put GC objects
into globals (assuming they are id or __strong types).
objc_assign_ivar() is for assigning to an ivar of an object (something
with an Objective-C Class). There are some additional details in the
GC programming guide, as well as the Leopard (10.5) release notes for
Objective-C and Garbage Collection (some GC stuff is nestled away
under the ObjC runtime)
But that's mostly for entertainment purposes. CFRetain and CFRelease
work well to manage "external references" (stuff holding a GC object
from non-GC memory) I've never needed to use any of the objc_assign*
functions directly for any strong references.
> The only remaining question I have pertains to compatibility with
> prior OS X versions. Clearly CFRetain/CFRelease work on Leopard.
> And as far as I know code with GC-write barriers can only work on
> Leopard, even when it is compiled to support both RR and GC, due to
> the simple fact that the objc_assign_* series of functions do not
> exist on older versions of OS X.
GC only works on Leopard, but several of the objc_assign_ functions
are available on Tiger (as their RR no-ops). I *believe* you can
compile mixed mode code for Leopard and Tiger so long as you do not
use any __weak references, but I'm not certain. I'd recommend trying
a small sample project on Leopard and testing the binary on Tiger.
> Keep in mind I am writing a library (wxCocoa) that must work in both
> RR and GC mode. Therefore my usage of CFRetain/CFRelease will be
> balanced appropriately so as to function at runtime correctly
> regardless of mode.
That is possible, although I haven't tried adding in the twist of
running on Tiger and Leopard. Nearly all the system frameworks on
Leopard run in both RR and GC mode.
> However, if compiling for pure-RR mode is it still reasonable to use
> CFRetain/CFRelease in lieu of retain/release or will this cause me
> problems when targeting older versions of OS X? Do I need to
> condition CFRetain/CFRelease on garbage collection being potentially
> supported or is it safe to just judiciously replace retain/release
> with CFRetain/CFRelease and expect the same code to run on prior
> versions of OS X?
There are a few factors at work here, so typically one does not use
CFRetain/CFRelease in lieu of retain/release wholesale.
(1) CFRetain and CFRelease crash on NULL so you cannot dispatch to
them blindly, as you can with -retain and -release. You have to check
for nil.
(2) You don't have to replace all your use of -retain/release with
CFRetain/CFRelease. You only need it when you want to assign into non-
GC memory or otherwise hold an external reference upon a block of GC
memory.
(3) You must balance a call to -retain/+alloc with -release/-
autorelease. You must balance a call to CFRetain/CFCreate... with a
call to CFRelease. You cannot balance -retain with CFRelease. You
cannot balance a call to CFCreate... with -release (this does work
under RR, but breaks dual mode or GC code)
(4) -retain, -release, -autorelease are no-ops under GC.
Rules 3 &4 combine into an interesting pattern:
id foo = [[NSObject alloc] init]; // RR count 1, GC external
reference count 0 because +alloc does not force an er under GC
if (foo) CFRetain((void*)foo); // RR count 2, GC er count 1
[foo release]; // RR count 1, GC er count 1 because -release is a no-
op under GC
At this point 'foo' has the same hard reference count under both RR
and GC. It can be freed equally well under either mode with
if (foo) CFRelease((void*)foo): // RR count 0; GC er count 0
// some point later either -dealloc or -finalize, if any, will get
called
You don't need this if 'foo' can be constructed by a CFCreate...
function since CF functions create objects that have an er count of 1
as they must be balanced by CFRelease.
This pattern is useful to deal with a problem encountered by dual
memory mode code. -finalize is called in a completely indeterminate,
random order. You absolutely cannot message another object in -
finalize unless you can guarantee it is alive. That is really hard to
know unless you put a CFRetain on it yourself, and CFRelease it in -
finalize. This includes your own ivars! Your ivars can be finalized
before you!
Now even if you do have a finalize method, most of your ivars won't
need to be messaged. Most messages to ivars in -dealloc are -release,
which you don't need at all in -finalize method. So this is a special
case. In hundreds of classes, with plenty of ivars apiece, I've only
used it 10 or 11 times.
The problem is an issue with dual mode code, because it's much easier
for GC only code to be structured with fewer (ideally zero) finalize
methods. finalize methods can be quite expensive so avoiding them is
best.
Writing a dual mode framework is not trivial.
You could override the new operator to allocate scanned collectible
memory. I don't have any experience writing Objective-C++ code under
GC, so I'm not sure it's any easier than the CFRetain approach.
- Ben
> On Feb 6, 2008, at 8:23 PM, Ben Trumbull wrote:
>
>> Dave,
>>
>> objc_assign_strongCast() will issue a write barrier, informing GC
>> that the destination value has changed. But if the only references
>> to this pointer are in unscanned (not GC) memory, than the GC
>> system will think it's dead as no references to that pointer exist
>> in scanned (GC live) memory.
>>
>> The C++ new operator allocates from malloc(), just as before.
>> malloc() memory is not GC scanned. It's probably easiest to
>> instead use CFRetain and balance it with CFRelease in delete/etc.
>>
>> In fact, I've written some pretty hairy hybrid memory model code,
>> and have yet to find a use for calling objc_assign_strongCast()
>> myself.
>>
>> The GC programming guide talks about using CFRetain/CFRelease in
>> the section on Core Foundation objects. The primary difference
>> with C++ objects is that C++'s new is from unscanned malloc memory
>> where as CF types (with the default allocator) are allocated from
>> scanned memory.
>> --
>>
>> -Ben
>
