Exceptions and autorelease pools
Exceptions and autorelease pools
- Subject: Exceptions and autorelease pools
- From: Larry Campbell <email@hidden>
- Date: Tue, 06 Dec 2011 15:47:36 -0500
(Mac OS X 10.6.8, Xcode 4.0.2, no GC)
I believe the following paragraph in the "Using Autorelease Pools" section of the "Memory Management Programming Guide" is wrong, or misleading:
"This behavior has implications for exceptional conditions. If an exception occurs, and the thread suddenly transfers out of the current context, the pool associated with that context is drained. However, if that pool is not the top pool on the thread’s stack, all the pools above the drained pool are also drained (releasing all their objects in the process). The top autorelease pool on the thread’s stack then becomes the pool previously underneath the drained pool associated with the exceptional condition. Because of this behavior, exception handlers do not need to release objects that were sent autorelease. Neither is it necessary or even desirable for an exception handler to send release to its autorelease pool, unless the handler is re-raising the exception."
In the following test program, if you leave the #define LEAK uncommented, the program leaks. Why? Is the above paragraph just wrong? Or am I missing something?
- lc
#import <Foundation/Foundation.h>
#import <malloc/malloc.h>
size_t heapSize()
{
malloc_statistics_t stats;
stats.size_in_use = 0;
malloc_zone_statistics(malloc_default_zone(), &stats);
return stats.size_in_use;
}
void allocAndRaise()
{
[[[NSString alloc] initWithString:@"foo bar and zot"] autorelease];
[NSException raise:@"foo" format:@"bar"];
}
//#define LEAK
void foo()
{
NSAutoreleasePool *pool = [NSAutoreleasePool new];
NS_DURING {
allocAndRaise();
} NS_HANDLER {
#ifdef LEAK
[localException raise];
#else
[localException retain]; // I don't think this workaround should be necessary, but it is
[pool release];
[[localException autorelease] raise];
#endif
} NS_ENDHANDLER
[pool release];
}
int main (int argc, const char * argv[])
{
NSAutoreleasePool * pool = [NSAutoreleasePool new];
const unsigned kRepeatCount = 20*1000;
unsigned i;
for (i = 0; i < kRepeatCount; i++) {
NSAutoreleasePool *pool = [NSAutoreleasePool new];
NS_DURING {
foo();
} NS_HANDLER {
} NS_ENDHANDLER
[pool release];
if (i && (i % 1000 == 0))
printf("Iteration %5u heap %zu\n", i, heapSize());
}
[pool release];
return 0;
}
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