[SOLVED] Unused parameter, but program won't function without it?

Unia · 2013-04-09 17:52:45

Hey all,

I've come across this situation a few times before but I have always ignored it because I came up with a different approach. This time though my curiosity got the better of me (and I don't yet have another approach ) so I want to know why this happens.

I have my program here: GXMS. When you compile it, it says:

gxms.c: In function ‘keypress’:
gxms.c:38:31: let op: unused parameter ‘widget’ [-Wunused-parameter]
 gboolean keypress (GtkWidget *widget, GdkEventKey *event) {

The thing is, when I remove that so-called "unused" parameter, the program doesn't function anymore! This function is supposed to bring up the searchbar, but it does not anymore. Here is the function:

gboolean keypress (GtkWidget *widget, GdkEventKey *event) {
    guint(g) = event->keyval;

    if ((event->state & GDK_CONTROL_MASK) == GDK_CONTROL_MASK) {
        switch (g) {
            case GDK_KEY_f:
                show_search(TRUE);
                return TRUE;
                break;
            default:
                return FALSE;
                break;
        }
    }
    if (gtk_widget_has_focus (searchbar)) {
        if (g == GDK_KEY_Escape) {
            show_search (FALSE);
            gtk_widget_grab_focus (view);
            return TRUE;
        }
        if ((g == GDK_KEY_Return) && (event->state & GDK_MOD1_MASK) == GDK_MOD1_MASK)
            search ();
    }
    return FALSE;
}

When I define *widget inside the function, like below, it doesn't work anymore either:

gboolean keypress (GdkEventKey *event) {
    GtkWidget *widget;

    <snip>

Is this a bug in my code, in the compiler, or just one of my mistakes?

Last edited by Unia (2013-04-09 18:39:42)

SahibBommelig · 2013-04-09 18:07:31

You likely pass keypress() as a callback to a function, do you? Well, this function will call your function with 2 parameters. If your function can take only one bad things happen. If you're lucky your compiler may warn you, but don't rely on that - just look up what the callback is supposed to look like. The Gtk+ Docs will tell you for example.

To suppress the warning you can do either:

// Poor man's approach
void func(int arg) {
    (void) arg;
}

// Place this in some header
#if !defined(SPARSE) && defined(__GNUC__)
   #define unused __attribute__((unused))
#else 
   #define unused
#endif


void func(unused int arg) {
}

See also this.

Unia · 2013-04-09 18:19:59

I call keypress() from within the create_window function, like so:

g_signal_connect (window, "key-press-event", G_CALLBACK(keypress), NULL);

AFAIK, that doesn't actually pass any parameters to keypress().

SahibBommelig · 2013-04-09 18:24:54

You seem to have trouble to understand callbacks/gtk's event system it seems.

You don't call keypress in this snippet. You pass it as a function pointer to g_signal_connect.
G_CALLBACK is just a macro that casts your function pointer into the GCallback Type, it does not call the function.
keypress() gets always called once "key-press-event" happens.

Unia · 2013-04-09 18:27:46

SahibBommelig wrote:

You seem to have trouble to understand callbacks/gtk's event system it seems.
You don't call keypress in this snippet. You pass it as a function pointer to g_signal_connect.
G_CALLBACK is just a macro that casts your function pointer into the GCallback Type, it does not call the function.
keypress() gets always called once "key-press-event" happens.

I had that vague idea, yes But I'm still not getting why I need that "unused" variable *widget. My keypress() function does not use it, but it won't work without it.

moetunes · 2013-04-09 18:31:59

Looks to me like you need to write your keypress function a little differently, but it's been ages since I've done any gtk programming.
Something like:
if widget == searchbar
do some stuff
else if widget == view
do other stuff

SahibBommelig · 2013-04-09 18:35:57

Well, let's summarize:

...your callback is registered by g_signal_connect, by passing a function pointer to it.
...your applications enters the gtk_main_loop() at some point.
...when the mainloop notices a key-press-event on the widget you registered the callback on, it will take your registered callback and call it with two parameters: The widget the event happened on, and the event that happened.
...The reason you have to take 2 arguments: Gtk+ demands it. If your function doesn't take 2 arguments the best thing that will happen is a crash.

