Why do I not see MSG_EOR while using SOCK_SEQPACKET?

[Jules Colding <colding at venalicium dot dk>]

Hi,

I have two processes which are communicating over a pair of sockets created with socketpair() and SOCK_SEQPACKET. Like this:

   int ipc_sockets[2];
   socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, ipc_sockets);

As I understand it, and as I read the manual page of recvmsg(), I should see MSG_EOR in the msg_flags member of "struct msghdr" when receiving a complete SOCK_SEQPACKET record. I am setting MSG_EOR in sendmsg() to be certain that the record is marked MSG_EOR, but I do not see it when receiving in recvmsg(). I've even tried to set MSG_EOR in the msg_flags field before sending the record (as you can see below), but that made no difference at all.

I think I should see MSG_EOR unless the record was cut short by, e.g. a signal, but I do not. Why is that?

I've pasted my sending and receiving code in below.

Thanks, 
  jules



int
send_fd(int fd,
        void *data,
        const uint32_t len,
        int fd_to_send,
        uint32_t * const bytes_sent)
{
    ssize_t n;
    struct msghdr msg;
    struct iovec iov;

    memset(&msg, 0, sizeof(struct msghdr));
    memset(&iov, 0, sizeof(struct iovec));

#ifdef HAVE_MSGHDR_MSG_CONTROL
    union {
        struct cmsghdr cm;
        char control[CMSG_SPACE_SIZEOF_INT];
    } control_un;
    struct cmsghdr *cmptr;

    msg.msg_control = control_un.control;
    msg.msg_controllen = sizeof(control_un.control);
    memset(msg.msg_control, 0, sizeof(control_un.control));

    cmptr = CMSG_FIRSTHDR(&msg);
    cmptr->cmsg_len = CMSG_LEN(sizeof(int));
    cmptr->cmsg_level = SOL_SOCKET;
    cmptr->cmsg_type = SCM_RIGHTS;
    *((int *) CMSG_DATA(cmptr)) = fd_to_send;
#else
    msg.msg_accrights = (caddr_t) &fd_to_send;
    msg.msg_accrightslen = sizeof(int);
#endif
    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    iov.iov_base = data;
    iov.iov_len = len;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    msg.msg_flags = MSG_EOR;

    n = sendmsg(fd, &msg, MSG_EOR);
    switch (n) {
    case EMSGSIZE:
        return EMSGSIZE;
    case -1:
        return 1;
    default:
        *bytes_sent = n;
    }

    return 0;
}

int
recv_fd(int fd,
        void *buf,
        const uint32_t len,
        int *recvfd,
        uint32_t * const bytes_recv)
{
    struct msghdr msg;
    struct iovec iov;
    ssize_t n = 0;
#ifndef HAVE_MSGHDR_MSG_CONTROL
    int newfd;
#endif
    memset(&msg, 0, sizeof(struct msghdr));
    memset(&iov, 0, sizeof(struct iovec));

#ifdef HAVE_MSGHDR_MSG_CONTROL
    union {
        struct cmsghdr  cm;
        char control[CMSG_SPACE_SIZEOF_INT];
    } control_un;
    struct cmsghdr *cmptr;

    msg.msg_control = control_un.control;
    msg.msg_controllen = sizeof(control_un.control);
    memset(msg.msg_control, 0, sizeof(control_un.control));
#else
    msg.msg_accrights = (caddr_t) &newfd;
    msg.msg_accrightslen = sizeof(int);
#endif
    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    iov.iov_base = buf;
    iov.iov_len = len;
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    if (recvfd)
        *recvfd = -1;

    n = recvmsg(fd, &msg, 0);
    if (msg.msg_flags) { // <== I believe I should see MSG_EOR here if the entire record was received but I do not...
        return 1;
    }
    if (bytes_recv)
        *bytes_recv = n;
    switch (n) {
    case 0:
        *bytes_recv = 0;
        return 0;
    case -1:
        return 1;
    default:
        break;
    }

#ifdef HAVE_MSGHDR_MSG_CONTROL
    if ((NULL != (cmptr = CMSG_FIRSTHDR(&msg))) 
        && cmptr->cmsg_len == CMSG_LEN(sizeof(int))) {
        if (SOL_SOCKET != cmptr->cmsg_level) {
            return 0;
        }
        if (SCM_RIGHTS != cmptr->cmsg_type) {
            return 0;
        }
        if (recvfd)
            *recvfd = *((int *) CMSG_DATA(cmptr));
    }
#else
    if (recvfd && (sizeof(int) == msg.msg_accrightslen))
        *recvfd = newfd;
#endif
    return 0;
}