Re: Launchd daemon

24 Mar 2013

      I just joined and saw this thread, might be able to fill in a couple of gaps (though as a result I don't have your email — hope you see this response).
...
- Communication will most probably be done with UNIX Domain Sockets.
- I will need to configure the plist, which is very well explained (in an other documentation).
I have also taken a look at SampleD.
- It gets their socket from launch_data_dict_lookup(sockets_dict, "MyListenerSocket");
Do I need to obtain a control socket from launchd or can I register it myself using ctl_register()?
UNIX domain sockets work great. You can even have launchd create the server end of it for you and grab it at checkin time.

Here's some code from a project of mine that might help with this. Does it cover what you meant by "obtaining a control socket from launchd"? It also grabs a timeout — my daemon exits if it's been idle for a while, yours may not need to. (Replace log() with calls to asl_log() or your own helpful logging function):

    int launchdSocketFd = 0;
    int launchdTimeout = 0;

    launch_data_t checkin_request;
    launch_data_t checkin_response;

    // Check in with launchd. Even if we weren't launched by launchd these first two calls should succeed
    if ((checkin_request = launch_data_new_string(LAUNCH_KEY_CHECKIN)) == NULL) {
        log(ASL_LEVEL_ERR, @"launch_data_new_string(\"" LAUNCH_KEY_CHECKIN "\") Unable to create string.");
        exit(EXIT_FAILURE);
    }
    if ((checkin_response = launch_msg(checkin_request)) == NULL) {
        log(ASL_LEVEL_ERR, @"launch_msg(\"" LAUNCH_KEY_CHECKIN "\") IPC failure: %s", strerror(errno));
        exit(EXIT_FAILURE);
    }

    // If we were started by launchd, get the file descriptor of the socket launchd opened for us
    if (LAUNCH_DATA_DICTIONARY == launch_data_get_type(checkin_response)) {
        launch_data_t sockets_dict = launch_data_dict_lookup(checkin_response, LAUNCH_JOBKEY_SOCKETS);
        size_t socketCount;
        if (sockets_dict && (socketCount = launch_data_dict_get_count(sockets_dict))) {
            if(socketCount > 1){
                log(ASL_LEVEL_WARNING, @"Launchd gave us extra sockets, ignoring them");
            }
            launch_data_t listening_fd_array = launch_data_dict_lookup(sockets_dict, "sockserverSocket");
            if (listening_fd_array && launch_data_array_get_count(listening_fd_array)) {
                launchdSocketFd = launch_data_get_fd(launch_data_array_get_index(listening_fd_array, 0));
            }
        }
        launch_data_t timeout = launch_data_dict_lookup(checkin_response, LAUNCH_JOBKEY_TIMEOUT);
        if(timeout){
            launchdTimeout = launch_data_get_integer(timeout);
        }
        if(!launchdSocketFd){
            log(ASL_LEVEL_ERR, @"Missing a socket to answer requests!");
            exit(EXIT_FAILURE);
        }
    } else {
        errno = launch_data_get_errno(checkin_response);
        log(ASL_LEVEL_ERR, @"Couldn't check in with launchd: %m");
    }
    launch_data_free(checkin_request);
    launch_data_free(checkin_response);
...
- How to set up my project to use Objective-C for my daemon.
- What does my Objective-C entry point look like (int main(…) ?)
- How do I handle SIGKILL
- If I can't use a for/while/etc. loop, how do I prevent my daemon from exiting once the main function returns?
- Do I need to obtain a control socket from launchd or can I register it myself using ctl_register()?
...
Daemos are by definition command line.
Do you know how to select objective-C as developing language?
Just create a Foundation command line tool! A launchd daemon can be *any* executable (though it shouldn't use AppKit), it isn't special. Your main()'ll probably look like this:

    int main () {
        [MyBigFancyController new];
        [[NSRunLoop currentRunLoop] run];
        return 0;
    }

Do whatever initialization you want from main() — it can be inline, a call to a C function, or a call to one or more ObjC methods, as above. It can start listening to the domain socket you got from launchd — you're welcome to wrap it in an NSFileHandle and -acceptConnectionInBackgroundAndNotify — or open files, connect to other programs, whatever.

The NSRunLoop is your infinite loop, the same one used by Cocoa apps, and the call to -run won't return while the it has input sources. Any evented ObjC call (NSTimer, -performSelector:afterDelay:, notification center, NSURLConnection, -acceptConnectionInBackgroundAndNotify, etc.) that can call you back at a later time registers itself with the run loop and counts as an input source, so don't worry about it.
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Re: Launchd daemon

Sidney San Martín