#define _GNU_SOURCE

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

#define pexit(msg) do { perror(msg); exit(1); } while (0)

static int sendfd(int sockfd, int fd)
{
	struct msghdr msg = { 0 };
	struct cmsghdr *cmsg;
	int err;
	char iobuf;
	struct iovec io = {
		.iov_base = &iobuf,
		.iov_len = sizeof(iobuf),
	};
	union {
		char buf[CMSG_SPACE(sizeof(fd))];
		struct cmsghdr align;
	} u;

	msg.msg_iov = &io;
	msg.msg_iovlen = 1;
	msg.msg_control = u.buf;
	msg.msg_controllen = sizeof(u.buf);
	cmsg = CMSG_FIRSTHDR(&msg);
	cmsg->cmsg_level = SOL_SOCKET;
	cmsg->cmsg_type = SCM_RIGHTS;
	cmsg->cmsg_len = CMSG_LEN(sizeof(fd));

	/* Send the file descriptor with an empty message
	 * containing only the ancillary data. */
	msg.msg_iov = NULL;
	msg.msg_iovlen = 0;

	memcpy(CMSG_DATA(cmsg), &fd, sizeof(fd));

	err = sendmsg(sockfd, &msg, 0);
	if (err < 0)
		pexit("sendmsg");

	if (err != msg.msg_iovlen)
		pexit("sendmsg");

	return 0;
}

static int recvfd(int sockfd)
{
	struct msghdr msg = { 0 };
	struct cmsghdr *cmsg;
	int err;
	int fd;
	union {
		char buf[CMSG_SPACE(sizeof(fd))];
		struct cmsghdr align;
	} u;

	msg.msg_control = u.buf;
	msg.msg_controllen = sizeof(u.buf);

	err = recvmsg(sockfd, &msg, 0);
	if (err < 0)
		pexit("recvmsg");

	cmsg = CMSG_FIRSTHDR(&msg);
	if (!cmsg ||
		cmsg->cmsg_len != CMSG_LEN(sizeof(fd)) ||
		cmsg->cmsg_level != SOL_SOCKET ||
		cmsg->cmsg_type != SCM_RIGHTS)
		pexit("Invalid cmsg");

	memcpy(&fd, CMSG_DATA(cmsg), sizeof(fd));

	return fd;
}

int main(void)
{
	int scm_fds[2], r;

	r = socketpair(AF_UNIX, SOCK_STREAM, 0, scm_fds);
	if (r < 0)
		pexit("socketpair");
	
	if (fork()) {
		int fd = open("/", O_PATH | O_DIRECTORY);

		if (fd < 0)
			pexit("open");

		r = sendfd(scm_fds[0], fd);
		if (r < 0)
			pexit("sendfd");

		puts("sent fd");

	} else {
		int fd = recvfd(scm_fds[1]);

		if (fd < 0)
			pexit("recvfd");

		puts("received fd");
		
		exit(0);
	}

	wait(NULL);
	puts("done");

	return 0;
}