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Core Foundation Framework Reference CFFileDescriptor Reference

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CFFileDescriptor Reference

The CFFileDescriptor provides an opaque type to monitor file descriptors for read and write activity via CFRunLoop.

You use CFFileDescriptor to monitor file descriptors for read and write activity via CFRunLoop using callbacks. Each call back is one-shot, and must be re-enabled if you want to get another one.

You can re-enable the callback in the callback function itself, but you must completely service the file descriptor before doing so. For example, if you create a CFFileDescriptor for a pipe and get a callback because there are bytes to be read, then if you don't read all of the bytes but nevertheless re-enable the CFFileDescriptor for read activity, you'll get called back again immediately.

You can monitor kqueue file descriptors for read activity to find out when an event the kqueue is filtering for has occurred. You are responsible for understanding the use of the kevent() API and inserting and removing filters from the kqueue file descriptor yourself.

The following example takes a UNIX process ID as argument, and watches up to 20 seconds, and reports if the process terminates in that time:

  1. // cc test.c -framework CoreFoundation -O
  2. #include <CoreFoundation/CoreFoundation.h>
  3. #include <unistd.h>
  4. #include <sys/event.h>
  5. static void noteProcDeath(CFFileDescriptorRef fdref, CFOptionFlags callBackTypes, void *info) {
  6. struct kevent kev;
  7. int fd = CFFileDescriptorGetNativeDescriptor(fdref);
  8. kevent(fd, NULL, 0, &kev, 1, NULL);
  9. // take action on death of process here
  10. printf("process with pid '%u' died\n", (unsigned int)kev.ident);
  11. CFFileDescriptorInvalidate(fdref);
  12. CFRelease(fdref); // the CFFileDescriptorRef is no longer of any use in this example
  13. }
  14. // one argument, an integer pid to watch, required
  15. int main(int argc, char *argv[]) {
  16. if (argc < 2) exit(1);
  17. int fd = kqueue();
  18. struct kevent kev;
  19. EV_SET(&kev, atoi(argv[1]), EVFILT_PROC, EV_ADD|EV_ENABLE, NOTE_EXIT, 0, NULL);
  20. kevent(fd, &kev, 1, NULL, 0, NULL);
  21. CFFileDescriptorRef fdref = CFFileDescriptorCreate(kCFAllocatorDefault, fd, true, noteProcDeath, NULL);
  22. CFFileDescriptorEnableCallBacks(fdref, kCFFileDescriptorReadCallBack);
  23. CFRunLoopSourceRef source = CFFileDescriptorCreateRunLoopSource(kCFAllocatorDefault, fdref, 0);
  24. CFRunLoopAddSource(CFRunLoopGetMain(), source, kCFRunLoopDefaultMode);
  25. CFRelease(source);
  26. // run the run loop for 20 seconds
  27. CFRunLoopRunInMode(kCFRunLoopDefaultMode, 20.0, false);
  28. return 0;
  29. }


  • Invalidates a CFFileDescriptor object.



    func CFFileDescriptorInvalidate(_ f: CFFileDescriptor!)


    void CFFileDescriptorInvalidate ( CFFileDescriptorRef f );



    A CFFileDescriptor.


    Once invalidated, the CFFileDescriptor object will no longer be read from or written to at the Core Fundation level.

    If you passed true for the closeOnInvalidate parameter when you called CFFileDescriptorCreate, this function also closes the underlying file descriptor. If you passed false, you must close the descriptor yourself after invalidating the CFFileDescriptor object.


    Available in iOS 2.0 and later.

Data Types


  • Constants that identify the read and write callbacks.



    var kCFFileDescriptorReadCallBack: CFOptionFlags { get } var kCFFileDescriptorWriteCallBack: CFOptionFlags { get }


    enum { kCFFileDescriptorReadCallBack = 1 << 0, kCFFileDescriptorWriteCallBack = 1 << 1 };


    • kCFFileDescriptorReadCallBack


      Identifies the read callback.

      Available in iOS 2.0 and later.

    • kCFFileDescriptorWriteCallBack


      Identifies the write callback.

      Available in iOS 2.0 and later.