C GTK+ Passing Data

[lewisforlife]

I need help passing data in a GTK+ program with g_signal_connect. Here is my example:


g_signal_connect (G_OBJECT (button), "clicked", G_CALLBACK (callback), (gpointer) "hello world");

Then I have the function callback:

static void callback( GtkWidget *widget,
gpointer data )
{
g_print ("%s", (gchar *) data);
}

This will print how "hello world" when the function callback is called. Can I pass other types of data besides a string, such as numbers, arrays, etc... I have tried passing other kinds of data, but get compilation errors when I pass anything else. Are there any examples I can look at?

[napsy]

The data argument of your callback works as a generic pointer (gpointer is void *). So you can pass any type you wish. Just cast the pointer into the appropriate type before using it in the callback.

[nvteighen]

It's usually very good to use a data structure when you need to pass more than one thing. A (void **) array can be a mess to use if you want a generic array.

[kahumba]

I'm somewhat confused that your example works.
I think that the example wouldn't work if it was called inside a non-main function?
Because writing directly "hello world" would create it on the function's stack?

[nvteighen]

I'm somewhat confused that your example works.
I think that the example wouldn't work if it was called inside a non-main function?
Because writing directly "hello world" would create it on the function's stack?

You're right. It wouldn't work.

[lewisforlife]

I'm somewhat confused that your example works.
I think that the example wouldn't work if it was called inside a non-main function?
Because writing directly "hello world" would create it on the function's stack?

Actually, in my program on my computer, it is in the main function, I should have posted that, but I wanted to make my example as simple as possible. I am wanting to pass an integer array, it is kind of working, but not all-the-way. Here is what I have:


/* inside main function */
int iArray [2] = {2, 1}
g_signal_connect (G_OBJECT (eventbox_main, "button_press_event", G_CALLBACK (make_move), (gpointer) *iArray);


static void callback( GtkWidget *widget,
gpointer data )
{
g_print ("%d", (int) data);
}


This will output the number "2", How can I output the second slot in the array? For instnance, if I try:


g_print ("%d", (int) data[1]);

I get: invalid use of void expression.

Thanks for your help.

[nvteighen]

/* inside main function */
int iArray [2] = {2, 1};
g_signal_connect (G_OBJECT (eventbox_main, "button_press_event", G_CALLBACK (make_move),
(gpointer) *iArray);


static void callback( GtkWidget *widget,
gpointer data )
{
g_print ("%d", (int) data);
}


This will output the number "2", How can I output the second slot in the array? For instnance, if I try:


g_print ("%d", (int) data[1]);

I get: invalid use of void expression.


The issue is that your casting in wrong way. The following is the correct code:


/* inside main function */
int iArray [2] = {2, 1}
g_signal_connect (G_OBJECT (eventbox_main, "button_press_event", G_CALLBACK (make_move),
(gpointer) iArray);

You don't need to cast to (gpointer *) because your dealing with a 1D array. gpointer is already a pointer, so it serves as (void *)... (gpointer *) = (void **)... i.e. a pointer-to-pointer.

Doing (int)data worked because of some pointer magic, but I'm sure (int)data[0] wouldn't have worked in your code as (int)data[1] didn't either.

[lewisforlife]

Ok, I think I figured it out, unless there is a better way of doing it. But here is how I did it:


int iArray [2] = {2, 1}
g_signal_connect (G_OBJECT (eventbox_main, "button_press_event", G_CALLBACK (make_move), (gpointer) iArray);

After that, instead of trying g_print ("%d", data[number]) which didn't work, I created an int pointer like so:



static void callback( GtkWidget *widget,
gpointer data )
{
int *test = data;
g_print ("%d, %d", test[0], test[1]);
}