#define _GNU_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>

#define NUM_THREADS 1

static pthread_attr_t attr;
static volatile int euid = -1, gen, num;

void *thr(void *arg) {
  while (euid == -1)
    ;
  int cur_gen = gen;
  int teuid = syscall(SYS_geteuid);
  if (teuid != euid) {
    printf("mismatch: %d != %d\n", teuid, euid);
    sleep(1000000);
  }
  __sync_fetch_and_add(&num, 1);
  while (cur_gen == gen)
    ;

  if (pthread_create(&(pthread_t){0}, &attr, thr, 0))
    abort();
  return 0;
}

int main(void) {
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
  for (int i = 0; i < NUM_THREADS; i++)
    if (pthread_create(&(pthread_t){0}, &attr, thr, 0))
      abort();

  printf("uid = %d euid = %d\n", getuid(), geteuid());
  int ruid = getuid();
  int cur_euid = 0;
  for (int it = 0;; it++) {
    printf("%d\n", it);
    cur_euid = cur_euid ? 0 : ruid;
    if (seteuid(cur_euid)) {
      perror("seteuid");
      abort();
    }
    num = 0;
    euid = cur_euid;
    while (num < NUM_THREADS)
      ;
    euid = -1;
    gen = 1 - gen;
  }
}