How do I stop a function?

I have a function running when I push a button in a GUI. I want to end this function when I press another button. example:

function first_callback (hObject, eventdata, handles)
    a = b + c
    while...
    ...
    ...
    ...
    end
end
function stopbuttonback_callback (hObject, eventdata, handles)
    %something that stops function first_callback
end

Answer 1

[Walter Roberson]

function first_callback (hObject, eventdata, handles)
    a = b + c
    set(handles.stopbutton, 'UserData', false);
    while true
      drawnow();
      if get(stopbutton, 'UserData'); break; end
      ...
      ...
      ...
    end
end
function stopbuttonback_callback (hObject, eventdata, handles)
    set(hObject, 'UserData', true);
end

[Renato Pereira]

Thank you so much!!!!

[Renato Pereira]

Dont know why but when I write the "while true" the function stopbuttonback_callback doesn't work... let me give you the all code:

function start_Callback(hObject, eventdata, handles)
a = 0;
while a < 100
  b = datestr(a/(24*60*60),'HH:MM:SS');
  set(handles.time,'String',b);
  a = a + 1;
  pause(1)
end 
function stop_Callback(hObject, eventdata, handles)
set(handles.time,'String','00:00:00');

[Walter Roberson]

function start_Callback(hObject, eventdata, handles)
a = 0;
set(handles.stop, 'UserData', false);
while a < 100
  pause(1);       %pause works as well as drawnow() to allow interrupts
  if get(handles.stop, 'UserData'); break; end
  b = datestr(a/(24*60*60),'HH:MM:SS');
  set(handles.time,'String',b);
  a = a + 1;
end 
function stop_Callback(hObject, eventdata, handles)
set(handles.time,'String','00:00:00');
set(hObject, 'UserData', true);