The Rules
  • Feel free to leave constructive criticism, or point out a better way to do something.
  • Personal attacks or flames, on me or anyone else, will be deleted.
  • Past history has shown that 99% of comments I can't read (i.e. those in other languages) to be spam. Therefore, any comment I can't read will be removed.
  • I'm pretty mellow concerning profanity, but excessive (as determined subjectively by me), bad language will be removed.

Thursday, May 22, 2008

NSMutableArray makes awesome Cocoa stacks and queues

A difference between Java and Objective-C/Cocoa? Java has ConcurrentLinkedQueue, PriorityBlockingQueue, ArrayBlockingQueue, blah blah blah.

Objective-C/Cocoa(or Foundation) has NSMutableArray. NSMutableArray has some nice instance methods that make it _extremely_ easy to build a queue, stack, priority queue (synchronized or otherwise) without loading up the API with a billion separate classes.

Need a stack?

-(void) push:(id) item {
[list addObject:item] // where list is the actual array in your stack

-(id) pop {
id r = [list lastObject];
[list removeLastObject];
return r;

How about a queue?

-(void) enqueue:(id) item {
[list insertObject:item atIndex:0];

-(id) dequeue {
id r = [list lastObject];
[list removeLastObject];
return r;

Priority queueing and/or synchronization for thread-safety will be left as an exercise for the reader. Man, I've always wanted to say that... :)

Wednesday, May 21, 2008

NSThreads when you need to wait.

For my webserver project I need to use queues. The main thread places connections that need to be serviced in a queue. "Worker" threads need to place log entries in a queue to be written out to the log file. These queues obviously need to be thread-safe. Writing a thread-safe queue is quite simple thanks to the NSLock and NSConditionLock classes. Just like any other semaphore, you create a NSLock object and surround your critical section with:

[myLock lock]
critical section...
[myLock unlock]

The real question I had to deal with here, though, was thoroughly testing my queues to make sure that they were _really_ thread safe. Unlike POSIX threads, Cocoa/Foundation has no mechanism for waiting on NSThread objects. But I needed to know that all of my testing threads were finished before I started comparing expected and actual results. The solution is not that difficult. The entry point/run loop for an NSThread object is a method with the mandatory signature:

(void) threadFuncName:(id)

So you create a struct or a class that has any data you need to pass in to your thread's main function. In addition, include in this struct or class an object of NSConditionLock. Immediately upon entry to the function, lock this with some condition code that will let your main program thread know that the thread is running. After spawning the thread, you want the main program execution to try and lock your condition lock when the run condition is no longer true. This will cause your main program thread to block and wait for your new thread to unlock with the condition you're waiting on. The last thing you'll want to do before your thread exits the run loop function is unlock the NSConditionLock with some condition that will let your main program thread know it's done. This will unblock your main thread of execution and you're off.
Sample code: (not complete, just the snippets that you'll need)

#import <Foundation/Foundation.h>
#define NOT_DONE 0
#define DONE 1

// this will need to be on some class that is calling the NSThread object
-(void) someFunc:(id) arg {
NSConditionLock* myLock = arg;
[myLock lock];
//do stuff that we need to know about in main
[myLock unlockWithCondition:DONE];

int main() {

NSConditionLock* finishedLock = [[NSConditionLock alloc]
initWithCondition: NOT_DONE];

NSThread* myThread = [[NSThread alloc] initWithTarget:self
selector:@selector(someFunc:) object:finishedLock];

[myThread start];

[finishedLock lockWhenCondition:DONE];

// now we can do whatever we need to do with the results from
// the thread we spawned


Wednesday, May 14, 2008

Is it love?

I've decided to use Objective-C and the Cocoa/Foundation frameworks to implement the main project for my internet programming class.

My mission: to build a multi-threaded (yet limited functionality) webserver with CGI capability by the 3rd week of June.

1. I'd like to work for Apple someday, and building a webserver with their technology seems a good resume-booster.
2. Objective-C (with the Foundation/Cocoa frameworks) is a pretty nifty language with some good features that I'm quite keen to learn about
3. I'll be learning a lot, which I care much more about than grades

1. I'll be learning a lot, so I'm not likely to get an A in this particular class
2. I've had to do a lot of research on threading/synchronization so I'm a little behind on a project. (2 days, but who's counting?)
3. Learning the quirks of a new language means I'll be working at the last minute most of the semester, whereas if I just used C and POSIX threading/synchronization libraries I could probably whip this together pretty quickly.

Is it love? I guess I'll know in a few weeks.