Desenvolvendo uma calculadora em C com GTK

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
GtkWidget *entry;
int first_number = -1;
int second_number = -1;
char symbol = '+';
int need_clear = 0;
int has_press_symbol = 0;
int has_press_number = 0;
int main(int argc, char *argv[])
{
    /*init window*/
    gtk_init(&argc, &argv);
    GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT(window), "delete_event", G_CALLBACK(delete_event), NULL);
    gtk_window_set_default_size(GTK_WINDOW(window), 400, 400);
    gtk_window_set_title(GTK_WINDOW(window), "calculator");
    gtk_container_set_border_width(GTK_CONTAINER(window), 10);

    /*init layout*/
    GtkWidget *vbox = gtk_vbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(window), vbox);
    gtk_widget_show(vbox);

    GtkWidget *hbox1 = gtk_hbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(vbox), hbox1);

    entry = malloc(sizeof(GtkWidget));
    memset(entry, 0, sizeof(GtkWidget));
    entry = gtk_entry_new();
    gtk_editable_set_editable(GTK_EDITABLE(entry), FALSE);
    gtk_widget_set_direction(entry, GTK_TEXT_DIR_RTL);
    gtk_entry_set_text(GTK_ENTRY(entry), "0");
    gtk_box_pack_start(GTK_BOX(hbox1), entry, TRUE, TRUE, 0);
    gtk_widget_show(entry);

    GtkWidget *button = gtk_button_new_with_label("清零");
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(clear), NULL);
    gtk_box_pack_start(GTK_BOX(hbox1), button, TRUE, TRUE, 0);
    gtk_widget_show(button);
    gtk_widget_show(hbox1);


    GtkWidget *hbox2 = gtk_hbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(vbox), hbox2);
    create_num_button(hbox2, button, "7");
    create_num_button(hbox2, button, "8");
    create_num_button(hbox2, button, "9");


    button = gtk_button_new_with_label("+");
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(click_symbol), (gpointer)'+');
    gtk_box_pack_start(GTK_BOX(hbox2), button, TRUE, TRUE, 0);
    gtk_widget_show(button);
    gtk_widget_show(hbox2);



    GtkWidget *hbox3 = gtk_hbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(vbox), hbox3);
    create_num_button(hbox3, button, "4");
    create_num_button(hbox3, button, "5");
    create_num_button(hbox3, button, "6");

    button = gtk_button_new_with_label("-");
    gtk_box_pack_start(GTK_BOX(hbox3), button, TRUE, TRUE, 0);
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(click_symbol), (gpointer)'-');
    gtk_widget_show(button);
    gtk_widget_show(hbox3);


    GtkWidget *hbox4 = gtk_hbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(vbox), hbox4);
    create_num_button(hbox4, button, "1");
    create_num_button(hbox4, button, "2");
    create_num_button(hbox4, button, "3");

    button = gtk_button_new_with_label("*");
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(click_symbol), (gpointer)'*');
    gtk_box_pack_start(GTK_BOX(hbox4), button, TRUE, TRUE, 0);
    gtk_widget_show(button);
    gtk_widget_show(hbox4);

    GtkWidget *hbox5 = gtk_hbox_new(FALSE, 0);
    gtk_container_add(GTK_CONTAINER(vbox), hbox5);
    create_num_button(hbox5, button, "0");
    button = gtk_button_new_with_label("=");
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(submit), NULL);
    gtk_box_pack_start(GTK_BOX(hbox5), button, TRUE, TRUE, 0);
    gtk_widget_show(button);


    button = gtk_button_new_with_label("/");
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(click_symbol), (gpointer)'/');
    gtk_box_pack_start(GTK_BOX(hbox5), button, TRUE, TRUE, 0);
    gtk_widget_show(button);
    gtk_widget_show(hbox5);

    gtk_widget_show(window);
    gtk_main();
    exit(0);
}



gint delete_event(GtkWidget *window, GdkEvent *event, gpointer data)
{
    gtk_main_quit();
    return FALSE;
}


