How to use g_timeout_add()? - c

Question

I'm having a problem using g_timeout_add. The function I'm calling is:
gboolean time_handler(GtkWidget *widget, GtkWidget *sum) {
if (widget == NULL) return FALSE;
gchar *display;
display = g_strdup_printf("%d", globalCounter); //convert num to str
gtk_label_set_text (GTK_LABEL(sum), display); //set label to "display"
g_free(display);
return TRUE;
}
main...
g_timeout_add(1000, (GSourceFunc) time_handler, (window, sum))
This is the warning I get:
gtkp2.c:66:60: warning: left-hand operand of comma expression has no effect [-Wunused-value]
g_timeout_add(1000, (GSourceFunc) time_handler, (window,sum));
While running the program I get a segmentation fault.

Answer 1

Let's take a look at the manual for g_timeout_add and GSourceFunc.
There we see:
gboolean
(*GSourceFunc) (gpointer user_data);
and
guint
g_timeout_add (guint interval,
GSourceFunc function,
gpointer data);
This means you have 2 basic errors in your code:
gboolean time_handler(GtkWidget *widget, GtkWidget *sum)
Your function does not follow the definition of GSourceFunc as it is only allowed to take 1 single parameter.
The compiler probably told you about this error becouse you have a cast to the proper type in your code. If you use the correct type, no cast should be required.
If GTK does not pass 2 parameters to your function (how should it know about your invalid function signature?) but you access that parameter and try to dereference it as a pointer, you invoke undefined behaviour which manifests as a segmentation fault in this case.
You try to pass 2 pointers via 1 parameter.
As the compiler told you, the left operand does not have any effect at all.
If you want to pass 2 values, you need to define a struct and pass a pointer to such a struct:
typedef struct{
GtkWidget *widget,
GtkWidget *sum
} myParameters;
gboolean time_handler(gpointer *par) {
myParams *params = par;
...
}
...
myParams param = {widget, sum};
g_timeout_add(1000, time_handler, &params);
// Make sure that lifetime of params does not end before your handler might be called! Local variables in main are OK for this.