Fl_Int_Input *int_input1, *int_input2; // These are initialized somewhere
 void button_callback(Fl_Widget *button, void *data)
 {
   // Button pressed, get values from inputs

   // You may want to do some sanity checks here
   if (int_input1->size() > 0 && int_input2->size() > 0) {
     long val1 = strtol(int_input1->value(),NULL,10); // Use 16 if hex value
     long val2 = strtol(int_input2->value(),NULL,10); // Use 16 if hex value
     do_calculation(val1, val2); // Perform some operation with values
   } else {
     fl_alert("Please fill both fields");
   }
 }

#include <FL/Fl.H>				
#include <FL/Fl_Window.H>
#include <Fl/Fl_Input.H>
#include <Fl/Fl_Output.H>
#include <Fl/Fl_Button.H>
#include <FL/Fl_Int_Input.H>


#include <string>
string firstname;
string greeting;


Fl_Int_Input  *nameInput;
Fl_Button *helloButton;
Fl_Output *outBox;

void doHelloButton(Fl_Widget* button, void* data) {
	 int val1 = strtol(nameInput->value(),NULL,10); 
//Stores inputinteger in variable, val1
   	 int val2=val1*4;                               
//Times val1 by four and stores in val2
 	 //output into outBox
}

int main() {
    Fl_Window *window = new Fl_Window(500,300,"My FLTK program");
  
	nameInput = new Fl_Int_Input(50,25,100,50,"Name");
	helloButton = new Fl_Button(50,150,150,50, "Say Hi");	 
	helloButton->callback(doHelloButton); 	    
  window->end();
  window->show();  
  return Fl::run();
}

void doHelloButton(Fl_Widget* button, void* data) {
  int val1 = strtol(nameInput->value(),NULL,10); 
  //Stores inputinteger in variable, val1
  int val2=val1*4;                               
  //Times val1 by four and stores in val2
  //output into outBox

  // Print value 'val2' to 'tmp' buffer and set it to outBox
  char tmp[16]; sprintf(tmp, "%d", val2);
  outBox->value(tmp);
}