Gtk+ Callback functions and signals help

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?.

Adam Rosenfield · Accepted Answer · 2010-12-05 05:33:32Z

3

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*

answered Dec 5, 2010 at 5:33

Adam Rosenfield

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3 Comments

silent Over a year ago

Thanks, another thing, Why would we include the GtkWidget *widget in the increase function?.

Adam Rosenfield Over a year ago

@sil3nt: GtkWidget *widget is included in the function because the function has to have a very specific prototype. The GtkButton::clicked callback requires the callback function to have that signature and it will pass you a GtkWidget* as the first parameter. If your callback has the wrong signature, you'll receive the wrong parameters and you will most likely crash.

silent Over a year ago

So the first parameter to the increase function is passed in automatically through g_signal_connect(plus,"clicked",G_CALLBACK(increase), label);?

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