为什么我在linux上看不到MSG_EOR for SOCK_SEQPACKET？

Question

我有两个进程通过socketpair()和SOCK_SEQPACKET创建的一对套接字进行通信.像这样:

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

据我了解,我应该在收到SOCK_SEQPACKET记录时在"struct msghdr"的msg_flags成员中看到MSG_EOR.我在sendmsg()中设置MSG_EOR以确保记录标记为MSG_EOR,但在recvmsg()中接收时我看不到它.我甚至尝试在发送记录之前在msg_flags字段中设置MSG_EOR,但这根本没有任何区别.

我想我应该看到MSG_EOR,除非记录被缩短,例如信号,但我没有.这是为什么？

我在下面粘贴了我的发送和接收代码.

谢谢,朱尔斯

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;

#ifdef __linux__
    msg.msg_flags = MSG_EOR;
    n = sendmsg(fd, &msg, MSG_EOR);
#elif defined __APPLE__
    n = sendmsg(fd, &msg, 0); /* MSG_EOR is not supported on Mac                                                                                                                                                                        
                               * OS X due to lack of                                                                                                                                                                                    
                               * SOCK_SEQPACKET support on                                                                                                                                                                              
                               * socketpair() */
#endif
    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 should see MSG_EOR here if the entire record was received
        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;
}

Answer 1

使用SOCK_SEQPACKET unix域套接字,消息被剪短的唯一方法是你给recvmsg()的缓冲区不够大(在这种情况下你会得到MSG_TRUNC).

POSIX说SOCK_SEQPACKET套接字必须在记录的末尾设置MSG_EOR,但Linux unix域套接字不能.

(参考:POSIX 2008 2.10.10表示SOCK_SEQPACKET必须支持记录,2.10.6表示接收器通过MSG_EOR标志可以看到记录边界.)

"记录"对于给定协议的意义取决于要定义的实现.

如果Linux确实为unix域套接字实现了MSG_EOR,我认为唯一明智的方法就是说每个数据包本身都是一个记录,所以总是设置MSG_EOR(或者在不设置MSG_TRUNC时总是设置它),所以它不会'无论如何都要提供信息.