Gtk+ Callback functions and signals help

I have the following example from here, it shows this under the "Increase - Decrease" title.

#include <gtk/gtk.h>

gint count = 0;
char buf[5];

void increase(GtkWidget *widget, gpointer label)
{
  count++;

  sprintf(buf, "%d", count);
  gtk_label_set_text(label, buf);
}

void decrease(GtkWidget *widget, gpointer label)
{
  count--;

  sprintf(buf, "%d", count);
  gtk_label_set_text(label, buf);
}

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

  GtkWidget *label;
  GtkWidget *window;
  GtkWidget *frame;
  GtkWidget *plus;
  GtkWidget *minus;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 250, 180);
  gtk_window_set_title(GTK_WINDOW(window), "+-");

  frame = gtk_fixed_new();
  gtk_container_add(GTK_CONTAINER(window), frame);

  plus = gtk_button_new_with_label("+");
  gtk_widget_set_size_request(plus, 80, 35);
  gtk_fixed_put(GTK_FIXED(frame), plus, 50, 20);

  minus = gtk_button_new_with_label("-");
  gtk_widget_set_size_request(minus, 80, 35);
  gtk_fixed_put(GTK_FIXED(frame), minus, 50, 80);

  label = gtk_label_new("0");
  gtk_fixed_put(GTK_FIXED(frame), label, 190, 58); 

  gtk_widget_show_all(window);

  g_signal_connect(window, "destroy",
      G_CALLBACK (gtk_main_quit), NULL);

  g_signal_connect(plus, "clicked", 
      G_CALLBACK(increase), label);

  g_signal_connect(minus, "clicked", 
      G_CALLBACK(decrease), label);

  gtk_main();

  return 0;
}

what I'm wondering is, the g_sig开发者_如何学运维nal_connect(plus, "clicked",G_CALLBACK(increase), label); function sends the "label" to the function increase, where its arguments are void increase(GtkWidget *widget, gpointer label). Now in the increase function the gtk_label_set_text() function requires a data type of GtkLabel as its first argument but I only see a GtkWidget variable and a void pointer label as the arguments to the increase function. If that is so how does the gtk_label_set_text() work?.

In C (but not C++), you can implicitly cast a void* to a pointer to any other type. It's very commonly seen when allocating memory with malloc, which returns a void*:

int *myIntArray = malloc(10 * sizeof(int));  // allocate array of 10 ints

Your code is doing the same thing, just with parameter passing:

void gtk_label_set_text(GtkLabel *label, const char *text);
void *label = ...;
gtk_label_set_text(label, "some string");  // label is implicitly cast from
                                           // void* to GtkLabel*