#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

static void client(int fd)
{
	int send_fd;
	struct msghdr msg;
	struct iovec iov[1];
	char buf[1];
	union {
		struct cmsghdr cm;
		char cntl[CMSG_SPACE(sizeof(int))];
	} cntl_u;
	struct cmsghdr *cmsgptr;

	send_fd = open("/dev/null", O_RDWR);
	
	iov[0].iov_base = buf;
	iov[0].iov_len = 1;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;
	msg.msg_name = NULL;
	msg.msg_namelen = 0;

	msg.msg_control = cntl_u.cntl;
	msg.msg_controllen = sizeof(cntl_u.cntl);

	cmsgptr = CMSG_FIRSTHDR(&msg);
	cmsgptr->cmsg_len = CMSG_LEN(sizeof(int));
	cmsgptr->cmsg_level = SOL_SOCKET;
	cmsgptr->cmsg_type = SCM_RIGHTS;
	*((int *)CMSG_DATA(cmsgptr)) = send_fd;

	if (sendmsg(fd, &msg, 0) <= 0)
		perror("sendmsg");
}

static void server(int fd)
{
	int recv_fd;
	struct stat statbuf;
	struct msghdr msg;
	struct iovec iov[1];
	char buf[1];
	union {
		struct cmsghdr cm;
		char cntl[CMSG_SPACE(sizeof(int))];
	} cntl_u;
	struct cmsghdr *cmsgptr;

	iov[0].iov_base = buf;
	iov[0].iov_len = 1;
	msg.msg_iov = iov;
	msg.msg_iovlen = 1;
	msg.msg_name = NULL;
	msg.msg_namelen = 0;

	msg.msg_control = cntl_u.cntl;
	msg.msg_controllen = sizeof(cntl_u.cntl);
	
	if (recvmsg(fd, &msg, 0) <= 0) {
		perror("recvmsg");
		exit(1);
	}
	cmsgptr = CMSG_FIRSTHDR(&msg);
	if (cmsgptr == NULL) {
		fprintf(stderr, "no control message\n");
		exit(1);
	}
	if (cmsgptr->cmsg_len != CMSG_LEN(sizeof(int)))
		fprintf(stderr, "bad length: %d\n", cmsgptr->cmsg_len);
	if (cmsgptr->cmsg_level != SOL_SOCKET)
		fprintf(stderr, "not SOL_SOCKET: %d\n", cmsgptr->cmsg_level);
	if (cmsgptr->cmsg_type != SCM_RIGHTS)
		fprintf(stderr, "not SCM_RIGHTS: %d\n", cmsgptr->cmsg_type);
	recv_fd = *((int *)CMSG_DATA(cmsgptr));
	if (fstat(recv_fd, &statbuf) == -1) {
		perror("stat");
		exit(1);
	}

	printf("fd: %d, dev: %llx, mode: %o\n",
	       recv_fd, statbuf.st_rdev, statbuf.st_mode);
}

int main()
{
	pid_t pid;
	int status, fd[2];

	if (socketpair(AF_UNIX, SOCK_STREAM, 0, fd) == -1) {
		perror("socketpair");
		exit(1);
	}

	pid = fork();
	if (pid == -1) {
		perror("fork");
		exit(1);
	}
	if (pid == 0) {
		close(fd[1]);
		client(fd[0]);
		exit(0);
	}

	close(fd[0]);
	if (waitpid(pid, &status, 0) != pid) {
		perror("waitpid");
		exit(1);
	}
	if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
		fprintf(stderr, "client abnormal end\n");
		exit(1);
	}

	server(fd[1]);

	return 0;
}