接下来我们做一个简单版的计算器。
1）获取按钮上的内容。
2）如果获取的内容是“ c ”，则代表进行退格操作，相当于删去最后一个字符。
3）如果获取的内容不是“ c ”，则把每一次获取的内容连接起来，如：
第一次按了 “ 1 ”， 第二次按了 “ + ”， 第三次按了 “ 1 ”， 连起来则变为 “1+1”。
4）如果获取的内容是“ = ”，则需要相应的运算。先把连起来的字符串拆出来，然后再进行相应操作，最后把运算结果重新显示到行编辑上。

完整代码如下：

#include <gtk/gtk.h>     // 头文件
#include <string.h>

char buf[50] = {0};    //全局变量，保存数据

// 按钮的回调函数
void deal_num(GtkButton *button, gpointer data)
{
    const char *txt = gtk_button_get_label( button ); // 获取点击按钮的内容

    // 退格操作
    if( 0 == strcmp("c", txt) ){ // 如果是退格键
        buf[strlen(buf) - 1] = 0; // 最后一个字符变为 0
        //gtk_entry_set_text(GTK_ENTRY(data), buf);    //重新给行编辑设定内容

    }else{
        int a = 0, b = 0;
        char c;
        strcat(buf, txt);    // 内容追加

        // 运算操作
        if(0 == strcmp("=", txt) ){    // 如果是 " = "
            sscanf(buf, "%d%c%d", &a, &c, &b);    // 拆包
            printf("a=%d, c = %c, d=%d\n", a, c, b);
            if('+' == c){
                sprintf(buf, "%d", a+b);
            }else if('-' == c){
                sprintf(buf, "%d", a-b);
            }else if('*' == c){
                sprintf(buf, "%d", a*b);
            }else if('/' == c){
                sprintf(buf, "%d", a/b);
            }
        }
    }

    gtk_entry_set_text(GTK_ENTRY(data), buf);    // 给行编辑设定内容


}


int main( int   argc, char *argv[] ) 
{
    gtk_init (&argc, &argv);     // 初始化

    // 窗口的操作
    GtkWidget *window = gtk_window_new (GTK_WINDOW_TOPLEVEL);      // 创建窗口 
    gtk_widget_set_size_request(window, 270, 320);

    // 表格的操作
    GtkWidget *table = gtk_table_new(5, 4, TRUE);   // 表格布局，5行4列  

    // 行编辑的操作
    GtkWidget *entry = gtk_entry_new();        // 行编辑的创建
    //gtk_entry_set_text(GTK_ENTRY(entry), "2+2=4");    // 给行编辑设置内容
    gtk_editable_set_editable(GTK_EDITABLE(entry), FALSE);// 设置行编辑不允许编辑，只做显示用

    // 按钮的操作
    GtkWidget *button0 = gtk_button_new_with_label("0");    // 按钮0
    GtkWidget *button1 = gtk_button_new_with_label("1");    // 按钮1
    GtkWidget *button2 = gtk_button_new_with_label("2");    // 按钮2
    GtkWidget *button3 = gtk_button_new_with_label("3");    // 按钮3
    GtkWidget *button4 = gtk_button_new_with_label("4");    // 按钮4
    GtkWidget *button5 = gtk_button_new_with_label("5");    // 按钮5
    GtkWidget *button6 = gtk_button_new_with_label("6");    // 按钮6
    GtkWidget *button7 = gtk_button_new_with_label("7");    // 按钮7
    GtkWidget *button8 = gtk_button_new_with_label("8");    // 按钮8
    GtkWidget *button9 = gtk_button_new_with_label("9");    // 按钮9
    GtkWidget *button_add = gtk_button_new_with_label("+");            // 加
    GtkWidget *button_minus = gtk_button_new_with_label("-");         // 减
    GtkWidget *button_multiply = gtk_button_new_with_label("*");    // 乘
    GtkWidget *button_divide = gtk_button_new_with_label("/");        // 除
    GtkWidget *button_equal = gtk_button_new_with_label("=");        // 等于
    GtkWidget *button_detele = gtk_button_new_with_label("c");        // 退格

    // 布局操作
    gtk_container_add(GTK_CONTAINER(window), table); // 表格放入窗口 
    gtk_table_attach_defaults(GTK_TABLE(table), entry, 0, 4, 0, 1);    // 行编辑放入表格
    gtk_table_attach_defaults(GTK_TABLE(table), button0, 0, 1, 4, 5);    // 按钮放入表格
    gtk_table_attach_defaults(GTK_TABLE(table), button1, 0, 1, 3, 4);
    gtk_table_attach_defaults(GTK_TABLE(table), button2, 1, 2, 3, 4);
    gtk_table_attach_defaults(GTK_TABLE(table), button3, 2, 3, 3, 4);
    gtk_table_attach_defaults(GTK_TABLE(table), button4, 0, 1, 2, 3);
    gtk_table_attach_defaults(GTK_TABLE(table), button5, 1, 2, 2, 3);
    gtk_table_attach_defaults(GTK_TABLE(table), button6, 2, 3, 2, 3);
    gtk_table_attach_defaults(GTK_TABLE(table), button7, 0, 1, 1, 2);
    gtk_table_attach_defaults(GTK_TABLE(table), button8, 1, 2, 1, 2);
    gtk_table_attach_defaults(GTK_TABLE(table), button9, 2, 3, 1, 2);
    gtk_table_attach_defaults(GTK_TABLE(table), button_add, 1, 2, 4, 5);
    gtk_table_attach_defaults(GTK_TABLE(table), button_minus, 2, 3, 4, 5);
    gtk_table_attach_defaults(GTK_TABLE(table), button_multiply, 3, 4, 2, 3);
    gtk_table_attach_defaults(GTK_TABLE(table), button_divide, 3, 4, 3, 4);
    gtk_table_attach_defaults(GTK_TABLE(table), button_equal, 3, 4, 4, 5);
    gtk_table_attach_defaults(GTK_TABLE(table), button_detele, 3, 4, 1, 2);

    // 注册信号函数，把 entry 传给回调函数 deal_num()
    g_signal_connect(button0, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button1, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button2, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button3, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button4, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button5, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button6, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button7, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button8, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button9, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_add, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_minus, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_multiply, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_divide, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_equal, "pressed", G_CALLBACK(deal_num), entry);
    g_signal_connect(button_detele, "pressed", G_CALLBACK(deal_num), entry);

    gtk_widget_show_all(window);  // 显示所有控件

    gtk_main();  //进入事件循环 

    return 0; 
} 

操作图如下：