Question

i have a non arc project. i’m trying to use dispatch_async to get data from server and save it in sqlite. the dispatch_async happens inside a method with callback. on calling the method the app crashes with exc bad access. here is how i’ve implemented the code.

- (void) HandleData:(const char*) receivedData WithSuccess:(void(^)(BOOL finishing))completed
{
dispatch_queue_t fetchQ = dispatch_queue_create("Refreshing", NULL);
dispatch_async(fetchQ, ^{

   [self write_data_in_sqlite]//    **<--crash happens here in the method which is called here**
    }
    dispatch_sync(dispatch_get_main_queue(), ^{
            completed(YES);
    });
});
dispatch_release(fetchQ);
}

and i call the method as follow:

HandleResponse *handleResponse = [[[HandleResponse alloc] init] autorelease];
                [handleResponse HandleData:aData WithSuccess:^(BOOL finishing) {
                 if(finishing)
                 {
                 //update the UI here
                 }
                 }];

if i remove the dispatch_async then it doesnt crash, but my UI gets blocked while writing to the sqlite.

what am i doing wrong?

edit:
removing the block and using dipatch_async produces the same exc_bad_access crash.

edit 2:
i tried example answer given below, it still crashes.

i thought to copy it then autorelease it. it crashes still but nit that often. i’m gonna check for memory leak. i’ll report.

HandleResponse *handleResponse = [[[HandleResponse alloc] init] autorelease];
        [handleResponse HandleData:aData WithSuccess: [[^(BOOL finishing) {
         if(finishing)
         {
         //update the UI here
         }
         } copy] autorelease];

edit 3:

the crash happens in strlen even the xml content is in xmlResopnse. but why this happen with dispatch and not without it

xmlDocPtr xml= xmlParseMemory(xmlResopnse, strlen(xmlResponse);

edit 4:
as in answer below suggested not to use c objects in dispatch async. so i converted xmlResponse from const char* to nsstring and it doesnt crash.

Answer 1

Everything you’ve shown seems to be okay in terms of blocks and memory management. It must be something else.

I notice that you’re passing in a C string (the char pointer receivedData) that you’re not using. If you’re not showing us the real code, and you are actually using the receivedData variable in the block, then that could be a problem, because the block simply captures the char pointer, but does not manage the memory of the string behind the pointer (it is not an Objective-C object). Therefore, it is possible that the C string is only valid in the calling scope (before the asynchronous operation), and no longer valid when the asynchronous operation runs. Your statement that something is crashing at strlen supports the idea that there is something wrong with some C string. You should try using NSString objects instead, since as objects they are properly memory-managed by blocks.