Objective-c – Easy example of Grand Central Dispatch

grand-central-dispatchmacosobjective c

I'm newbie programming for mac and i'm really surprised on Grand Central Dispatch. I read about that and looks like the perfect solution for parallel programming. I worked with POSIX threads and want to move to GCD.

I saw the samples codes in the Apple Developer Connection, but It confused me so much. I searched for an easy example with two threads to start but i can't find it.

How can I do this sample code using GCD ???

#include <stdio.h>       /* standard I/O routines                 */
#include <pthread.h>     /* pthread functions and data structures */

/* function to be executed by the new thread */
void* do_loop(void* data)
{
int i;          /* counter, to print numbers */
int j;          /* counter, for delay        */
int me = *((int*)data);     /* thread identifying number */

for (i=0; i<10; i++) 
{
    for (j=0; j<500000; j++) /* delay loop */
    ;
    printf("'%d' - Got '%d'\n", me, i);
}

/* terminate the thread */
pthread_exit(NULL);
}
void* th2(void* data)
{
cout << "Thread nº 2" << endl;
}

int main(int argc, char* argv[])
{
int        thr_id;         /* thread ID for the newly created thread */
pthread_t  p_thread1;
pthread_t  p_thread2;       /* thread's structure                     */
int        a         = 1;  /* thread 1 identifying number            */
int        b         = 2;  /* thread 2 identifying number            */

/* create a new thread that will execute 'do_loop()' */
thr_id = pthread_create(&p_thread1, NULL, do_loop, (void*)&a);
/* run 'do_loop()' in the main thread as well */
thr_id = pthread_create(&p_thread2, NULL, th2, (void*)&b);


return 0;
}

Thanks in advance

Best Answer

It would be something like this:

// The block replaces your doLoop function, it basically does the same thing
dispatch_block_t myBlock = ^{
    int i;          /* counter, to print numbers */
    int j;          /* counter, for delay        */

    dispatch_queue_t me = dispatch_get_current_queue();     /* The queue which currently runs this block */

    for (i=0; i<10; i++) 
    {
        for (j=0; j<500000; j++) /* delay loop */
            ;


        printf("'%s' - Got '%d'\n", dispatch_queue_get_label(me), i); // Print the name of the queue
    }
};


// Create two queues
dispatch_queue_t queue1 = dispatch_queue_create("my.totally.unique.and.reverse.dns.identifier.1", NULL);
dispatch_queue_t queue2 = dispatch_queue_create("my.totally.unique.and.reverse.dns.identifier.2", NULL);

// Let both execute the block
dispatch_async(queue1, myBlock);
dispatch_async(queue2, myBlock);

// And then release the queues because we are great citizens who care about memory
dispatch_release(queue1);
dispatch_release(queue2);
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