Question

I am using dispatch_semaphore_wait to stop my current thread but it looks like it stops all my threads.

Code:

SampleReader *reader = [[SampleReader alloc] initWithHostname:hostname andFilePath:filepath];
reader.endHandler = endHandler;

[reader start];

dispatch_semaphore_wait(reader->mSem, DISPATCH_TIME_FOREVER);

My start method has something like:

mFileStream = [[NSOutputStream outputStreamToFileAtPath:[fileurl path] append:FALSE] retain];
[mFileStream open];

mNetworkStream = (NSInputStream *)CFReadStreamCreateWithFTPURL(NULL, (CFURLRef)ftpurl);
mNetworkStream.delegate = self;
[mNetworkStream scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
[mNetworkStream open];

I get callback in one of the delegate methods wherein I signal the semaphore

- (void)stream:(NSStream *)aStream handleEvent:(NSStreamEvent)eventCode{

  switch (eventCode)
  Case NSStreamEventErrorOccurred:
                    dispatch_semaphore_signal(mSem);
        break;
    case NSStreamEventEndEncountered:
        dispatch_semaphore_signal(mSem);
        break;

}

However, when i send wait on semaphore, the delegate method is not called.
Its called only when i comment out the line

 //dispatch_semaphore_signal(mSem);

Any help will be greatly appreciated.

Answer 1

If you’re calling [reader start] from your main thread then you are creating a deadlock. Your stream is being associated with the main thread here:

[mNetworkStream scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

That means for it to work the main run loop must be spinning. If the dispatch_semaphore_wait is on the main thread though, you’re stopping the run loop and preventing the stream from handling its events.

I don’t see how commenting out the dispatch_semaphore_signal would do anything besides make the wait never return.