RFC 2292 CMSG_FIRSTHDR macro...

From: George V. Neville-Neil (gnn_at_neville-neil.com)
Date: 04/29/04

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    Date: Wed, 28 Apr 2004 17:09:30 -0700
To: net@freebsd.org

    Howdy,

            This should be a minor nit but, is there any reason we
            implement this this way:

    #define CMSG_FIRSTHDR(mhdr) ((struct cmsghdr *)(mhdr)->msg_control)

    instead of this way:

    #define CMSG_FIRSTHDR(mhdr) \
              ( (mhdr)->msg_controllen >= sizeof(struct cmsghdr) ? \
               (struct cmsghdr *)(mhdr)->msg_control : \
               (struct cmsghdr *)NULL )

    as suggested in RFC 2292?

    A coworker of mine asked about this and then I got to wondering
    myself.

    The RFC says:

       (Note: Most existing implementations do not test the value of
       msg_controllen, and just return the value of msg_control. The value
       of msg_controllen must be tested, because if the application asks
       recvmsg() to return ancillary data, by setting msg_control to point
       to the application's buffer and setting msg_controllen to the length
       of this buffer, the kernel indicates that no ancillary data is
       available by setting msg_controllen to 0 on return. It is also
       easier to put this test into this macro, than making the application
       perform the test.)

    Later,
    George
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