#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <assert.h>
#include <err.h>
#include <stdlib.h>

static int own_child(int *us)
{
        int pid;
        int s[2];
        struct msghdr mh;
        char crap[1024];
        struct iovec iov;
        struct cmsghdr *c;
        int *fd;
        int rc;

        pid = fork();
        if (pid == -1)
                err(1, "fork()");

        if (pid) {
              close(us[1]);

                return pid;
        }

        close(us[0]);

        memset(&mh, 0, sizeof(mh));
        iov.iov_base = "a";
        iov.iov_len  = 1;

        mh.msg_iov        = &iov;
        mh.msg_iovlen    = 1;
        mh.msg_control    = crap;
        mh.msg_controllen = sizeof(crap);

        c = CMSG_FIRSTHDR(&mh);
        assert(c);

        c->cmsg_level = SOL_SOCKET;
        c->cmsg_type  = SCM_RIGHTS;

        fd = (int*) CMSG_DATA(c);
        assert(fd);

        c->cmsg_len = CMSG_LEN(sizeof(int));
        mh.msg_controllen = c->cmsg_len;

        while (1) {
                if (socketpair(PF_UNIX, SOCK_STREAM, 0, s) == -1)
                        err(1, "socketpair()");

                *fd = s[0];

                rc = sendmsg(us[1], &mh, 0);
                if (rc == -1)
                        err(1, "sendmsg()");

                if (rc != iov.iov_len)
                        errx(1, "sent short");

                close(s[0]);
                close(us[1]);
                us[1] = s[1];
        }
}

static void own(void)
{     
        static int pid;
        static int us[2];
        char crap[1024];
        char morte[1024];
        struct cmsghdr *c;
        int rc;
        struct msghdr mh;
        struct iovec iov;
        int *fds;

        if (!pid) {
                if (socketpair(PF_UNIX, SOCK_STREAM, 0, us) == -1)
                        err(1, "socketpair()");
                pid = own_child(us);
        }

        iov.iov_base = morte;
        iov.iov_len  = sizeof(morte);

        memset(&mh, 0, sizeof(mh));
        mh.msg_iov        = &iov;
        mh.msg_iovlen    = 1;
        mh.msg_control    = crap;
        mh.msg_controllen = sizeof(crap);

        rc = recvmsg(us[0], &mh, 0);
        if (rc == -1)
                err(1, "recvmsg()");

        if (rc == 0)
                errx(1, "EOF");

        c = CMSG_FIRSTHDR(&mh);
        assert(c);
        assert(c->cmsg_type == SCM_RIGHTS);

        fds = (int*) CMSG_DATA(c);
        assert(fds);

        close(us[0]);
        us[0] = *fds;
}

int main(int argc, char *argv[])
{
   own();
   exit(0);
}