After installing Leopard, I have noticed that some of the applications I have written have begun to misbehave after running for a while. It turns out that they are no longer able to create new threads, despite having plenty of free memory and only a handful of other threads running.

The culprit is a change in behaviour when calling pthread_create on Tiger vs. calling it on Leopard. On Tiger, it is possible to create millions of threads without worrying about cleaning up after them. This has changed in Leopard: Now, it is only possible to create 11002 (I wonder were that magical constant came from) threads before additional new threads can not be created. The following snippet of code demonstrates the issue:

//    A thread that just exits
void*    mythread(void *arg) { return 0; }

int main (int argc, char *argv[]) {
    int       err,
              count;
    pthread_t thread;
    while (1) {
        err = pthread_create(&thread, 0, mythread, 0);
        if (err != 0) {
            printf("Count: %d Error: %d '%s'\n", count, err, strerror(err));
            sleep(5);
        }
        count++;
        if (count % 1000 == 0)
            printf("Count: %d\n", count);
    }
    return 0;
}

To compile the code, put it in a file called pthread-test.c and compile it by running gcc -Wall -o pthread-test pthread-test.c from a terminal. Then run the binary by typing ./pthread-test . After a few seconds, the program will give the following output:

[ fourlights ] ./pthread-test
...
Count: 10000
Count: 11000
Count: 11002 Error: 35 'Resource temporarily unavailable'

The fix to this particular problem is to always explicitly detach threads that you do not intend to pthread_join() with later on. That is, once the thread has been created, it should be detached as follows:

err = pthread_create(&thread, 0, mythread, 0);
if (err != 0) {
    printf("Count: %d Error: %d '%s'\n", count, err, strerror(err));
    sleep(5);
}
pthread_detach(thread);

With this fix, the example no longer fails after the aforementioned 11002 threads.