#include <assert.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/wait.h>

int main(void)
{
	union {
		struct cmsghdr cmsghdr;
		char buf[CMSG_SPACE(sizeof(int))];
	} control = {};

	int fd;
	struct iovec iov = {
		.iov_base = &fd,
		.iov_len = sizeof(iov)
	};

	struct msghdr mh = {
		.msg_iov = &iov,
		.msg_iovlen = 1,
		.msg_control = &control,
		.msg_controllen = sizeof(control)
	};

	while ((fd = open("/dev/null", O_RDWR)) < 3)
		assert(fd >= 0);
	(void) close(3);

	int sv[2];
	assert(socketpair(AF_UNIX, SOCK_STREAM, 0, sv) == 0);

	pid_t pid = fork();
	assert(pid >= 0);

	if (pid) {
		assert(close(sv[0]) == 0);
		assert(dup2(sv[1], 1) == 1);
		assert(close(sv[1]) == 0);

		assert((fd = open("/dev/null", O_RDWR)) == 3);

		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&mh);
		cmsg->cmsg_level = SOL_SOCKET;
		cmsg->cmsg_type = SCM_RIGHTS;
		cmsg->cmsg_len = CMSG_LEN(sizeof fd);
		memcpy(CMSG_DATA(cmsg), &fd, sizeof fd);
		mh.msg_controllen = cmsg->cmsg_len;

		assert(sendmsg(1, &mh, 0) == sizeof(iov));
		assert(close(1) == 0);

                int status;
		assert(waitpid(pid, &status, 0) == pid);
		assert(status == 0);
	} else {
		assert(close(sv[1]) == 0);
		assert(dup2(sv[0], 0) == 0);
		assert(close(sv[0]) == 0);

		assert(recvmsg(0, &mh, 0) == sizeof(iov));
		assert(close(0) == 0);
	}

	return 0;
}