FSEvents C++ Example

Question

I need to create FSEvents watcher for a Folder in Mac. I'm comfortable with C++ and is there a way to get FSEvents notifications in C++ code, rather than Objective-C. Is there some example code to start with and any libraries i need to include ..?

I'm already on this page. http://developer.apple.com/library/mac/#featuredarticles/FileSystemEvents/_index.html

But there seems to be only Objective C, can i have CPP version of it

Answer 1

Yes, it is possible in C. You should look for Kernel Queues.

Here's a small sample to watch the directory:

#include <errno.h>       // for errno
#include <fcntl.h>       // for O_RDONLY
#include <stdio.h>       // for fprintf()
#include <stdlib.h>      // for EXIT_SUCCESS
#include <string.h>      // for strerror()
#include <sys/event.h>   // for kqueue() etc.
#include <unistd.h>      // for close()

int main (int argc, const char *argv[])
{
   int kq = kqueue ();
   // dir name is in argv[1], NO checks for errors here
   int dirfd = open (argv[1], O_RDONLY);

   struct kevent direvent;
    EV_SET (&direvent, dirfd, EVFILT_VNODE, EV_ADD | EV_CLEAR | EV_ENABLE,
            NOTE_WRITE, 0, (void *)dirname);

   kevent(kq, &direvent, 1, NULL, 0, NULL);

   // Register interest in SIGINT with the queue.  The user data
   // is NULL, which is how we'll differentiate between
   // a directory-modification event and a SIGINT-received event.
   struct kevent sigevent;
   EV_SET (&sigevent, SIGINT, EVFILT_SIGNAL, EV_ADD | EV_ENABLE, 0, 0, NULL);
   // kqueue event handling happens after the legacy API, so make
   // sure it doesn eat the signal before the kqueue can see it.
   signal (SIGINT, SIG_IGN);

   // Register the signal event.
   kevent(kq, &sigevent, 1, NULL, 0, NULL);

   while (1) {
       // camp on kevent() until something interesting happens
       struct kevent change;
       if (kevent(kq, NULL, 0, &change, 1, NULL) == -1) { exit(1); }
       // The signal event has NULL in the user data.  Check for that first.
       if (change.udata == NULL) {
           break;
       } else {
        // udata is non-null, so it's the name of the directory
        printf ("%s\n", (char*)change.udata);
       }
   }
   close (kq);
   return 0;
}

The details can be found in ch. 16 (kqueues and FSEvents) of "Advanced Mac OSX Programming" by Mark Dalrymple. The additional info may be found in *BSD documentation for kqueues.

Or use this API from FSEvents (it's mostly C-based).

FSEventStreamRef FSEventStreamCreate (CFAllocatorRef allocator,
                                  FSEventStreamCallback callback,
                                  FSEventStreamContext *context,
                                  CFArrayRef pathsToWatch,
                                  FSEventStreamEventId sinceWhen,
                                  CFTimeInterval latency,
                                  FSEventStreamCreateFlags flags);

to create the FSEvents event stream with pure-C callback.

Then attach this event stream to your runloop using the

void FSEventStreamScheduleWithRunLoop (FSEventStreamRef streamRef,
                                   CFRunLoopRef runLoop,
                                   CFStringRef runLoopMode);

Yes, here you probably should use a line of Obj-C to get the RunLoop handle: get the CFRunLoop from an NSRunLoop by using -getCFRunLoop

CFRunLoop* loopRef = [[NSRunLoop currentRunLoop] getCFRunLoop];

or use the pure C call

CFRunLoop* loopRef =  CFRunLoopGetCurrent();

Start the event stream with

Boolean FSEventStreamStart (FSEventStreamRef streamRef);

Stop the event stream with

void FSEventStreamStop (FSEventStreamRef streamRef);

And then unschedule it from the runloop with this:

void FSEventStreamUnscheduleFromRunLoop (FSEventStreamRef streamRef,
                                     CFRunLoopRef runLoop,
                                     CFStringRef runLoopMode);

Invalidate the stream (cleanup):

void FSEventStreamInvalidate (FSEventStreamRef streamRef);

Hope this will get you started.