updating interface in GTK - c

Question

I want to make an application that displays the time (ultimately I am trying to hack together my own simple smart mirror without using the existing smart mirror APIs). I am using GTK3 for the UI however I am having trouble figuring out a solution to make the UI update the time (I am not experienced in front end or GTK so bear with me).
I have tried placing loops around parts of the view3 code shown below however I have discovered that once gtk_main() is called I can't get out of the gtk main loop so that the while loop starts over and recalls the time function I wrote.
I have tried using functions like gtk_main_iteration_do(gtk_false()) (false so that it doesn't block) but I clearly don't understand enough about these functions because it's not working. If I leave gtk_main() out of the loop obviously gtk_main() never gets called and my application window won't even open up.
I have shown the relevant code in main below and following that the definition of the get_time_string() function I wrote.
int
main (int argc,
char *argv[])
{
// initialization and setting up window, packing widgets, etc
// view3
// populate buffer with time string
// and insert into text view
view3 = gtk_text_view_new();
gtk_widget_set_name(view3, "view3");
buffer = gtk_text_view_get_buffer(GTK_TEXT_VIEW(view3));
gtk_text_buffer_get_iter_at_offset(buffer, &iter, 0);
gtk_text_buffer_insert(buffer, &iter, get_time_string(), -1);
gtk_text_view_set_editable(GTK_TEXT_VIEW(view3), FALSE);
gtk_text_view_set_cursor_visible(GTK_TEXT_VIEW(view3), FALSE);
// More widget packing, setting up UI
gtk_widget_show_all(window);
gtk_main();
return 0;
}
Definition of get_time_string()
char* get_time_string(){
time_t time_var = time(NULL);
struct tm *info;
char *time_string = calloc(100, sizeof(char));
info = localtime( &time_var );
strftime(time_string, 100, "%I:%M %p", info);
return time_string;
}

Answer 1

You should not play with mainloop iterations unless you really need or know what to do.
The trick is to use g_timeout_add or g_idle_add and their variants. As you want the time to update at regular intervals (for minute resolution you will update every 60 seconds) then you can use g_timeout_add_seconds.
For illustration purposes, i'll add the seconds to your time string and update every second, using your get_time_string function but creating a very simple window to show just a time label:
#include <time.h>
#include <stdlib.h>
#include <gtk/gtk.h>
char* get_time_string(){
time_t time_var = time(NULL);
struct tm *info;
char *time_string = calloc(100, sizeof(char));
info = localtime( &time_var );
strftime(time_string, 100, "%I:%M:%S %p", info);
return time_string;
}
gboolean update_label_time (gpointer user_data) {
gchar *t = get_time_string();
gtk_label_set_text(GTK_LABEL(user_data), t);
g_free (t);
return G_SOURCE_CONTINUE;
}
int main (int argc, char *argv[])
{
gchar *t;
GtkWidget *window;
GtkWidget *label_time;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_default_size (GTK_WINDOW(window), 300, 200);
t = get_time_string();
label_time = gtk_label_new (t);
g_free (t);
g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(gtk_main_quit), NULL);
gtk_container_add (GTK_CONTAINER(window), label_time);
g_timeout_add_seconds(0.5, update_label_time, label_time);
gtk_widget_show_all(window);
gtk_main();
return 0;
}
The result should be a window with a label updating every second:
GLib has its own time functions which you should use.