using g_timeout_add and updating GUI

I am just getting back into programming and need some help. I am totally new to GTK3 and dusting off the coding cobwebs.

The program I am writing basically reads the serial port for GPS GPGGA data strings that come in every sec, parses the data and calculates the distance between strings and triggers an output when a user enter distance is reached and repeats. I am writing in C on a raspberry PI, used Glade to make interface and Code::Block IDE.

Right now, I have the time g_timeout_add working, Kinda. If I remove the gtk_label_set_text from the calc_display function it works and runs that function every 500mS (had a LED on the GPIO to confirm that - code removed to make reading simple). But the GUI is unresponsive and doesn't update.

If I have the gtk_label_set_text and the gtk_main_iteration() in the function, it runs once and freezes up (updates display with GPS but not blinking LED). What am I screwing up here, I have read through the GTK reference but nothing jumps out, I suspect it's a newbie dumb error. Any thoughts?

Here is the code:

#include <gtk/gtk.h>
#include <stdio.h>
#include <stdlib.h>
#include <wiringPi.h>
#include "serial_gps.h"
#include "pin_names.h"
#include "gps_distance.h"

typedef struct
{
    GtkWidget *w_shot_spacing_ent;      // generic pointer for entered text
    GtkWidget *w_num_shots_val;
    GtkWidget *w_gps_q_val;
    GtkWidget *w_total_dist_val;
    GtkWidget *w_avg_space_val;
    GtkWidget *w_lat_val;
    GtkWidget *w_long_val;
    GtkWidget *w_pause_but;
    GtkWidget *w_start_but;
    GtkWidget *w_serial_in;
} app_widgets;

int start =0;           // used in the "Start button clicked" function
int port_fd = 0;    // file descriptor for the serial port
float spacing = 0;      // global for the shor spacing value
gint input_type = 1;    // for picking USB or DB9 serial
guint event_id =  0;
char port_data[100];   // big buffer for the port data
int toggle = FALSE;

void flash_leds(void);
void ini_data(void);
void calc_display(app_widgets *app_wdgts);
void calc_display_f(void);

int main (int argc, char *argv[])
{

    GtkBuilder *builder;
    GtkWidget *window;

    // instantiate structure, allocating memory for it
    app_widgets *widgets = g_slice_new(app_widgets);

    wiringPiSetup();  // initialise the wiring Pi library
    flash_leds();
    ini_data();     // ini the data structure
    gtk_init(&argc, &argv);

    builder = gtk_builder_new_from_file ("glade/ShotMaster.glade");

    window = GTK_WIDGET(gtk_builder_get_object (builder, "window_main"));
     // get pointers to label widgets
    widgets->w_shot_spacing_ent= GTK_WIDGET(gtk_builder_get_object(builder, "shot_spacing_ent"));
    widgets->w_num_shots_val= GTK_WIDGET(gtk_builder_get_object(builder, "shot_num_val"));
    widgets->w_total_dist_val= GTK_WIDGET(gtk_builder_get_object(builder, "total_dist_val"));
    widgets->w_avg_space_val= GTK_WIDGET(gtk_builder_get_object(builder, "avg_space_val"));
    widgets->w_gps_q_val= GTK_WIDGET(gtk_builder_get_object(builder, "gps_q_val"));
    widgets->w_lat_val= GTK_WIDGET(gtk_builder_get_object(builder, "lat_val"));
    widgets->w_long_val= GTK_WIDGET(gtk_builder_get_object(builder, "long_val"));
    widgets->w_pause_but= GTK_WIDGET(gtk_builder_get_object(builder, "pause_but"));
    widgets->w_start_but= GTK_WIDGET(gtk_builder_get_object(builder, "start_but"));
    widgets->w_serial_in= GTK_WIDGET(gtk_builder_get_object(builder, "serial_in"));

    gtk_builder_connect_signals(builder,widgets);
    g_object_unref(builder);
    gtk_widget_show(window);
    gtk_main();
    g_slice_free(app_widgets, widgets);  // free up memory, usually done by OS, but good pratice
    return 0;
}


void on_reset_but_clicked (GtkButton *button, app_widgets *app_wdgts)
{
    // just some test code
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_num_shots_val), "3");
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_total_dist_val), "2");
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_avg_space_val), "1");

