# 10 - Advanced FLTK

This chapter explains advanced programming and design topics that will help you to get the most out of FLTK.

## Multithreading

FLTK supports multithreaded application using a locking mechanism based on "pthreads". We do not provide a threading interface as part of the library. However a simple example how threads can be implemented for all supported platforms can be found in `test/threads.h` and `test/threads.cxx`.

To use the locking mechanism, FLTK must be compiled with `--enable-threads` set during the `configure` process. IDE-based versions of FLTK are automatically compiled with locking enabled if possible.

In `main()`, call `Fl::lock()` before `Fl::run()` or `Fl::wait()` to start the runtime multithreading support for your program. All callbacks and derived functions like `handle()` and `draw()` will now be properly locked:

        int main() {
          Fl::lock();
          /* run thread */
          while (Fl::wait() > 0) {
            if (Fl::thread_message()) {
              /* process your data */
            }
          }
        }

You can now start as many threads as you like. From within a thread (other than the main thread) FLTK calls must be wrapped with calls to `Fl::lock()` and `Fl::unlock()`:

        Fl::lock();      // avoid conflicting calls
        ...              // your code here
        Fl::unlock();    // allow other threads to access FLTK again

You can send messages from child threads to the main thread using `Fl::awake(msg)`:

        void *msg;       // "msg" is a pointer to your message
        Fl::awake(msg);  // send "msg" to main thread

You can also tell the main thread to call a function for you as soon as possible by using `Fl::awake(callback, userdata)`:

        void do_something(void *userdata) {
          // running with the main thread
        }

        // running in another thread
        void *data;       // "data" is a pointer to your user data
        Fl::awake(do_something, data);  // call something in main thread

FLTK supports multiple platforms, some of them which do not allow any other but the main thread to handle system events and open or close windows. The safe thing to do is to adhere to the following rules for threads on all operating systems:

- Don't `show()` or `hide()`anything that contains widgets derived from `Fl_Window`, including dialogs, file choosers, subwindows or `Fl_GL_Window`s
- Don't call `Fl::wait()`, `Fl::flush()` or any related methods that will handle system messages
- Don't start or cancel timers
- Don't change window decorations or titles
- The `make_current()` method may or may not work well for regular windows, but should always work for `Fl_GL_Window`s to allow for high speed rendering on graphics cards with multiple pipelines

See also: void awake(void *message), void lock(), void *thread_message(), void unlock().