How to use Fl::awake in the FLTK Library

Question

I would like to use the Fl::awake(callback) function to call functions from my main loop in an FLTK program. I am calling Fl::awake from a child thread, it returns 1 (success), but my function never gets called. I am calling Fl::wait() in a loop in the main thread. Is there any way i can troubleshoot this?

Answer 1

Code below is an example of a relatively simple FLTK 1.3 application which uses Fl::awake(callback) . The code is from the FLTK's test directory. Just fetch the source, and you'll find numerous examples. You did not give us your code, but I assume you did not call Fl::lock(), and later Fl::unlock() in your thread. It is essential because you should not access widgets outside that section...

include

#if HAVE_PTHREAD || defined(WIN32)
#  include <FL/Fl.H>
#  include <FL/Fl_Double_Window.H>
#  include <FL/Fl_Browser.H>
#  include <FL/Fl_Value_Output.H>
#  include <FL/fl_ask.H>
#  include "threads.h"
#  include <stdio.h>
#  include <math.h>

Fl_Thread prime_thread;

Fl_Browser *browser1, *browser2;
Fl_Value_Output *value1, *value2;
int start2 = 3;

void magic_number_cb(void *p)
{
  Fl_Value_Output *w = (Fl_Value_Output*)p;
  w->labelcolor(FL_RED);
  w->redraw_label();
}

void* prime_func(void* p)
{
  Fl_Browser* browser = (Fl_Browser*) p;
  Fl_Value_Output *value;
  int n;
  int step;
  char proud = 0;

  if (browser == browser2) {
n      = start2;
start2 += 2;
step   = 12;
value  = value2;
  } else {
n     = 3;
step  = 2;
value = value1;
  }

  // very simple prime number calculator !
  //
  // The return at the end of this function can never be reached and thus 
  // will generate a warning with some compilers, however we need to have 
  // a return statement or other compilers will complain there is no return 
  // statement. To avoid warnings on all compilers, we fool the smart ones 
  // into beleiving that there is a chance that we reach the end by testing 
  // n>=0, knowing that logically, n will never be negative in this context.
  if (n>=0) for (;;) {
int pp;
int hn = (int)sqrt((double)n);

for (pp=3; pp<=hn; pp+=2) if ( n%pp == 0 ) break;
if (pp >= hn) {
  char s[128];
  sprintf(s, "%d", n);

  // Obtain a lock before we access the browser widget...
  Fl::lock();

  browser->add(s);
  browser->bottomline(browser->size());
  if (n > value->value()) value->value(n);
  n += step;

  // Release the lock...
  Fl::unlock();

  // Send a message to the main thread, at which point it will
  // process any pending redraws for our browser widget.  The
  // message we pass here isn't used for anything, so we could also
  // just pass NULL.
  Fl::awake(p);
  if (n>10000 && !proud) {
    proud = 1;
    Fl::awake(magic_number_cb, value);
  }
} else {
  // This should not be necessary since "n" and "step" are local variables,
  // however it appears that at least MacOS X has some threading issues
  // that cause semi-random corruption of the (stack) variables.
  Fl::lock();
  n += step;
  Fl::unlock();
}
  }
  return 0L;
}

int main(int argc, char **argv)
{
  Fl_Double_Window* w = new Fl_Double_Window(200, 200, "Single Thread");
  browser1 = new Fl_Browser(0, 0, 200, 175);
  w->resizable(browser1);
  value1 = new Fl_Value_Output(100, 175, 200, 25, "Max Prime:");
  w->end();
  w->show(argc, argv);
  w = new Fl_Double_Window(200, 200, "Six Threads");
  browser2 = new Fl_Browser(0, 0, 200, 175);
  w->resizable(browser2);
  value2 = new Fl_Value_Output(100, 175, 200, 25, "Max Prime:");
  w->end();
  w->show();

  browser1->add("Prime numbers:");
  browser2->add("Prime numbers:");

  // Enable multi-thread support by locking from the main
  // thread.  Fl::wait() and Fl::run() call Fl::unlock() and
  // Fl::lock() as needed to release control to the child threads
  // when it is safe to do so...
  Fl::lock();

  // Start threads...

  // One thread displaying in one browser
  fl_create_thread(prime_thread, prime_func, browser1);

  // Several threads displaying in another browser
  fl_create_thread(prime_thread, prime_func, browser2);
  fl_create_thread(prime_thread, prime_func, browser2);
  fl_create_thread(prime_thread, prime_func, browser2);
  fl_create_thread(prime_thread, prime_func, browser2);
  fl_create_thread(prime_thread, prime_func, browser2);
  fl_create_thread(prime_thread, prime_func, browser2);

  Fl::run();

  return 0;
}
#else
#  include <FL/fl_ask.H>

int main() {
  fl_alert("Sorry, threading not supported on this platform!");
}
#endif // HAVE_PTHREAD || WIN32