gtk_adjustment_get_value segmentation fault - c

Question

I'm trying to code some simple stuff in C, with GTK+ and Cairo, but I'm having some troubles using the gtk_adjustment_get_value function.
This is my code:
#include <cairo.h>
#include <gtk/gtk.h>
#include <math.h>
#include <stdlib.h>
#include <gtk/gtkhscale.h>
#include <math.h>
typedef struct
{
GtkAdjustment *adj[2]; //Adjustments para as escalas
GtkWidget *scale[2]; //Escalas
} ProgInfo;
gboolean
on_expose_event (GtkWidget *widget,
GdkEventExpose *event,
gpointer data)
{
cairo_t *cr;
double pos1x, pos1y;
static gdouble pos2x = 450., pos2y = 290.; //Coordenadas Espelho
static gdouble pos3x = 450., pos3y = 250.;
ProgInfo *pinfo;
cr = gdk_cairo_create(widget->window);
pinfo = (ProgInfo *) data;
pos1x = gtk_adjustment_get_value (pinfo->adj[0]);
//Objecto
cairo_set_source_rgb (cr, 0, 0, 0);
cairo_set_line_width (cr, 1.0);
cairo_rectangle (cr, (double) pos1x, (double) pos1y, 20, 80);
cairo_stroke_preserve (cr);
cairo_set_source_rgb (cr, 1, 1, 1);
cairo_fill (cr);
cairo_set_source_rgb (cr, 0, 0, 0);
cairo_set_line_width (cr, 1.0);
cairo_rectangle (cr, (gint) pos3x, (gint) pos3y, 5, 80);
cairo_stroke_preserve (cr);
cairo_set_source_rgb (cr, 1, 1, 1);
cairo_fill (cr);
cairo_stroke(cr);
cairo_destroy(cr);
return FALSE;
}
int main (int argc, char *argv[])
{
GtkWidget *window, *vbox, *vbox1, *frame, *frame1, *frame2, *label, *label1, *label2, *hbox, *hbox1, *hscale;
char string[40] = "Posição Objecto", string1[30] = "Posição Espelho";
ProgInfo *pinfo;
gdouble pos1x;
pinfo = (ProgInfo *) calloc (1, sizeof (ProgInfo));
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
g_signal_connect (window, "expose-event", G_CALLBACK(on_expose_event), NULL);
g_signal_connect (window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
gtk_window_set_position (GTK_WINDOW(window), GTK_WIN_POS_CENTER);
gtk_window_set_title (GTK_WINDOW(window), "Projecto Óptica");
gtk_window_set_default_size (GTK_WINDOW(window), 800, 600);
gtk_widget_set_app_paintable (window, TRUE);
vbox = gtk_vbox_new (FALSE, 0);
gtk_container_add (GTK_CONTAINER (window), vbox);
hbox1 = gtk_hbox_new (FALSE, 0);
gtk_container_add (GTK_CONTAINER (vbox), hbox1);
vbox1 = gtk_vbox_new (FALSE, 0);
gtk_box_pack_end (GTK_BOX (hbox1), vbox1, FALSE, FALSE, 20);
//Posição Objecto
label2 = gtk_label_new (string);
gtk_box_pack_start (GTK_BOX (vbox1), label2, FALSE, FALSE, 20);
pinfo->adj[0] = (GtkAdjustment *) gtk_adjustment_new (0.7, 0.0, 1.0, 0.001, 1.0, 0.0);
//Os parâmetros são, por ordem: valor inicial, valor mínimo e valor máximo
pinfo->scale[0] = gtk_hscale_new (GTK_ADJUSTMENT (pinfo->adj[0]));
gtk_box_pack_start (GTK_BOX (vbox1), pinfo->scale[0], FALSE, TRUE, 5);
//Posição Espelho
label1 = gtk_label_new (string1);
gtk_box_pack_start (GTK_BOX (vbox1), label1, FALSE, FALSE, 20);
pinfo->adj[1] = (GtkAdjustment *)gtk_adjustment_new (0.0, 0.0, 101.0, 0.1, 1.0, 1.0);
pinfo->scale[1] = gtk_hscale_new (GTK_ADJUSTMENT (pinfo->adj[1]));
gtk_box_pack_start (GTK_BOX (vbox1), pinfo->scale[1], FALSE, FALSE, 0);
gtk_widget_show_all(window);
gtk_main();
return 0;
}
I created a couple of sliders with the hscale widget, and they show up fine in the window, I can move them, everything's alright. But I want to make my variable, pos1x, change when I move the slider. I tried using the gtk_adjustment_get_value function to do it, but I'm getting a segmentation fault. I've noticed that if I comment out this line:
pos1x = gtk_adjustment_get_value (pinfo->adj[0]);
The segmentation fault does not happen anymore, but the app doesn't do what I want either. I realize that this is probably very simple, and my doubts derive from scarce understanding of how GTK+ works, but I've been trying for the last few hours and I can't figure out what's wrong. I'd appreciate any help you have to offer.
Oh, and sorry for putting such a big block of code here, but I wasn't sure which parts you'd need.

Answer 1

You connect to the expose-event here with the userdata pointer set to NULL:
g_signal_connect (window, "expose-event", G_CALLBACK(on_expose_event), NULL);
but in the handler you expect it to be a pointer to ProgInfo. You should replace NULL with pinfo.