segmentation fault on clicking a GtkButton

Question

Depending on the location of the init_prog(user) call in my program I get a segmentation fault, I don't understand the reason. If I put the call in the start() callback function after clicking btn_go it crashes:

Thread 1 "./a.out" received signal SIGSEGV, Segmentation fault. 0x00007ffff7812526 in gdk_window_get_display (window=0x55555555b1c0) at ../../../../gdk/gdkwindow.c:2303 2303 ../../../../gdk/gdkwindow.c: No such file or directory.

This is the back trace:

Thread 1 "./a.out" received signal SIGSEGV, Segmentation fault.
0x00007ffff7812526 in gdk_window_get_display (window=0x55555555b1c0) at ../../../../gdk/gdkwindow.c:2303
2303    ../../../../gdk/gdkwindow.c: No such file or directory.
(gdb) bt
#0  0x00007ffff7812526 in gdk_window_get_display (window=0x55555555b1c0) at ../../../../gdk/gdkwindow.c:2303
#1  0x00007ffff78022ce in event_get_display (event=0x7fffe800add0) at ../../../../gdk/gdkevents.c:457
#2  0x00007ffff78022ce in gdk_event_free (event=0x7fffe800add0) at ../../../../gdk/gdkevents.c:839
#3  0x00007ffff783311a in gdk_event_source_dispatch
    (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>)
    at ../../../../../gdk/x11/gdkeventsource.c:369
#4  0x00007ffff730cf2e in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#5  0x00007ffff730d1c8 in  () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#6  0x00007ffff730d4c2 in g_main_loop_run () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#7  0x00007ffff7affb15 in gtk_main () at ../../../../gtk/gtkmain.c:1323
#8  0x0000555555556da1 in create_window () at gui.c:118
#9  0x000055555555644d in main (argc=1, argv=0x7fffffffe328) at main.c:26

This is the code:

#include <glib.h>
#include <stdio.h>
#include <gtk/gtk.h>

void start (GtkWidget *widget, struct prog_data *user);

void create_window() {

    GtkWidget *window;
    GtkWidget *headbar;
    GtkWidget *vbox;
    GtkWidget *hbox_superself;
    GtkWidget *hbox_my_struct;
    GtkWidget *hbox_self;
    GtkWidget *btn_about;
    GtkWidget *btn_go;
    GtkWidget *evnt_box[3];

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    headbar = gtk_header_bar_new();
    btn_about = gtk_button_new_with_label("About");
    btn_go = gtk_button_new_with_label("Start");
    evnt_box[0] = gtk_evnt_box_new ();
    evnt_box[1] = gtk_evnt_box_new ();
    evnt_box[2] = gtk_evnt_box_new ();
    vbox = gtk_box_new (GTK_ORIENTATION_VERTICAL, 0);
    hbox_superself = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 35);
    hbox_my_struct = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
    hbox_self = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 35);
    my_struct.label_user[USR0] = gtk_label_new ("0");
    my_struct.label_user[USR1] = gtk_label_new ("0");
    my_struct.lbl_pings_left = gtk_label_new ("");

    my_struct.img_show[0] = gtk_image_new ();
    my_struct.img_show[1] = gtk_image_new ();

    my_struct.image_deck_pile = gtk_image_new_from_file ("1.png");

my_struct.self_image[0] = gtk_image_new_from_file ("2.png");
my_struct.self_image[1] = gtk_image_new_from_file ("2.png");
my_struct.self_image[2] = gtk_image_new_from_file ("2.png");

    my_struct.image_orca = gtk_image_new ();

    my_struct.super_self[0] = gtk_image_new();
    my_struct.super_self[1] = gtk_image_new();
    my_struct.super_self[2] = gtk_image_new();

    gtk_header_bar_set_title (GTK_HEADER_BAR (headbar), "Orca");
    gtk_window_set_title (GTK_WINDOW (window), "Orca");    
    gtk_window_set_titlebar (GTK_WINDOW (window), headbar);
    gtk_header_bar_set_show_close_button (GTK_HEADER_BAR (headbar), TRUE);
    gtk_window_maximize (GTK_WINDOW (window));

    GtkCssProvider *css_provider = gtk_css_provider_new();
    gtk_css_provider_load_from_path (css_provider, "style.css", NULL);

