You will need to edit your main code considerably to get it to link with the output from FLUID. If you are not interested in this you may have more immediate luck with the forms compatibility header, <FL/forms.H>.
You will also have to compile your Forms or XForms program using a C++ compiler. The FLTK library does not provide C bindings or header files.
Although FLTK was designed to be compatible with the GL Forms library (version 0.3 or so), XForms has bloated severely and it's interface is X-specific. Therefore, XForms compatibility is no longer a goal of FLTK. Compatibility was limited to things that were free, or that would add code that would not be linked in if the feature is unused, or that was not X-specific.
To use any new features of FLTK, you should rewrite your code to not use the inline functions and instead use "pure" FLTK. This will make it a lot cleaner and make it easier to figure out how to call the FLTK functions. Unfortunately this conversion is harder than expected and even Digital Domain's inhouse code still uses forms.H a lot.
Many parts of XForms use X-specific structures like XEvent in their interface. I did not emulate these! Unfortunately these features (such as the "canvas" widget) are needed by most large programs. You will need to rewrite these to use FLTK subclasses.
Fl_Free widgets emulate the old Forms "free" widget. It may be useful for porting programs that change the handle() function on widgets, but you will still need to rewrite things.
Fl_Timer widgets are provided to emulate the XForms timer. These work, but are quite inefficient and inaccurate compared to using Fl::add_timeout().
All instance variables are hidden. If you directly refer to the x, y, w, h, label, or other fields of your Forms widgets you will have to add empty parenthesis after each reference. The easiest way to do this is to globally replace "->x" with "->x()", etc. Replace "boxtype" with "box()".
const char * arguments to most FLTK methods are simply stored, while Forms would strdup() the passed string. This is most noticable with the label of widgets. Your program must always pass static data such as a string constant or malloc'd buffer to label(). If you are using labels to display program output you may want to try the Fl_Output widget.
The default fonts and sizes are matched to the older GL version of Forms, so all labels will draw somewhat larger than an XForms program does.
fdesign outputs a setting of a "fdui" instance variable to the main window. I did not emulate this because I wanted all instance variables to be hidden. You can store the same information in the user_data() field of a window. To do this, search through the fdesign output for all occurances of "->fdui" and edit to use "->user_data()" instead. This will require casts and is not trivial.
The prototype for the functions passed to fl_add_timeout() and fl_set_idle_callback() callback are different.
All the following XForms calls are missing:
None of this works with FLTK. Nor will it compile, the necessary calls are not in the interface.
You have to make a subclass of Fl_Gl_Window and write a draw() method and handle() method. This may require anywhere from a trivial to a major rewrite.
If you draw into the overlay planes you will have to also write a draw_overlay() method and call redraw_overlay() on the OpenGL window.
One easy way to hack your program so it works is to make the draw() and handle() methods on your window set some static variables, storing what event happened. Then in the main loop of your program, call Fl::wait() and then check these variables, acting on them as though they are events read from fl_queue.
The file <FL/gl.h> defines replacements for a lot of IRISGL calls, translating them to OpenGL. There are much better translators available that you might want to investigate.
An attempt has been made to emulate the "free" widget. This appears to work quite well. It may be quicker to modify your subclass into a "free" widget, since the "handle" functions match.
If your subclass draws into the overlay you are in trouble and will have to rewrite things a lot.
fl_font_name(3,"*courier-medium-r-no*"); fl_font_name(4,"*courier-bold-r-no*"); fl_font_name(5,"*courier-medium-o-no*"); fl_font_name(6,"*times-medium-r-no*"); fl_font_name(7,"*times-bold-r-no*"); fl_font_name(8,"*times-medium-i-no*"); fl_font_name(9,"*bookman-light-r-no*"); fl_font_name(10,"*bookman-demi-r-no*"); fl_font_name(11,"*bookman-light-i-no*");