one useless futex example, hopefully correct

userland/linux/futex.c

@@ -0,0 +1,40 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <linux/futex.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+
+#include <lkmc/futex.h>
+
+static int futex1, futex2;
+
+void* main_thread(void *arg) {
+    (void)arg;
+    puts("child start");
+    lkmc_futex(&futex1, FUTEX_WAKE, 1, NULL, NULL, 0);
+    lkmc_futex(&futex2, FUTEX_WAIT, 0, NULL, NULL, 0);
+    puts("child end");
+    return NULL;
+}
+
+int main(void) {
+    pthread_t thread;
+
+    puts("main start");
+    assert(!pthread_create(
+        &thread,
+        NULL,
+        main_thread,
+        NULL
+    ));
+    lkmc_futex(&futex1, FUTEX_WAIT, 0, NULL, NULL, 0);
+    sleep(1);
+    puts("main after sleep");
+    lkmc_futex(&futex2, FUTEX_WAKE, 1, NULL, NULL, 0);
+    assert(!pthread_join(thread, NULL));
+}