void create_num_button(GtkWidget *box, GtkWidget *button, char* button_num)
{
    button = gtk_button_new_with_label(button_num);
    g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(click_number), (gpointer)button_num);
    gtk_box_pack_start(GTK_BOX(box), button, TRUE, TRUE, 0);
    gtk_widget_show(button);
}

void click_number(GtkWidget *widget, gpointer data)
{
    if(need_clear)
    {
        gtk_entry_set_text(GTK_ENTRY(entry), "0");
        need_clear = 0;
    }
    char* button_num = (char*)data;
    int entry_int = get_entry_value();
    if(entry_int == 0)
        gtk_entry_set_text(GTK_ENTRY(entry), button_num);
    else
        gtk_entry_append_text(GTK_ENTRY(entry), button_num);
    has_press_number = 1;
    has_press_symbol = 0;
}



void clear(GtkWidget *widget, gpointer data)
{
    gtk_entry_set_text(GTK_ENTRY(entry), "0");
    first_number = -1;
    second_number = -1;
    has_press_number = 0;
    has_press_symbol = 0;
    need_clear = 0;
    symbol = '+';
}

void click_symbol(GtkWidget *widget, gpointer data)
{
    if(has_press_symbol == 0)
    {
        if(second_number == -1 && first_number == -1 && has_press_number == 0)
        {}
        else if(second_number == -1 && first_number == -1 && has_press_number == 1)
        {
            first_number = get_entry_value();
            has_press_number = 0;
        }
        else if(second_number == -1 && first_number != -1 && has_press_number == 1)
        {
            second_number = get_entry_value();
            first_number = calculator(symbol);
            second_number = -1;
            char* result_string = malloc(sizeof(char) * 10);
            memset(result_string, 0, 10);
            sprintf(result_string, "%d", first_number);
            gtk_entry_set_text(GTK_ENTRY(entry), result_string);
        }
        symbol = (char)data;
        need_clear = 1;
        has_press_symbol = 1;
    }
}

void submit(GtkWidget *widget, gpointer data)
{
    second_number = get_entry_value();
    int result = calculator(symbol);
    second_number = -1;
    has_press_symbol = 0;
    has_press_number = 0;
    first_number = result;
    char* result_string = malloc(sizeof(char) * 10);
    memset(result_string, 0, 10);
    sprintf(result_string, "%d", result);
    gtk_entry_set_text(GTK_ENTRY(entry), result_string);
}

int calculator(char symbol)
{
    int result;
    switch(symbol)
    {
        case '+':
            result = first_number + second_number;
            break;
        case '-':
            result = first_number - second_number;
            break;
        case '*':
            result = first_number * second_number;
            break;
        case '/':
            result = first_number / second_number;
            break;
        default:
            printf("error\n");
            return;
    }
    return result;
}


int get_entry_value()
{
    const char* entry_string = gtk_entry_get_text(GTK_ENTRY(entry));
    int entry_int = *entry_string - '0';
    entry_string++;
    while(*entry_string != '\0')
    {
        entry_int = entry_int * 10 + (*entry_string - '0');
        entry_string++;
    }
    return entry_int;
}
#ifndef CALCULATOR_H
#define CALCULATOR_H
#include <gtk/gtk.h>
typedef struct calbutton
{
    char *value;
    GtkWidget *button;
}calbutton;

/*function prototype*/
gint delete_event(GtkWidget *window, GdkEvent *event, gpointer data);
void create_num_button(GtkWidget *box, GtkWidget *button, char* button_num);
void click_number(GtkWidget *widget, gpointer data);
void clear(GtkWidget *widget, gpointer data);
void submit(GtkWidget *widget, gpointer data);
void add(GtkWidget *widget, gpointer data);
void sub(GtkWidget *widget, gpointer data);
void mul(GtkWidget *widget, gpointer data);
void division(GtkWidget *widget, gpointer data);
void click_symbol(GtkWidget *widget, gpointer data);
int calculator(char symbol);
#endif

Alguém pode me ajudar a terminar essa calculadora? acho que só falta alguns detalhes.