GTK 3 GUI updated from separate thread text countdown in text box - c

Question

First post, so will try and be brief until need to add more. Ported an app from macOS to NI LabWindows/CVI, in "C", then to port to GTK3 and trying to grasp the updating GUI from external thread concept. I've read the gnome documentation and searched on here, and everywhere, but not finding similar usage, or not grasping the updating the GUI from separate thread. I have experimented with g_idle_add() as follows.
int main(int argc, char* argv[])
{
gtk_init(&argc, &argv); // init Gtk
gtk_start_button = GTK_WIDGET(gtk_builder_get_object(builder,"start_button"));
gtk_main();
return EXIT_SUCCESS;
}
void start_button_clicked_cb(GtkWidget *widget, gpointer data)
{
printf("\nStart Button Pressed\n");
run_tests();
}
void run_tests( void )
{
GThread *start_testing_thread;
start_testing_thread = g_thread_new("", &start_testing, NULL);
}
void *start_testing (void *data)
{
pause(5);
}
void pause( double pause_time)
{
char string[33];
while( (double)pause_time > (double)0 )
{
sprintf( string, "Pausing %02.1f", pause_time );
printf(string);
//test_name( string );
g_idle_add(test_name_gui, string);
g_usleep(100000); // uSecs for 100 mSecs
pause_time -= 0.1;
}
}
void test_name_gui(gpointer user_data)
{
GtkTextBuffer* buffer = gtk_text_buffer_new(NULL);
char temp[99];
int error = -1;
buffer = gtk_text_view_get_buffer(GTK_TEXT_VIEW (gtktextview_test_name));
sprintf(temp,"\n%s gui\n",(char*)user_data);
printf(temp);
if(!g_utf8_validate(user_data,-1,NULL))
{
error = 3;
}
gtk_text_buffer_set_text (buffer, (char*)user_data, -1);
printf("\nTest Name gui\n");
g_object_unref(buffer);
return G_SOURCE_REMOVE;
}