What does the prefix in NSLog mean?

Question

When I use NSLog, I get output similar to the following:

2012-01-24 17:05:32:860 App[21856:71939] {logging goes here}

I recognize that '2012-01-24 17:05:32:860' is the date, 'App' is the app name, but I have no clue what '[21856:71939]' means. Can someone fill me in on what that is and where it's generated at?

All I'm trying to do is get logging that lines up nicely so it's easy to read, but the '[21856:71939]' varies in digits enough to mess up any alignment attempts. If I knew how the numbers in '[21856:71939]' were generated, I could add spaces as needed to make it line up correctly, but that's my only idea at this point.

Any help would be much appreciated :)

Answer 1

21856 is the process id. 71939 is the thread id.

You can generate this portion of the log on your own using:

[NSString stringWithFormat:@"[%ld,%lx]",
    (long) getpid(),
    (long) pthread_mach_thread_np(pthread_self())];

---

Edit 2014-09-23:

At least on the simulator in iOS 8, the second number is now the pthread_threadid_np of the thread.

    __uint64_t threadId;
    if (pthread_threadid_np(0, &threadId)) {
        threadId = pthread_mach_thread_np(pthread_self());
    }

    [NSString stringWithFormat:@"[%ld,%llu]", (long) getpid(), threadId]

Answer 2

IIRC, the 21856 is the process PID, and the 71939 is some sort of thread identifier.