Design pattern: First Class Abstract Data Type

gui.c

#include <gtk/gtk.h>                                                                                                                                                                                                                         
#include <string.h>
#include <stdlib.h>

typedef struct Window* WindowPtr;

struct Window {
    char *title;
    GtkWidget *win;
};  
    
static void print_msg (GtkButton *button, WindowPtr window) {
    puts(window->title);
}
    
WindowPtr create_window(const char *title) {
    
    WindowPtr window = malloc(sizeof *window);

    if (window) {
        window->title = malloc(strlen(title)+1);
        strcpy(window->title, title);
        GtkWidget *button;

        window->win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
        gtk_window_set_type_hint(GTK_WINDOW(window->win), GDK_WINDOW_TYPE_HINT_DIALOG);
        gtk_window_set_title(GTK_WINDOW(window->win), title);
        gtk_window_set_default_size(GTK_WINDOW(window->win), 200, 200);
        g_signal_connect (GTK_WIDGET (window->win), "destroy", G_CALLBACK (gtk_main_quit), NULL);


        button = gtk_button_new_with_label("Click me");
        gtk_container_add(GTK_CONTAINER(window->win), button); 
        g_signal_connect(button, "clicked", G_CALLBACK(print_msg), window);

        gtk_widget_show_all(window->win);
    }
    return window;
}

void destroy_window(WindowPtr window) {
    if (gtk_main_level() !=0 && window->win != NULL)
        gtk_widget_destroy(window->win);
    free(window);
}

main.c

#include <gtk/gtk.h>                                                                                                                                                                                                                         

typedef struct Window* WindowPtr;
WindowPtr create_window(const char *title);
void destroy_window(WindowPtr window);                                                                                

int main(int argc, char *argv[]) {
    gtk_init(&argc, &argv);                                                         
                
    WindowPtr win1 = create_window("The Title");
    WindowPtr win2= create_window("Another windy!");                                

    gtk_main();     

    destroy_window(win1);                                                           
    destroy_window(win2);
    

makefile

CC=gcc
CFLAGS=-std=c99 -g -Wall 
GTKFLAGS=`pkg-config --cflags --libs gtk+-3.0`
GTKLIBS=`pkg-config --cflags --libs gtk+-3.0`
 
gui: main.c gui.c
    $(CC) main.c gui.c -o gui $(EXECUTABLE) $(CFLAGS) $(GTKFLAGS)

.PHONY: clean
clean:
    rm -f gui