What causes segfault on Xlib XGetInputFocus call?

[Chef Boyardee]

Hello, this is my first post here.

I'm playing around with Gtk+ and Xlib. I'm trying to get the X window ID of the active window, and then display that ID in a Gtk label. However, when I run it, it seg faults. I've narrowed it down to XGetInputFocus, but I'm stuck, because it seems like I'm supplying it with the parameter types it needs. All three parameters are *Display, *Window and *int, which is what I'm supplying it, according to https://tronche.com/gui/x/xlib/input/XGetInputFocus.html

1. What am I doing wrong?

2. Also, when I compile with gcc -Wallpkg-config --cflags gtk+-3.0-o inputfocus inputfocus.cpkg-config --libs gtk+-3.0-lX11 I get a warning because sprintf expects an unsigned int because I provide %u as a format specifier. However, the source "focus" is of type Window. I referenced the definition of Window, and it is typedefed as XID, and XID is typedefed as unsigned long int.

How should I properly convert or cast that to a type sprintf is satisfied with?

#include <gtk/gtk.h>
#include <gtk/gtkx.h>
#include <X11/Xlib.h>
#include <stdlib.h>

static void activate (GtkApplication* app, gpointer user_data) {
  GtkWidget     *window;
  GtkWidget     *glblLabel = gtk_label_new("Hello!");
  GdkWindow     *gdkwin; 
  GdkDisplay    *gdkdisplay;
  Display       *dpy;
  Window        xwindow;
  Window        *focus;
  char          *text = malloc(sizeof(Window) +1);
  int           *revert_to = 0;

  // Set up the application, window, and widgets
  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200); 
  gtk_container_add(GTK_CONTAINER(window), glblLabel);
  gtk_widget_show_all (window);

  gdkwin = gtk_widget_get_window(window); // Get GtkWidget *window's GdkWindow
  gdkdisplay = gdk_window_get_display(gdkwin); // Then use it to get its GdkDisplay
  dpy = GDK_DISPLAY_XDISPLAY(gdkdisplay); // Use that to get its X11 display

  xwindow = GDK_WINDOW_XID(gdkwin);     // Get GtkWidget *window's X window ID
  focus = &xwindow;

  // XGetInputFocus returns the active X window in focus
  XGetInputFocus(dpy, focus, revert_to);

  sprintf(text, "%u", focus);           // Convert active window's ID to string
  gtk_label_set_text(GTK_LABEL(glblLabel), text); // Then display it in glblLabel
}

int main (int  argc, char **argv) {
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.techstuffonly.inputtest", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

[Chef Boyardee]

Nevermind, I fixed it.

#include <gtk/gtk.h>
#include <gtk/gtkx.h>
#include <X11/Xlib.h>
#include <stdlib.h>
//#include <gdk/gdkx.h>
//#include <gdk/gdkprivate.h>

static void activate (GtkApplication* app, gpointer user_data) {
  GtkWidget     *window;
  GtkWidget     *glblLabel = gtk_label_new("Hello!");
  GdkWindow     *gdkwin; 
  GdkDisplay    *gdkdisplay;
  Display       *dpy;
  Window         xwindow;
  Window        *focus;
  char          *text = malloc(sizeof(Window) +1);
  int            intvar = 0;
  int           *revert_to;

  // Set up the application, window, and widgets
  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200); 
  gtk_container_add(GTK_CONTAINER(window), glblLabel);
  gtk_widget_show_all (window);

  gdkwin = gtk_widget_get_window(window); // Get GtkWidget *window's GdkWindow
  gdkdisplay = gdk_window_get_display(gdkwin); // Then use it to get its GdkDisplay
  dpy = GDK_DISPLAY_XDISPLAY(gdkdisplay); // Use that to get its X11 display

  xwindow = GDK_WINDOW_XID(gdkwin);     // Get GtkWidget *window's X window ID
  focus = &xwindow;

  revert_to = &intvar;
  // XGetInputFocus returns the active X window in focus
  XGetInputFocus(dpy, &xwindow, revert_to);

  sprintf(text, "%u", focus);           // Convert active window's ID to string
  gtk_label_set_text(GTK_LABEL(glblLabel), text); // Then display it in glblLabel
}

int main (int  argc, char **argv) {
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.techstuffonly.inputtest", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}