SpinButton.addOnOutput

The ::output signal can be used to change to formatting of the value that is displayed in the spin buttons entry.

// show leading zeros
static gboolean
on_output (GtkSpinButton *spin,
gpointer       data)
{
GtkAdjustment *adjustment;
gchar *text;
int value;

adjustment = gtk_spin_button_get_adjustment (spin);
value = (int)gtk_adjustment_get_value (adjustment);
text = g_strdup_printf ("`02d`", value);
gtk_entry_set_text (GTK_ENTRY (spin), text);
g_free (text);

return TRUE;
}