EXC_BAD_ACCESS crash when releasing NSXMLParser

Question

I'm seeing an intermittent crash on [parser release]. I'd say I see it about 5% of the time, and the data I am parsing varies between each crash. I can't for the life of me figure out why.

Before I submit a bug report to Apple (which, with my luck, will not be reproducible in sample code), has anyone run into this and know what might be going on?

    NSData *d = [data copy]; // data is typically 2K-13K bytes
    @synchronized (xmlParserLock) {
        [[NSURLCache sharedURLCache] setMemoryCapacity:0];
        [[NSURLCache sharedURLCache] setDiskCapacity:0];

        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        NSXMLParser *parser = [[NSXMLParser alloc] initWithData:d];
        [parser setDelegate:self];
        [parser setShouldProcessNamespaces:NO];
        [parser setShouldReportNamespacePrefixes:NO];
        [parser setShouldResolveExternalEntities:NO];
        [parser parse];
        [parser release];
        [pool release];
    }
    [d release];

And here's the gdb 'where' output, which points to [parser release]:

#0  0x93d08d12 in xmlCharEncCloseFunc ()
#1  0x93cfc0e3 in xmlFreeParserInputBuffer ()
#2  0x93cfc08f in xmlFreeInputStream ()
#3  0x93cfbdac in xmlFreeParserCtxt ()
#4  0x961384d6 in -[NSXMLParser dealloc] ()
#5  0x00149de7 in -[MyParserClass parseResponse] (self=0x104e9f0, _cmd=0x1766dc) at /Users/mike/Documents/MyApp/Classes/MyParserClass.m:60

Thanks in advance for any help!

Answer 1

It looks like you are using multiple threads. There could be an issue with that. Threading bugs often do present themselves sporadically. You might also have a bug in your parser delegate methods, which you didn't post here.

Answer 2

I think I figured it out - some code elsewhere in the app uses XML functions such as:

xmlCtxtReadMemory()
xmlClearParserCtxt();
xmlFreeParserCtxt();
xmlCleanupParser();
xmlFreeDoc();

These functions are likely executing in another thread at the same time I am executing the code fragment I posted. NSXMLParser obviously uses the same functions under the hood.

I've added a synchronized block to the other code using the same lock object as the one I use for my NSXMLParser usage, and the crashes seem to have gone away. So I guess the lesson here is that these XML functions are totally not thread-safe - use with caution!

Answer 3

You need to delete your data only when parsing is complete. In the delegate method called parserDidEndDocument: release data.

Hope this help.

thierry

Answer 4

You need to delete your data only when parsing is complete. In the delegate method called parserDidEndDocument: release data.

Hope this help.

thierry