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

void fatal(char *s) {
	perror(s);
	exit(1);
}

int main(){
	int n, fd, pair[2];
	char fdbuf[CMSG_SPACE(sizeof(int))];
	struct cmsghdr *cmsg;
	struct msghdr msg;
	char buf[1024];

	/* dummy */
	struct iovec vec;
	char ch = '\0';

	memset(&msg, 0, sizeof(msg));

	/* having zero msg_iovlen or iov_len doesnt seem to work */
	vec.iov_base = &ch;
	vec.iov_len = 1;
	msg.msg_iov = &vec;
	msg.msg_iovlen = 1;

	msg.msg_control = fdbuf;
	msg.msg_controllen = CMSG_LEN(sizeof(int));
	cmsg = CMSG_FIRSTHDR(&msg);
	cmsg->cmsg_len = CMSG_LEN(sizeof(int));

	if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair))
		fatal("socketpair");

	switch (fork()) {
	case -1:
		fatal("fork");
	case 0:
		fd = open("/etc/passwd", O_RDONLY);
		if (fd < 0) {
			perror("/etc/passwd");
			exit(1);
		}
		printf("child: sending fd=%d for /etc/passwd\n", fd);

		cmsg->cmsg_level = SOL_SOCKET;
		cmsg->cmsg_type = SCM_RIGHTS;
		*(int *)CMSG_DATA(cmsg) = fd;
		if (sendmsg(pair[0], &msg, 0) < 0)
			fatal("sendmsg");
		exit(0);
	default:
		fd = open("/etc/motd", O_RDONLY);
		printf("parent: got fd=%d for /etc/motd\n", fd);

		if (recvmsg(pair[1], &msg, 0) < 0)
			fatal("recvmsg");
		if (cmsg->cmsg_type != SCM_RIGHTS)
			fatal("didnt get a fd?\n");
		fd = *(int *)CMSG_DATA(cmsg);
		printf("parent: received fd=%d\n", fd);
		n = read(fd, buf, sizeof(buf));
		if (n < 0)
			perror("read");
		printf("read %d bytes\n\n", n);
		fflush(stdout);
		write(1,buf,n);
	}

	return 0;
}