Pass received argument to a callback function

Question

I am working on a Gtk project in C.

From the main.c I call a function1 with an int address as a parameter.

In that function1, I can access that first value, but then at the end (inside) of that function1, I call another function2 (which is a callback function to a click event) and pass it the address I got from the function1 parameter.

But in function2, the address have changed, definitely can't figure out why...

My project looks like this :

[main.c]

int main(...) {

    int a = 50;
    function1(&a);

}

[function1.c]

void function1(int* nb) {
    ...
    g_signal_connect(G_OBJECT(button),"clicked", G_CALLBACK(function2), &nb);
    // I know that the 4th arg expects void*, but even though I give the address of that _nb_ parameter, still can't get that 50 in function2
}

[function2.c]

void function2(void* nb) {
    ...
    printf("should got 50 : %d ", *(int*)nb);
    // shows random 8 digits number like 60035152
}

EDIT: Forgot to mention that each function is in a separate file, I don't know if that matters as long as I do the includes and gives the prototypes...

Thank you in advance...

You're passing the address of a local variable. That address becomes invalid when the function returns. — Barmar
– Barmar, Commented Dec 24, 2019 at 22:07
So in function1, in the g_signal_connect, i passed nb but it still don't give me that 50 in function2 — Narek
– Narek, Commented Dec 24, 2019 at 22:13

Siva Guru · Accepted Answer · 2019-12-25 17:58:32Z

1

The problem's in your code are:-

1) you are passing the address of the variable to the callback function so instead of &nb, it should be nb.

2) this is the callback function for clicked signal (https://developer.gnome.org/gtk3/stable/GtkButton.html#GtkButton-clicked_

void
user_function (GtkButton *button,
               gpointer   user_data)

you are missing an argument in your callback function

answered Dec 25, 2019 at 17:58

Siva Guru

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1 Comment

Narek Over a year ago

😍😍😍😍😍😍😍 Oh. My. Goodness. Thank you soooo much

Barmar · Accepted Answer · 2019-12-24 22:14:09Z

0

You have two problems:

First, you're passing the address of a local variable, but this cannot be used after the function returns.

Second, function2 expects nb to be a pointer to int, but you're passing a pointer to a pointer to int to g_signal_connect().

void function1(int* nb) {
    ...
    int *nb_copy = malloc(sizeof(int));
    *nb_copy = *nb;
    g_signal_connect(G_OBJECT(button),"clicked", G_CALLBACK(function2), nb_copy);
    // I know that the 4th arg expects void*, but even though I give the address of that _nb_ parameter, still can't get that 50 in function2
}

function_2() should free(nb); after it's done with it to prevent a memory leak.

answered Dec 24, 2019 at 22:14

Barmar

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3 Comments

Narek Over a year ago

Hi Thanks for taking the time to answer, I tried your solution but it still shows me a bunch of random digits, am I right to have the following for function2 : function2(void* nb) {... ? Thanks !

Barmar Over a year ago

Maybe you should use GINT_TO_POINTER?

Narek Over a year ago

Oh I didn't know about that Im going to try it out Thanks !

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