Thread: How do I g_print a number using a function?

In the following code, how do I print the variable a (local to main function) using the print_number function?

Code:

#include <gtk/gtk.h>


static void print_number (GtkWidget *widget, gpointer data) {
  g_print ("Number = %i\n",a);
}






int main (int   argc,char *argv[]) {


//Declare and assign a value to variable a
  int a=15;	
	
  GtkBuilder *builder;
  GObject *window;
  GObject *button;
  GObject *GTKMenuItem;


  gtk_init (&argc, &argv);


  /* Construct a GtkBuilder instance and load our UI description */
  builder = gtk_builder_new ();
  gtk_builder_add_from_file (builder, "demo.ui", NULL);


  /* Connect signal handlers to the constructed widgets. */
  window = gtk_builder_get_object (builder, "main_window");
  g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);


  button = gtk_builder_get_object (builder, "start_button");
  g_signal_connect (button, "clicked", G_CALLBACK (print_number), NULL);


  GTKMenuItem = gtk_builder_get_object (builder, "file_quit");
  g_signal_connect (GTKMenuItem, "activate", G_CALLBACK (gtk_main_quit), NULL);


  gtk_main ();


  return 0;
}

I get the error: ‘a’ undeclared (first use in this function. I understand why I get this error, since when I call the function in G_CALLBACK (print_number), I am not passing any arguments to it. So I tried this:

Code:

#include <gtk/gtk.h>


static void print_number (GtkWidget *widget, gpointer data, int a) {
  g_print ("Number = %i\n",a);
}


int main (int   argc,char *argv[]) {


//Declare and assign a value to variable a
  int a=15;	
	
  GtkBuilder *builder;
  GObject *window;
  GObject *button;
  GObject *GTKMenuItem;


  gtk_init (&argc, &argv);


  /* Construct a GtkBuilder instance and load our UI description */
  builder = gtk_builder_new ();
  gtk_builder_add_from_file (builder, "demo.ui", NULL);


  /* Connect signal handlers to the constructed widgets. */
  window = gtk_builder_get_object (builder, "main_window");
  g_signal_connect (window, "destroy", G_CALLBACK (gtk_main_quit), NULL);


  button = gtk_builder_get_object (builder, "start_button");
  g_signal_connect (button, "clicked", G_CALLBACK (print_number(a)), NULL);


  GTKMenuItem = gtk_builder_get_object (builder, "file_quit");
  g_signal_connect (GTKMenuItem, "activate", G_CALLBACK (gtk_main_quit), NULL);


  gtk_main ();


  return 0;
}

and I get the error: too few arguments to function ‘print_number’. How do I pass the value of a into the function print_number?

The way you have it you are simply calling print_number directly (with the wrong number of parameters). Instead, you need to pass the data to the gpointer argument by passing it's address as the last argument of g_signal_connect. You can't just add parameters to the callback function. It can only have the two allowed parameters.

Code:

static void print_number(GtkWidget *widget, gpointer data) {
  g_print("Number = %d\n", *(int*)data);
}

// in main
  g_signal_connect(button, "clicked", G_CALLBACK (print_number), &a);