Passing an integer stored with mmap to GTK - c

Question

EDIT
This question ended up being two problems packed into one. Yet, I cannot delete the question. The scope of the original question regarding pointers was solved by #David Ranieri. The mmap/fork/gtk problem will be the scope of a new question and will not be addressed here.
I want to print a value I have stored in memory in a GTK window. The integer must be stored using mmap to be retained during a fork elsewhere in the code. I cannot reference this memory mapped address from GTK without getting a SegFault. Am I doing something wrong here? Is there a better way?
My current strategy:
Reserve memory for an int with mmap at *VAL
Fork process, one half modifies VAL the other half runs GTK
Pass *VAL to app in userdata slot
The userdata now called localval in activate()
Print the value at address localval by converting from gpointer to int.
MWE (this causes a segfault, run at your own risk):
/*
* MMAP Variable SegFault
*/
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <gtk/gtk.h>
static void activate (GtkApplication *app, gpointer *localval) {
GtkWidget *window;
// Button Containers
GtkWidget *button_box_quit;
// Buttons
GtkWidget *exit_button;
// Text
GtkWidget *text_status;
// Define Window, dynamic size for screen.
window = gtk_application_window_new (app);
gtk_window_set_title (GTK_WINDOW (window), "test");
gtk_window_set_default_size(GTK_WINDOW(window), 400, 300);
// Define Button Boxes.
button_box_quit = gtk_button_box_new(GTK_ORIENTATION_HORIZONTAL);
// Define Exit Button, put it in a box, put box in window
exit_button = gtk_button_new_with_label ("Exit");
gtk_container_add(GTK_CONTAINER (button_box_quit), exit_button);
gtk_container_add(GTK_CONTAINER (window), button_box_quit);
// Connect signals to buttons
g_signal_connect_swapped (exit_button, "clicked", G_CALLBACK (gtk_widget_destroy), window);
// Define text status
char msg[32]={0};
// The "print" line
g_snprintf(msg, sizeof msg, "val: %d\n", GPOINTER_TO_INT(*localval));
text_status = gtk_label_new(msg);
gtk_container_add(GTK_CONTAINER (button_box_quit), text_status);
//Activate!
gtk_widget_show_all (window);
}
int main (int argc, char **argv) {
GtkApplication *app;
int status;
int *VAL = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
int *ABORT = mmap(NULL, sizeof(int), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
int pid = fork();
if (pid == 0) {
while(!*ABORT) {
printf("%d\n", *VAL);
*VAL = *VAL + 1;
usleep(1000000);
}
exit(0);
} else {
app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
// The passing line
g_signal_connect (app, "activate", G_CALLBACK (activate), (gpointer *)*VAL);
status = g_application_run (G_APPLICATION (app), argc, argv);
g_object_unref (app);
*ABORT = 1;
}
*ABORT = 1;
return status;
}
In a non-working alternative to this code, I tried changing the print line to:
g_snprintf(msg, sizeof msg, "val: %d\n", &GPOINTER_TO_INT(*localval));
But the compiler thinks I want to use the & as a comparator.

Answer 1

You are passing a dereferenced pointer (a value) to a function expecting a pointer:
int *VAL = ...;
...
g_signal_connect (app, "activate", G_CALLBACK (activate), (gpointer *)*VAL);
switch to
g_signal_connect (app, "activate", G_CALLBACK (activate), VAL); // Do not use a wrong cast (void **)
also, gpointer is an alias of void *, using gpointer *data you get a void **data, not what you want, so
static void activate (GtkApplication *app, gpointer *localval) {
should be
static void activate (GtkApplication *app, gpointer localval) { // without *
finally, to print the value of the pointer use
g_snprintf(msg, sizeof msg, "val: %d\n", *(int *)localval);