problem about gdk_input_add()

["gaof" <gaof guoguang com cn>]

My dear friends,

I want to redirect the ouput of a shell command(ping) to a gtk widget(text).
I use FIFO, the code is something like this:
    
    void
    on_my_ping_show    (GtkWidget    *widget,
                        gpointer      user_data)
    {
        int fd;

        fd = open("/root/fifo", O_RDONLY | O_NONBLOCK);

        gdk_input_add(fd, GDK_INPUT_READ, func, NULL);
    }

    void
    on_button4_clicked    (GtkButton    *button,
                           gpointer      user_data)
    {
        int pid;

        pid = fork();

        if (pid == 0) {
            system("ping -c 4 192.168.3.3 > /root/fifo");
            _exit(0);
        }
    }

    static void
    func(gpointer data, gint fd, GdkInputCondition conditon)
    {
        fprintf(stderr, "func####################\n");
    }

When I click "button4", the function of "func" is not called immediately but waits until the function of 
"system" returns.
Why?
Thanks!