}

void calc_display(app_widgets *app_wdgts)
{
    char    buffer[20];
    int     num_buf;
    double   F_buff;

    shot_trigger();   // this code works, has no GTK functions in it  works with the "data" struct for collecting data
    GString *temp = g_string_new("0000");
    num_buf = data.num_shots;
    F_buff = data.latitude;
    F_buff = data.longitude;


    sprintf(buffer, "%i", data.num_shots);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_num_shots_val),buffer);

    sprintf(buffer, "%i", data.gps_q);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_gps_q_val),buffer );

    sprintf(buffer, "%f", data.total_dist);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_total_dist_val),buffer );

    sprintf(buffer, "%f", data.avg_space);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_avg_space_val),buffer );

    sprintf(buffer, "%f", data.latitude);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_lat_val),buffer );

    sprintf(buffer, "%f", data.longitude);
    gtk_label_set_text(GTK_LABEL (app_wdgts->w_long_val),buffer );



    }

    gtk_main_iteration();   // update the display
}

void on_start_but_clicked(GtkButton *button, app_widgets *app_wdgts)  // Main function that starts serial data grabbing/ calculations
{
    int     port_err = 0;

    data.spacing = gtk_spin_button_get_value(GTK_SPIN_BUTTON(app_wdgts->w_shot_spacing_ent));

    input_type = gtk_combo_box_get_active(GTK_COMBO_BOX(app_wdgts->w_serial_in)); // returns a 0 for USB and 1 for DBc
    port_fd = start_serial(input_type, 9600);   //open the port
    flush_port(port_fd);  // start with a clean buffer

    port_err= get_serial_string(port_data,port_fd);  // get a line of GPS data
    data.run=TRUE;
    if (port_err != 0)
        {
        // error found,  -1 = buffer overflow
        }

    if(start == 0)     // initial condition
    {
        event_id = g_timeout_add(500,(GSourceFunc) calc_display,app_wdgts);
        start = 1;
        gtk_button_set_label(button, "Change");  //once started you can only change things..
    }
    else   // after program has Started,  button in "Change" mode
    {
        spacing = gtk_spin_button_get_value(GTK_SPIN_BUTTON(app_wdgts->w_shot_spacing_ent));  // get new value

    }
}

void on_pause_but_clicked (GtkButton *button, app_widgets *app_wdgts)
{
        if(data.run == TRUE)
        {
            gtk_button_set_label(button, "RUN");// change button label
            data.run = FALSE;
        }
        else
        {
            data.run = TRUE;
            gtk_button_set_label(button, "PAUSE");// change button label
        }
}
void on_quit_but_clicked(void)
{
    stop_serial( port_fd);    // close the serial port
    gtk_main_quit();
}
void on_window_main_destroy(void)
{
    stop_serial( port_fd);  // close the serial port
    gtk_main_quit();
}


void flash_leds(void)
{
    pinMode(test_LED,OUTPUT);  // set pin(s) to outputs
    pinMode(trig_LED,OUTPUT);  // set pin(s) to outputs
    pinMode(gps_LED,OUTPUT);  // set pin(s) to outputs
    pinMode(error_LED,OUTPUT);  // set pin(s) to outputs
    digitalWrite(test_LED, HIGH); // high is LED off
    digitalWrite(trig_LED, HIGH); // high is LED off
    digitalWrite(gps_LED, HIGH); // high is LED off
    digitalWrite(error_LED, HIGH); // high is LED off

    digitalWrite(test_LED, LOW);
    digitalWrite(trig_LED, LOW);
    digitalWrite(gps_LED, LOW);
    digitalWrite(error_LED, LOW);
    delay(1000);
    digitalWrite(test_LED, HIGH); // high is LED off
    digitalWrite(trig_LED, HIGH); // high is LED off
    digitalWrite(gps_LED, HIGH); // high is LED off
    digitalWrite(error_LED, HIGH); // high is LED off


}

void ini_data(void)
{
data.spacing = 0;
data.num_shots =0;
data.gps_q = 0;
data.total_dist = 0;
data.avg_space = 0;
data.latitude = 0;
data.longitude = 0;
data.run = FALSE;
}