On Oct 15, 2009, at 3:54 PM, Ben Haller wrote:
Hi all. I'm getting a crash in auto_zone_root_write_barrier() that
I don't understand. I suspect it has to do with this little blurb
in the Garbage Collection Programming Guide:
Limitations on Mac OS X v10.5: You may pass addresses of strong
globals or statics into routines expecting
pointers to object pointers (such as id* or NSError**) only if they
have first been assigned to directly, rather
than through a pointer dereference. You should never take the
address of a weak global, static or instance
variable, as assigning or reading through that pointer will bypass
the weak barriers and expose your programs
to race conditions.
But I'm not really sure. :-> Here's the situation. My app
generates floods of instances of an NSObject subclass called
AKIndividual. So many, in fact, that I don't want to alloc and
dealloc them all, as that just thrashes the allocator madly (as
verified with Sampler). Instead, I want to allocate a pool of them,
and then throw them in the pool when I'm done with them, and get new
ones out of the pool. (When I reuse them, I don't call -init again,
I just wipe the ivars I'm using and put new values in, which I
believe is OK.) When I'm messing about with them, and when they're
in the pool, I don't want to keep them in Cocoa collections like
NSMutableArray, because again that introduces too much overhead.
Countless billions upon billions of these little guys I'm making,
and the runtimes of my app will be measured in days to weeks, so
optimizing this bottleneck really is important.
So my solution was to keep pointers to them in malloced buffers
instead. The "unused pool" is a malloced buffer, the pools of ones
that are doing various things are also malloced buffers, and
everything is nice fast C code at this level. But I guess pointers
to objects kept in malloced buffers are weak references, so my
objects would be collected if I didn't have a strong reference
somewhere. So when I first allocate them, I throw them into an
NSMutableArray, and I don't ever take them out. That array is kept
by the controller of the whole shebang; so when that controller gets
collected, then all the individuals will be collected, but until
then, they should always be strong-referenced.
This seems like it ought to work; and yet I get that crash in
auto_zone_root_write_barrier(). After some puzzling, I found the
above paragraph in the GC guide. I am indeed on 10.5. Is this
what's biting me? I guess all the pointers to my AKIndividuals that
are kept in my malloced arrays are all weak references, and so I
guess the pointer to the malloced array itself, for example, is a
pointer to a weak reference such as I am not supposed to use, and
whenever I do something like "individuals[i]" to get an instance
from my malloced buffer I guess I'm violating the weak barriers. I
find it hard to believe that I'm not allowed to keep an array of
pointers to objects, though; is that really what this blurb is
saying? (On the other hand, the GC guide also says that "the malloc
zone is never scanned", which would imply that this is fine, and
that the references I put in my malloced arrays are not even weak;
but then I'm puzzled by the crash in a write barrier function...) I
don't really understand what is prohibited and exactly why; that
blurb is way too short, and the example is too short and cryptic.
Can some explain what is actually prohibited, and why, and whether
what I'm doing is prohibited?
A pointer value stored in an ordinary malloc block is neither a
"strong" nor a "weak" reference. It is a dangling pointer. It can be
used safely, but requires great care because the garbage collector has
no knowledge of what you're doing.
The auto_zone_root_write_barrier() crash can occur when you take the
address of a global variable, then store into the global indirectly
via that address. What does the crashed line of code look like?
--
Greg Parker gpar...@apple.com Runtime Wrangler
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