rfc2292.txt

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   is being read or written.  The inclusion of the msg_control and
   msg_controllen members of the msghdr structure along with the cmsghdr
   structure that is pointed to by the msg_control member is required by
   the Posix.1g sockets API standard (which should be completed during
   1997).

   In this document ancillary data is used to exchange the following
   optional information between the application and the kernel:

       1.  the send/receive interface and source/destination address,
       2.  the hop limit,
       3.  next hop address,
       4.  Hop-by-Hop options,
       5.  Destination options, and
       6.  Routing header.

   Before describing these uses in detail, we review the definition of
   the msghdr structure itself, the cmsghdr structure that defines an
   ancillary data object, and some functions that operate on the
   ancillary data objects.






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4.1.  The msghdr Structure

   The msghdr structure is used by the recvmsg() and sendmsg()
   functions.  Its Posix.1g definition is:

    struct msghdr {
      void      *msg_name;        /* ptr to socket address structure */
      socklen_t  msg_namelen;     /* size of socket address structure */
      struct iovec  *msg_iov;     /* scatter/gather array */
      size_t     msg_iovlen;      /* # elements in msg_iov */
      void      *msg_control;     /* ancillary data */
      socklen_t  msg_controllen;  /* ancillary data buffer length */
      int        msg_flags;       /* flags on received message */
    };

   The structure is declared as a result of including <sys/socket.h>.

   (Note: Before Posix.1g the two "void *" pointers were typically "char
   *", and the two socklen_t members and the size_t member were
   typically integers.  Earlier drafts of Posix.1g had the two socklen_t
   members as size_t, but Draft 6.6 of Posix.1g, apparently the final
   draft, changed these to socklen_t to simplify binary portability for
   64-bit implementations and to align Posix.1g with X/Open's Networking
   Services, Issue 5.  The change in msg_control to a "void *" pointer
   affects any code that increments this pointer.)

   Most Berkeley-derived implementations limit the amount of ancillary
   data in a call to sendmsg() to no more than 108 bytes (an mbuf).
   This API requires a minimum of 10240 bytes of ancillary data, but it
   is recommended that the amount be limited only by the buffer space
   reserved by the socket (which can be modified by the SO_SNDBUF socket
   option).  (Note: This magic number 10240 was picked as a value that
   should always be large enough.  108 bytes is clearly too small as the
   maximum size of a Type 0 Routing header is 376 bytes.)

4.2.  The cmsghdr Structure

   The cmsghdr structure describes ancillary data objects transferred by
   recvmsg() and sendmsg().  Its Posix.1g definition is:

    struct cmsghdr {
      socklen_t  cmsg_len;   /* #bytes, including this header */
      int        cmsg_level; /* originating protocol */
      int        cmsg_type;  /* protocol-specific type */
                 /* followed by unsigned char cmsg_data[]; */
    };

   This structure is declared as a result of including <sys/socket.h>.



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   As shown in this definition, normally there is no member with the
   name cmsg_data[].  Instead, the data portion is accessed using the
   CMSG_xxx() macros, as described shortly.  Nevertheless, it is common
   to refer to the cmsg_data[] member.

   (Note: Before Posix.1g the cmsg_len member was an integer, and not a
   socklen_t.  See the Note in the previous section for why socklen_t is
   used here.)

   When ancillary data is sent or received, any number of ancillary data
   objects can be specified by the msg_control and msg_controllen
   members of the msghdr structure, because each object is preceded by a
   cmsghdr structure defining the object's length (the cmsg_len member).
   Historically Berkeley-derived implementations have passed only one
   object at a time, but this API allows multiple objects to be passed
   in a single call to sendmsg() or recvmsg().  The following example
   shows two ancillary data objects in a control buffer.

|<--------------------------- msg_controllen -------------------------->|
|                                                                       |
|<----- ancillary data object ----->|<----- ancillary data object ----->|
|<---------- CMSG_SPACE() --------->|<---------- CMSG_SPACE() --------->|
|                                   |                                   |
|<---------- cmsg_len ---------->|  |<--------- cmsg_len ----------->|  |
|<--------- CMSG_LEN() --------->|  |<-------- CMSG_LEN() ---------->|  |
|                                |  |                                |  |
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
|cmsg_|cmsg_|cmsg_|XX|           |XX|cmsg_|cmsg_|cmsg_|XX|           |XX|
|len  |level|type |XX|cmsg_data[]|XX|len  |level|type |XX|cmsg_data[]|XX|
+-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
 ^
 |
msg_control
points here

   The fields shown as "XX" are possible padding, between the cmsghdr
   structure and the data, and between the data and the next cmsghdr
   structure, if required by the implementation.

