The handles structure that your callback receives is meant exactly for this:
%It's a good idea to create the variable used in the callback when the gui is first created. e.g. in the OpeningFcn callback
function mygui_OpeningFcn(hObject, eventdata, handles)
handles.a = NaN;
%...
end
function a_Callback(hObject, eventdata, handles)
handles.a = 2+2;
guidata(hObject, handles) %save new state of handles
end
function c_Callback(hObject, eventdata, handles)
c = 2 + handles.a;
%...
end
