Many questions when I drew a line

["Lazy Fox" <lazy fox wu gmail com>]

/***************************************************************************
I'm new to GTK+, please help me with following simple questions.
Thanks for your patience.

I wrote a simple programme to draw a line, the whole file is below
these questions:
1. I use gtk_widget_set_size_request() to resize the drawing_area,
   but it's useless. The size of the drawing_area will resize to it's
   parent's size automatically.How to make it's size fixed?
2. The bg_color I set in redraw() is useless too. Each time I run the
application,
   the background color is different. Looks like random color.
3. Where can find introduction of GrStyle? It's too simple in GTK+ Manual Page.
4. How to set the line color and line width?
5. How to use color? The common RGB color is like (255, 255, 255),
   but in GTK+ this can be (0xffff, 0xffff, 0xffff).
   I was confused, how to set a right RGB color? Is there a bunch of build in
   color I can use?
 ***************************************************************************/
#include <gtk/gtk.h>

gboolean redraw(GtkWidget *widget, GdkEvent *event, gpointer data)
{
        GdkGC *gc = gdk_gc_new(widget->window);
        GdkColor bg_color;

        // Draw background
        bg_color.red = 0xffff;
        bg_color.green = 0;
        bg_color.blue = 0;
        gdk_gc_set_foreground(gc, &bg_color);
        gdk_draw_rectangle(widget->window, gc, TRUE,
                        0, 0, widget->allocation.width, widget->allocation.height);
        gdk_draw_line(widget->window, widget->style->fg_gc[GTK_WIDGET_STATE(widget)],
                        0, 0, 120, 100);

        return TRUE;
}

int main(int argc, char *argv[])
{
        GtkWidget *w;
        GtkWidget *drawing_area;

        gtk_init(&argc, &argv);

        w = gtk_window_new(GTK_WINDOW_TOPLEVEL);
        gtk_window_set_default_size(GTK_WINDOW(w), 300, 200);
        g_signal_connect(GTK_OBJECT(w), "destroy", G_CALLBACK(gtk_main_quit), NULL);

        drawing_area = gtk_drawing_area_new();
        gtk_widget_set_size_request(GTK_WIDGET(drawing_area), 100, 100);
        g_signal_connect(G_OBJECT(drawing_area), "expose-event",
G_CALLBACK(redraw), NULL);
        gtk_container_add(GTK_CONTAINER(w), drawing_area);

        gtk_widget_show_all(w);
        gtk_main();
        return 0;
}