how to correctly draw a line in Cairo gtk3.0 after pressing a button

Question

I am following the zetecode (http://zetcode.com/gfx/cairo/basicdrawing/) and trying to build my first example of drawing a line after the button is pressed, but it failed and I just couldn't figure out why. I got a segmentation fault error when click the button.

#include <cairo.h>
#include <gtk/gtk.h>

static void do_drawing(cairo_t *);

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr, 
    gpointer user_data)
{
    do_drawing(cr);
    return FALSE;
}


static void do_drawing(cairo_t *cr)
{
    cairo_set_source_rgb(cr,0,0,0);
    cairo_set_line_width(cr,0.5);

    cairo_move_to(cr,400,400);
    cairo_line_to(cr,400,200);

    cairo_stroke(cr);


}


int main(int argc, char *argv[])
{
    GtkWidget *window;
    GtkWidget *darea;

    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size(GTK_WINDOW(window), 800, 480);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);

    darea = gtk_drawing_area_new();
    GtkWidget *btn_draw = gtk_button_new_with_label("Draw a line");


    GtkWidget *mainwindow = gtk_grid_new();
        gtk_grid_set_row_spacing (GTK_GRID (mainwindow), 16);
        gtk_grid_set_column_spacing (GTK_GRID (mainwindow), 16);
    gtk_grid_set_row_homogeneous(GTK_GRID(mainwindow), TRUE);
    gtk_grid_set_column_homogeneous(GTK_GRID(mainwindow), TRUE);

    gtk_widget_set_margin_left(mainwindow,20);
    gtk_widget_set_margin_right(mainwindow,20);
    gtk_widget_set_margin_top(mainwindow,20);
    gtk_widget_set_margin_bottom(mainwindow,20);

    gtk_grid_attach(GTK_GRID(mainwindow),btn_draw,0,0,1,1);  
    gtk_grid_attach(GTK_GRID(mainwindow),darea,1,0,5,1);  

    gtk_container_add(GTK_CONTAINER(window),mainwindow);

    g_signal_connect(G_OBJECT(btn_draw),"clicked",G_CALLBACK(on_draw_event),NULL);

    gtk_widget_show_all(window);

        gtk_main ();

        return(0);

}

This is how it looks :

PS:

I followed @jku 's advice and use a gboolean draw_a_line to record the button status, and gtk_widget_queue_draw(widget) for redrawing. The only problem I noticed is that when I click the button, it does not draw immediate, but I have to hide the window or stretch the window for it to show up. I think I need to add some kind of automatic_update() function but I am a newbie to GUI design, so could anybody to me how to make the line show up immediately after I click the button?

#include <cairo.h>
#include <gtk/gtk.h>

static void do_drawing(cairo_t *);
GtkWidget *window;
GtkWidget *darea;
gboolean draw_a_line = false;

static gboolean on_draw_event(GtkWidget *widget, GdkEventExpose *event, 
    gpointer user_data)
{
    cairo_t *cr = gdk_cairo_create(gtk_widget_get_window(GTK_WIDGET(widget)));
    do_drawing(cr);

    return FALSE;
}


static void do_drawing(cairo_t *cr)
{
    cairo_set_source_rgb(cr,0,0,0);
    cairo_set_line_width(cr,0.5);
    if (draw_a_line){
        cairo_move_to(cr,400,400);
        cairo_line_to(cr,400,200);
        cairo_stroke(cr);
    }

}


static void on_clicked(GtkWidget *widget, gpointer data)
{
    draw_a_line = true;
    gtk_widget_queue_draw(widget);

}

int main(int argc, char *argv[])
{


    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size(GTK_WINDOW(window), 800, 480);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);

    darea = gtk_drawing_area_new();
    GtkWidget *btn_draw = gtk_button_new_with_label("Draw a line");


    GtkWidget *mainwindow = gtk_grid_new();
        gtk_grid_set_row_spacing (GTK_GRID (mainwindow), 16);
        gtk_grid_set_column_spacing (GTK_GRID (mainwindow), 16);
    gtk_grid_set_row_homogeneous(GTK_GRID(mainwindow), TRUE);
    gtk_grid_set_column_homogeneous(GTK_GRID(mainwindow), TRUE);

    gtk_widget_set_margin_left(mainwindow,20);
    gtk_widget_set_margin_right(mainwindow,20);
    gtk_widget_set_margin_top(mainwindow,20);
    gtk_widget_set_margin_bottom(mainwindow,20);

    gtk_grid_attach(GTK_GRID(mainwindow),btn_draw,0,0,1,1);  
    gtk_grid_attach(GTK_GRID(mainwindow),darea,1,0,5,1);  

    gtk_container_add(GTK_CONTAINER(window),mainwindow);


    g_signal_connect(G_OBJECT(darea), "draw", G_CALLBACK(on_draw_event), NULL); 
    g_signal_connect(G_OBJECT(btn_draw),"clicked",G_CALLBACK(on_clicked),darea);

    gtk_widget_show_all(window);

        gtk_main ();

        return(0);

}

Answer 1

You have written a draw handler for (presumably) the drawing area but connected it to the clicked signal of the button. These two have completely different function signatures so a segfault is not a surprise.

Don't draw directly on the clicked handler for the button, instead change some application state (like a gboolean should_draw_line) and use gtk_widget_queue_draw () to mark the drawing area as needing a redraw. Then in the draw handler of the drawing area decide what to draw based on application state (like should_draw_line).

After looking at the example you linked to, it seems to be doing almost exactly what I suggested...