#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>

static void lock(int fd, const char *where, const char *expected)
{
    struct flock lf = { .l_type = F_WRLCK, 0 };
    int err = fcntl(fd, F_SETLK, &lf);
    if (err < 0)
	fprintf(stderr, "lock failed in %s (%sexpected) %d\n", where, expected, errno);
}

static void unlock(int fd, const char *where, const char *expected)
{
    struct flock lf = { .l_type = F_UNLCK, 0 };
    int err = fcntl(fd, F_SETLKW, &lf);
    if (err < 0)
	fprintf(stderr, "unlock failed in %s (%sexpected) %d\n", where, expected, errno);
}

int main(int argc, char**argv)
{
  // int fd = open("foo", O_WRONLY | O_CREAT);
  int fd = fileno(fopen("foo", "w"));
  int err, pid = fork();
  if (pid < 0) {
	fprintf(stderr, "fork error\n");
	return -1;
  }

  // in child
  if (pid == 0) {
    lock(fd, "child", "not ");
    sleep(2);
    unlock(fd, "child", "not ");
    _exit(0);
  }

  sleep(1);
  lock(fd, "parent", "");
  // file isn't locked in child, but there should not be an error on cygwin
  unlock(fd, "parent", "not ");
  sleep(2);

  return 0;
}