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Re: NSMessagePort crashes
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Re: NSMessagePort crashes


  • Subject: Re: NSMessagePort crashes
  • From: Gwynne <email@hidden>
  • Date: Sun, 23 Jan 2005 13:30:02 -0500

On Jan 23, 2005, at 11:15 AM, Stephan Lichtenauer wrote:
I have written a small test program to use NSMessagePorts/NSPortMessages but at best the message is not received and in the worst case the program crashes with a EXC_BAD_ACCESS.

Below is my code, when I initialize the message as it can be seen with the distantPort, the app crashes, when I init. it with a send port of nil it does not crash but the message is not received nevertheless (push is attached to a button in the IB)... I looked everywhere on the net and in my "Core Mac OS X and Unix Programming" and Cocoa books and could not find much example code that would help, all the code I found looked more or less what I did...

I suggest taking a look at the ThreadMessage code by Dustin Mierau, which can be found at <http://www.blackholemedia.com/code/>. A quick summary of what his code does differently than yours:


- Use [[NSPort port] retain] instead of NSMessagePort. NSPort creates an NSMachPort by default, which will work better for OS X-local messaging. I'm not sure if NSMessagePort by itself works at all.

- Use [localPort scheduleInRunLoop:forMode:] I know the headers say not to do that, but he does it and it works.

- Use localPort as the send port and don't use any receive port. It's a one-way message; the receive port is for replies.

- Don't use nil for the components. Send something, even if you just put [NSArray arrayWithObject:[NSNull null]].

- Don't send before [NSDate distantPast]. Send before [NSDate distantFuture]. You're basically telling it to fail instantly. The timeout in that parameter only waits for a lock on the port's message queue to become available. It won't give you a timeout in communication with another thread.

- Finally, I suggest using a separate thread to test this. I doubt NSPort semantics are well-defined for intra-thread messaging, and they're certainly not necessary for it. Again, see Dustin's code.

Remember that a port you schedule on a given thread RECEIVES messages from other threads that SEND to the same port. This can get a little confusing if you're not careful. I got the semantics mixed up many times.

A somewhat more complicated example of inter-thread messaging (two-way) can be found in NDRunLoopMessenger, which you can get at <http://homepage.mac.com/nathan_day/pages/source.html>.

Both ThreadMessage and NDRunLoopMessenger are light-weight forms of the much more complicated Distributed Objects architecture, which I found to be severely overdesigned and much too complex for my purposes. I definitely recommend either one of these, or your own implementation, depending on your needs.

-- Gwynne, key to the Code
Email: email@hidden
Website: http://musicimage.plasticchicken.com/
"This whole world is an asylum for the incurable."

@implementation Test

-(id)init {
[super init];

localPort = [[NSMessagePort alloc] init];
[localPort setDelegate:self]; // do I need this?
distantPort = [[NSMessagePort alloc] init];

[[NSRunLoop currentRunLoop] addPort:localPort forMode:NSDefaultRunLoopMode];


	return self;
}

- (void)handlePortMessage:(NSPortMessage *)portMessage {
	NSLog(@"Msg received");
}

-(IBAction)push:(id)sender {
	NSPortMessage *msg;
	msg = [[NSPortMessage alloc] initWithSendPort:distantPort
									  receivePort:localPort
									   components:nil];
	[msg setMsgid:10];
	[msg sendBeforeDate:[NSDate distantPast]];
}


@end

Anybody any idea?

Thanks a lot!

Stephan

PS I know there are memory leaks but I wanted to keep the code as simple as possible...

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