gistfile1.txt

~ $ curl -s https://gist.githubusercontent.com/bendem/60cd81c4a3aa1961fc94fd8c7557d702/raw/fe3a9bbceb9408fb86b6150553948100cc3ce197/i3ipc-glib-test.c | gcc $(pkg-config --libs --cflags i3ipc-glib-1.0) -x c -
~ $ ./a.out 
id: 2577442560

$ i3-msg -t get_tree | jq . | grep 'id"' | sed -E 's/\s+//' | sort
"id": 94341533615152,
"id": 94341533617280,
"id": 94341533617712,
"id": 94341533703184,
"id": 94341534014560,
"id": 94341534026752,
"id": 94341534037296,
"id": 94341534037952,
"id": 94341534038512,
"id": 94341534052576,
"id": 94341534054000,
"id": 94341534054432,
"id": 94341534055136,
"id": 94341534055840,
"id": 94341534056544,
"id": 94341534058176,
"id": 94341534059040,
"id": 94341534092576,
"id": 94341534099200,

i3ipc-glib-test.c

#include <glib/gprintf.h>
#include <i3ipc-glib/i3ipc-glib.h>
#include <inttypes.h>

gint main() {
  i3ipcConnection *conn;
  gchar *reply;

  conn = i3ipc_connection_new(NULL, NULL);

  i3ipcCon *root = i3ipc_connection_get_tree(conn, NULL);
  i3ipcCon *focused = i3ipc_con_find_focused(root);

  uintptr_t id;
  g_object_get(focused, "id", &id, NULL);
  printf("id: %" PRIuPTR "\n", id);

  return 0;
}