Gtk+ Callback functions and signals help - c

Question

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_signal_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?.

Answer 1

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*