4.3.  Ancillary Data Object Macros

   To aid in the manipulation of ancillary data objects, three macros
   from 4.4BSD are defined by Posix.1g: CMSG_DATA(), CMSG_NXTHDR(), and
   CMSG_FIRSTHDR().  Before describing these macros, we show the
   following example of how they might be used with a call to recvmsg().

    struct msghdr   msg;
    struct cmsghdr  *cmsgptr;



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    /* fill in msg */

    /* call recvmsg() */

    for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
         cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
        if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
            u_char  *ptr;

            ptr = CMSG_DATA(cmsgptr);
            /* process data pointed to by ptr */
        }
    }

   We now describe the three Posix.1g macros, followed by two more that
   are new with this API: CMSG_SPACE() and CMSG_LEN().  All these macros
   are defined as a result of including <sys/socket.h>.

4.3.1.  CMSG_FIRSTHDR

       struct cmsghdr *CMSG_FIRSTHDR(const struct msghdr *mhdr);

   CMSG_FIRSTHDR() returns a pointer to the first cmsghdr structure in
   the msghdr structure pointed to by mhdr.  The macro returns NULL if
   there is no ancillary data pointed to the by msghdr structure (that
   is, if either msg_control is NULL or if msg_controllen is less than
   the size of a cmsghdr structure).

   One possible implementation could be

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

   (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.)







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4.3.2.  CMSG_NXTHDR

       struct cmsghdr *CMSG_NXTHDR(const struct msghdr *mhdr,
                                   const struct cmsghdr *cmsg);

   CMSG_NXTHDR() returns a pointer to the cmsghdr structure describing
   the next ancillary data object.  mhdr is a pointer to a msghdr
   structure and cmsg is a pointer to a cmsghdr structure.  If there is
   not another ancillary data object, the return value is NULL.

   The following behavior of this macro is new to this API: if the value
   of the cmsg pointer is NULL, a pointer to the cmsghdr structure
   describing the first ancillary data object is returned.  That is,
   CMSG_NXTHDR(mhdr, NULL) is equivalent to CMSG_FIRSTHDR(mhdr).  If
   there are no ancillary data objects, the return value is NULL.  This
   provides an alternative way of coding the processing loop shown
   earlier:

struct msghdr  msg;
struct cmsghdr  *cmsgptr = NULL;

/* fill in msg */

/* call recvmsg() */

while ((cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) != NULL) {
    if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
        u_char  *ptr;

        ptr = CMSG_DATA(cmsgptr);
        /* process data pointed to by ptr */
    }
}

   One possible implementation could be:

    #define CMSG_NXTHDR(mhdr, cmsg) \
        ( ((cmsg) == NULL) ? CMSG_FIRSTHDR(mhdr) : \
          (((u_char *)(cmsg) + ALIGN((cmsg)->cmsg_len) \
                             + ALIGN(sizeof(struct cmsghdr)) > \
            (u_char *)((mhdr)->msg_control) + (mhdr)->msg_controllen) ? \
           (struct cmsghdr *)NULL : \
           (struct cmsghdr *)((u_char *)(cmsg) + ALIGN((cmsg)->cmsg_len))) )

   The macro ALIGN(), which is implementation dependent, rounds its
   argument up to the next even multiple of whatever alignment is
   required (probably a multiple of 4 or 8 bytes).




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4.3.3.  CMSG_DATA

       unsigned char *CMSG_DATA(const struct cmsghdr *cmsg);

   CMSG_DATA() returns a pointer to the data (what is called the
   cmsg_data[] member, even though such a member is not defined in the
   structure) following a cmsghdr structure.

   One possible implementation could be:

       #define CMSG_DATA(cmsg) ( (u_char *)(cmsg) + \
                                 ALIGN(sizeof(struct cmsghdr)) )

4.3.4.  CMSG_SPACE

       unsigned int CMSG_SPACE(unsigned int length);

   This macro is new with this API.  Given the length of an ancillary
   data object, CMSG_SPACE() returns the space required by the object
   and its cmsghdr structure, including any padding needed to satisfy
   alignment requirements.  This macro can be used, for example, to
   allocate space dynamically for the ancillary data.  This macro should
   not be used to initialize the cmsg_len member of a cmsghdr structure;
   instead use the CMSG_LEN() macro.

   One possible implementation could be:

       #define CMSG_SPACE(length) ( ALIGN(sizeof(struct cmsghdr)) + \
                                    ALIGN(length) )

4.3.5.  CMSG_LEN

       unsigned int CMSG_LEN(unsigned int length);

   This macro is new with this API.  Given the length of an ancillary
   data object, CMSG_LEN() returns the value to store in the cmsg_len
   member of the cmsghdr structure, taking into account any padding
   needed to satisfy alignment requirements.

   One possible implementation could be:

       #define CMSG_LEN(length) ( ALIGN(sizeof(struct cmsghdr)) + length
       )








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