Detecting connection errors when using CFStreamCreatePairWithSocketToCFHost

Question

I am finding the doc for CFStreamCreatePairWithSocketToCFHost confusing:

Specifically, its not clear to me how the function can set the readStream pointer to null on error. as far as I understand, the pointer is passed by value - so the function can only change the objected pointed to by the pointer. Right now I can't figure out how to detect connection errors.

Relevant doc snippet:

---

Creates readable and writable streams connected to a given CFHost object.

void CFStreamCreatePairWithSocketToCFHost (
   CFAllocatorRef alloc,
   CFHostRef host,
   SInt32 port,
   CFReadStreamRef *readStream,
   CFWriteStreamRef *writeStream
);

readStream

Upon return, contains a CFReadStream object connected to the host host on port port, or NULL if there is a failure during creation. If you pass NULL, the function will not create a readable stream. Ownership follows the Create Rule.

---

This is my connecting code, it goes all the way to NSLog(@"Connected") even when the server is down.

NSLog(@"Attempting to (re)connect to %@:%d", m_host, m_port);
while(TRUE)
{
    CFHostRef host = CFHostCreateWithName(kCFAllocatorDefault, (CFStringRef)m_host);
    if (!host)
    {
        NSLog(@"Error resolving host %@", m_host);
        [NSThread sleepForTimeInterval:5.0];
        continue;
    }
    CFStreamCreatePairWithSocketToCFHost(kCFAllocatorDefault, host , m_port, &m_in, &m_out);
    CFRelease(host);

    if (!m_in)
    {
        NSLog(@"Error");
    }

    CFStreamClientContext context = {0, self,nil,nil,nil};

    if (CFReadStreamSetClient(m_in, kCFStreamEventHasBytesAvailable | kCFStreamEventErrorOccurred | kCFStreamEventEndEncountered, networkReadEvent, &context))
    {
        CFReadStreamScheduleWithRunLoop(m_in, CFRunLoopGetCurrent(),kCFRunLoopCommonModes);
    }

    if (CFWriteStreamSetClient(m_out, kCFStreamEventErrorOccurred | kCFStreamEventEndEncountered, networkWriteEvent, &context))
    {
        CFWriteStreamScheduleWithRunLoop(m_out, CFRunLoopGetCurrent(),kCFRunLoopCommonModes);
    }


    BOOL success = CFReadStreamOpen(m_in);
    CFErrorRef error = CFReadStreamCopyError(m_in);
    if (!success || (error && CFErrorGetCode(error) != 0))
    {
        NSLog(@"Connect error %s : %d", CFErrorGetDomain(error), CFErrorGetCode(error));
        [NSThread sleepForTimeInterval:5.0];
    }
    else 
    {
        NSLog(@"Connected");
        break;
    }
}

Answer 1

From the "CFNetwork Programming Guide":

Opening a stream can be a lengthy process, so the CFReadStreamOpen and CFWriteStreamOpen functions avoid blocking by returning TRUE to indicate that the process of opening the stream has begun. To check the status of the open, call the functions CFReadStreamGetStatus and CFWriteStreamGetStatus, which returnkCFStreamStatusOpening if the open is still in progress, kCFStreamStatusOpen if the open is complete, orkCFStreamStatusErrorOccurred if the open has completed but failed. In most cases, it doesn’t matter whether the open is complete because the CFStream functions that read and write will block until the stream is open.

Also check out the kCFStreamEventOpenCompleted, (http://developer.apple.com/library/ios/#documentation/CoreFoundation/Reference/CFStreamConstants/Reference/reference.html) : a stream event that reports the successful completion of the opening process. So to conclude, after calling CFReadStreamOpen (or Write), which will probably succeed, register to listen to the "OpenCompleted" event to identify a "real" success.

Answer 2

Surely after you call CFStreamCreatePairWithSocketToCFHost() just test readstream to see if it's NULL?

As you're passing in the memory location of the readstream pointer, the function can easily set that to whatever value it chooses (either a reference to a created object, or alternatively NULL).

Edit

I've tried your code, and I agree, it's very confusing. It appears that the CFReadStreamRef is readily created and opened, even for a nonsense host (I literally used "nonsense"). I don't believe this function will return NULL pointers for an unreachable host.

I suppose this makes sense, in as far as until one tries to open the stream, whether it will work or not is unknown.

Answer 3

So, the readStream param is a pointer to the CFReadStreamRef and, as such, can definitely be set to NULL by the function. &foo means "address of foo" and if you have the address you can set the value.

My reading of the documentation for CFStreamCreatePairWithSocketToCFHost is that they will be set to NULL on failure, but that failure is not about connection failure, but other kinds of failure (memory, etc). So not likely you'll get an error there.

Looks to me like the issue is that CFReadStreamOpen can return immediately with true when it can open the stream in the background and so this code is not really opening the stream or testing that it's been opened, merely queuing it for opening). From the documentation for CFReadStreamOpen:

" If the stream can open in the background without blocking, this function always returns true."

So I think you will need to follow the rest of the instructions for CFReadStreamOpen and schedule the stream on a run loop, or perhaps poll (though obviously polling in a tight loop isn't likely to work).

Answer 4

In the documentation for CFReadStreamOpen we see:

Opening a stream causes it to reserve all the system resources it requires. If the stream can open in the background without blocking, this function always returns true.

I suspect that the stream is opening in the background, and thus you are saying "Connected" before it actually opens. You've already scheduled the stream with a runloop, so if you let the run loop run, you'll probably get a callback with the event type set to kCFStreamEventErrorOccurred, and from there you can process the error appropriately.