Unia · 2013-04-09 18:38:41

Moetunes: Indeed, that works

gboolean keypress (GtkWidget *widget, GdkEventKey *event) {
    guint(g) = event->keyval;

    if ((event->state & GDK_CONTROL_MASK) == GDK_CONTROL_MASK) {
        switch (g) {
            case GDK_KEY_f:
                show_search(TRUE);
                return TRUE;
                break;
            default:
                return FALSE;
                break;
        }
    }
+   if (widget != searchbar) {
        if (gtk_widget_has_focus (searchbar)) {
            if (g == GDK_KEY_Escape) {
                show_search (FALSE);
                gtk_widget_grab_focus (view);
                return TRUE;
            }
            if ((g == GDK_KEY_Return) && (event->state & GDK_MOD1_MASK) == GDK_MOD1_MASK)
                search ();
        }
+   }
    return FALSE;
}

SahibBommelig: And thanks to you too! Now I know why the above works and I understand the system I'm using a bit more!

Thanks guys!

SahibBommelig · 2013-04-09 18:41:01

One last thing: You don't need this check actually, since you could most likely do:

g_signal_connect (searchbar, "key-press-event", G_CALLBACK(keypress), NULL);

Unia · 2013-04-09 18:43:14

SahibBommelig wrote:

One last thing: You don't need this check actually, since you could most likely do:
g_signal_connect (searchbar, "key-press-event", G_CALLBACK(keypress), NULL);

I cant, since I need to capture CTRL+F in view, and not in searchbar, right?

SahibBommelig · 2013-04-09 18:46:39

Well, okay. Misinterpreted your code then.

moetunes · 2013-04-09 18:59:16

The main point being you have multiple widgets defined so you have to do something to check which one the keypress comes from.

Unia · 2013-04-09 19:09:26

moetunes wrote:

The main point being you have multiple widgets defined so you have to do something to check which one the keypress comes from.

Exactly, I get that now

Boogie · 2013-04-10 07:30:25

Is it really necessary to break; switch statement. you return just before break?

    if ((event->state & GDK_CONTROL_MASK) == GDK_CONTROL_MASK) {
        switch (g) {
            case GDK_KEY_f:
                show_search(TRUE);
                return TRUE;
                break;               // This is not needed
            default:
                return FALSE;
                break;               // This is not needed
        }
    }

Unia · 2013-04-10 10:27:32

Will see about that! Thanks for your suggestion

Unia · 2013-04-10 11:27:25

You were right Boogie, code is updated

Arch Linux

#1 2013-04-09 17:52:45

[SOLVED] Unused parameter, but program won't function without it?

#2 2013-04-09 18:07:31

Re: [SOLVED] Unused parameter, but program won't function without it?

#3 2013-04-09 18:19:59

Re: [SOLVED] Unused parameter, but program won't function without it?

#4 2013-04-09 18:24:54

Re: [SOLVED] Unused parameter, but program won't function without it?

#5 2013-04-09 18:27:46

Re: [SOLVED] Unused parameter, but program won't function without it?

#6 2013-04-09 18:31:59

Re: [SOLVED] Unused parameter, but program won't function without it?

#7 2013-04-09 18:35:57

Re: [SOLVED] Unused parameter, but program won't function without it?

#8 2013-04-09 18:38:41

Re: [SOLVED] Unused parameter, but program won't function without it?

#9 2013-04-09 18:41:01

Re: [SOLVED] Unused parameter, but program won't function without it?

#10 2013-04-09 18:43:14

Re: [SOLVED] Unused parameter, but program won't function without it?

#11 2013-04-09 18:46:39

Re: [SOLVED] Unused parameter, but program won't function without it?

#12 2013-04-09 18:59:16

Re: [SOLVED] Unused parameter, but program won't function without it?

#13 2013-04-09 19:09:26

Re: [SOLVED] Unused parameter, but program won't function without it?

#14 2013-04-10 07:30:25

Re: [SOLVED] Unused parameter, but program won't function without it?

#15 2013-04-10 10:27:32

Re: [SOLVED] Unused parameter, but program won't function without it?

#16 2013-04-10 11:27:25

Re: [SOLVED] Unused parameter, but program won't function without it?

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