    GdkScreen *myScreen = gdk_screen_get_default();
    gtk_style_context_add_provider_for_screen (myScreen, GTK_STYLE_PROVIDER (css_provider), GTK_STYLE_PROVIDER_PRIORITY_USER);

    struct prog_data user[2];

    gtk_container_add(GTK_CONTAINER (headbar), btn_about);
    gtk_container_add(GTK_CONTAINER (window), vbox);
    gtk_container_add(GTK_CONTAINER (vbox), hbox_superself);
    gtk_container_add(GTK_CONTAINER (vbox), hbox_my_struct);
    gtk_container_add(GTK_CONTAINER (vbox), hbox_self);
    gtk_container_add(GTK_CONTAINER (hbox_superself), my_struct.self_image[0]);
    gtk_container_add(GTK_CONTAINER (hbox_superself), my_struct.self_image[1]);
    gtk_container_add(GTK_CONTAINER (hbox_superself), my_struct.self_image[2]);
    gtk_container_add(GTK_CONTAINER (hbox_superself), my_struct.label_user[USR1]);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), my_struct.image_orca);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), my_struct.image_deck_pile);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), my_struct.lbl_pings_left);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), btn_go);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), my_struct.img_show[0]);
    gtk_container_add(GTK_CONTAINER (hbox_my_struct), my_struct.img_show[1]);
    gtk_container_add(GTK_CONTAINER (hbox_self), evnt_box[0]);
    gtk_container_add(GTK_CONTAINER (hbox_self), evnt_box[1]);
    gtk_container_add(GTK_CONTAINER (hbox_self), evnt_box[2]);
    gtk_container_add(GTK_CONTAINER (evnt_box[0]), my_struct.super_self[0]);
    gtk_container_add(GTK_CONTAINER (evnt_box[1]), my_struct.super_self[1]);
    gtk_container_add(GTK_CONTAINER (evnt_box[2]), my_struct.super_self[2]);
    gtk_container_add(GTK_CONTAINER (hbox_self), my_struct.label_user[USR0]);

    GtkStyleContext *context1;                                      
    context1 = gtk_widget_get_style_context(hbox_self);         
    gtk_style_context_add_class(context1, "my_hbox_self");

    GtkStyleContext *context2;
    context2 = gtk_widget_get_style_context(hbox_superself);    
    gtk_style_context_add_class(context2, "my_hbox_superself");

    GtkStyleContext *context3;
    context3 = gtk_widget_get_style_context(hbox_my_struct);    
    gtk_style_context_add_class(context3, "my_hbox_my_struct");

    gtk_widget_set_name (my_struct.img_show[0], "user1");

    g_signal_connect (btn_about, "clicked", G_CALLBACK (activate_about), NULL);
    g_signal_connect (G_OBJECT (evnt_box[0]), "button_press_event", G_CALLBACK (ping1_clicked), user);
    g_signal_connect (G_OBJECT (evnt_box[1]), "button_press_event", G_CALLBACK (ping2_clicked), user);
    g_signal_connect (G_OBJECT (evnt_box[2]), "button_press_event", G_CALLBACK (ping3_clicked), user);
    g_signal_connect (G_OBJECT (btn_go), "button_press_event", G_CALLBACK (start), user);
    g_signal_connect (G_OBJECT (window), "destroy", G_CALLBACK (destroy), NULL);

    gtk_widget_show_all (window);

    gtk_main();
}

void start (GtkWidget *btn_go, struct prog_data *user) {
    gtk_widget_hide(btn_go);
    printf("Right\n");
    init_prog(user);    
}

If I move the call init_prog(user) in the create_window() function after gtk_widget_show_all (window);, I do not experience any crash. I've tested the init_prot(user) function separately and it works well.

Answer 1

You are cheating GTK by providing an incorrect callback function.

The manual tells us how your callback shall look like for "button_press_event":

gboolean
user_function (GtkWidget *widget,
               GdkEvent  *event,
               gpointer   user_data)

You access event pointer and treat is as user_data pointer. That causes undefined behaviour. You also fail to provide mandatory boolean return value.

You must adjust the signature of your callback function to match the requirements.

Or maybe you might change to use "clicked" or "pressed" event of the button widget instead of the "button-pressed-event".

You also should check the other callback functions that were not present in the question for the same error.