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copied!<p><code>dispatch_read</code> and <code>dispatch_write</code> are not synchronous calls -- that's their whole point. In other words, the way you have it set up here, you <code>close</code> the file descriptor right after calling <code>dispatch_write</code>. By the time GCD goes to perform the write on a background thread, the file descriptor is already closed. Same for the read operation. You have to wait until the write operation is finished before you close the file.</p> <p>I reworked your code a little bit to use a <code>dispatch_semaphore</code> to wait for the writes and reads to complete before closing the file:</p> <pre><code>int main() { dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); int intbuffer[] = { 1, 2, 3, 4 }; dispatch_data_t data = dispatch_data_create(intbuffer, 4 * sizeof(int), queue, NULL); dispatch_semaphore_t sem = dispatch_semaphore_create(0); // Write dispatch_fd_t fd = open("/tmp/data.dat", O_RDWR | O_CREAT | O_TRUNC, S_IRWXU | S_IRWXG | S_IRWXO); printf("FD: %d\n", fd); dispatch_write(fd, data, queue,^(dispatch_data_t d, int e) { printf("Written %zu bytes!\n", dispatch_data_get_size(data) - (d ? dispatch_data_get_size(d) : 0)); printf("\tError: %d\n", e); dispatch_semaphore_signal(sem); }); dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER); close(fd); // Read fd = open("/tmp/data.dat", O_RDWR); dispatch_read(fd, 4 * sizeof(int), queue, ^(dispatch_data_t d, int e) { printf("Read %zu bytes!\n", dispatch_data_get_size(d)); printf("\tError: %d\n", e); dispatch_semaphore_signal(sem); }); dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER); close(fd); // Exit confirmation getchar(); return 0; } </code></pre>

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