Question

I want to make a simples cliente of TCP. But I’m getting one error. When I make inputStream = (NSInputStream *)readStream; and outputStream = (NSOutputStream *)writeStream; it suggests me to introduce the prefix __bridge or _bridge_transfer.

First, what is it? Second, I tried both and still can’t send messages. I followed this tutorial and I’ve the send messages and stream too. I installed Wireshark and the send message is been called, but it’s not sending any packet to the ip.

I’ve just posted here the initNetworkCommunication because is where I get the “bridge” error.

- (void) initNetworkCommunication {

    CFReadStreamRef readStream;
    CFWriteStreamRef writeStream;
    CFStreamCreatePairWithSocketToHost(NULL, (CFStringRef)@"54.xxx.xxx.xxx", 1333, &readStream, &writeStream);

    inputStream = (NSInputStream *)readStream;
    outputStream = (NSOutputStream *)writeStream;
    [inputStream setDelegate:self];
    [outputStream setDelegate:self];
    [inputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
    [outputStream scheduleInRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
    [inputStream open];
    [outputStream open];

}

The server is fine, because i’ve tried the sample code and I get a response.

Can you help me?

Answer 1

As H2CO3 mentioned, it’s an ARC thing.

If you don’t know what ARC is, read this. To summarise, it’s a deterministic way of automating the memory management stuff (retain/release statements etc) that Objective-C programmers previously had to do manually. It’s well worth using, and has few downsides. However, it does have a few gotcha’s.

ARC doesn’t work on Core Foundation objects. They’re still subject to the old rules, where you have to do the memory management yourself. ARC only applies to Objective-C objects. However, some Core Foundation objects are actually toll-free bridged to their Cocoa equivalent. You’re using toll-free bridging in your code sample, to create a CFReadStreamRef and then work with it as an NSInputStream.

So what do you do? Apple’s docs say the following:

If you cast between Objective-C and Core Foundation-style objects, you need to tell the compiler about the ownership semantics of the object using either a cast (defined in objc/runtime.h) or a Core Foundation-style macro (defined in NSObject.h):

• __bridge transfers a pointer between Objective-C and Core Foundation with no transfer of ownership.
• __bridge_retained or CFBridgingRetain casts an Objective-C pointer to a Core Foundation pointer and also transfers ownership to you.
  You are responsible for calling CFRelease or a related function to relinquish ownership of the object.
• __bridge_transfer or CFBridgingRelease moves a non-Objective-C pointer to Objective-C and also transfers ownership to ARC.
  ARC is responsible for relinquishing ownership of the object.

You’re moving from Core Foundation to Objective-C, so ignore the second bullet point (that’s for going in the other direction). The question is what do you want to happen — if after the transfer you want to hand that object to ARC, use it only from the Objective-C side, and have ARC deal with the memory management, use __bridge_transfer. That’s probably what you want based on your code sample.

If you just use __bridge, or if you don’t use ARC, you’ll need to clear up the objects yourself, using either CFRelease() or by sending them a release message (the latter only works if you’re not using ARC).