recvmsg man page on 4.4BSD

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RECV(2)			    BSD Programmer's Manual		       RECV(2)

     recv, recvfrom, recvmsg - receive a message from a socket

     #include <sys/types.h>
     #include <sys/socket.h>

     recv(int s, void *buf, size_t len, int flags);

     recvfrom(int s, void *buf, size_t len, int flags, struct sockaddr *from,
	     int *fromlen);

     recvmsg(int s, struct msghdr *msg, int flags);

     Recvfrom() and recvmsg() are used to receive messages from a socket, and
     may be used to receive data on a socket whether or not it is connection-

     If from is non-nil, and the socket is not connection-oriented, the source
     address of the message is filled in.  Fromlen is a value-result parame-
     ter, initialized to the size of the buffer associated with from, and mod-
     ified on return to indicate the actual size of the address stored there.

     The recv() call is normally used only on a connected socket (see
     connect(2))  and is identical to recvfrom() with a nil from parameter.
     As it is redundant, it may not be supported in future releases.

     All three routines return the length of the message on successful comple-
     tion.  If a message is too long to fit in the supplied buffer, excess
     bytes may be discarded depending on the type of socket the message is re-
     ceived from (see socket(2)).

     If no messages are available at the socket, the receive call waits for a
     message to arrive, unless the socket is nonblocking (see fcntl(2))	 in
     which case the value -1 is returned and the external variable errno set
     to EAGAIN. The receive calls normally return any data available, up to
     the requested amount, rather than waiting for receipt of the full amount
     requested; this behavior is affected by the socket-level options
     SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).

     The select(2) call may be used to determine when more data arrive.

     The flags argument to a recv call is formed by or Ap ing one or more of
     the values:

	   MSG_OOB	  process out-of-band data
	   MSG_PEEK	  peek at incoming message
	   MSG_WAITALL	  wait for full request or error
     The MSG_OOB flag requests receipt of out-of-band data that would not be
     received in the normal data stream.  Some protocols place expedited data
     at the head of the normal data queue, and thus this flag cannot be used
     with such protocols.  The MSG_PEEK flag causes the receive operation to
     return data from the beginning of the receive queue without removing that
     data from the queue.  Thus, a subsequent receive call will return the
     same data.	 The MSG_WAITALL flag requests that the operation block until
     the full request is satisfied.  However, the call may still return less
     data than requested if a signal is caught, an error or disconnect occurs,
     or the next data to be received is of a different type than that re-

     The recvmsg() call uses a msghdr structure to minimize the number of di-
     rectly supplied parameters.  This structure has the following form, as
     defined in <sys/socket.h>:

     struct msghdr {
	     caddr_t msg_name;	     /* optional address */
	     u_int   msg_namelen;    /* size of address */
	     struct  iovec *msg_iov; /* scatter/gather array */
	     u_int   msg_iovlen;     /* # elements in msg_iov */
	     caddr_t msg_control;    /* ancillary data, see below */
	     u_int   msg_controllen; /* ancillary data buffer len */
	     int     msg_flags;	     /* flags on received message */

     Here msg_name and msg_namelen specify the destination address if the
     socket is unconnected; msg_name may be given as a null pointer if no
     names are desired or required.  Msg_iov and msg_iovlen describe scatter
     gather locations, as discussed in read(2).	 Msg_control, which has length
     msg_controllen, points to a buffer for other protocol control related
     messages or other miscellaneous ancillary data.  The messages are of the

     struct cmsghdr {
	     u_int   cmsg_len;	     /* data byte count, including hdr */
	     int     cmsg_level;     /* originating protocol */
	     int     cmsg_type;	     /* protocol-specific type */
     /* followed by
	     u_char  cmsg_data[]; */
     As an example, one could use this to learn of changes in the data-stream
     in XNS/SPP, or in ISO, to obtain user-connection-request data by request-
     ing a recvmsg with no data buffer provided immediately after an accept()

     Open file descriptors are now passed as ancillary data for AF_UNIX domain
     sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to

     The msg_flags field is set on return according to the message received.
     MSG_EOR indicates end-of-record; the data returned completed a record
     (generally used with sockets of type SOCK_SEQPACKET). MSG_TRUNC indicates
     that the trailing portion of a datagram was discarded because the data-
     gram was larger than the buffer supplied.	MSG_CTRUNC indicates that some
     control data were discarded due to lack of space in the buffer for ancil-
     lary data.	 MSG_OOB is returned to indicate that expedited or out-of-band
     data were received.

     These calls return the number of bytes received, or -1 if an error oc-

     The calls fail if:

     [EBADF]	 The argument s is an invalid descriptor.

     [ENOTCONN]	 The socket is associated with a connection-oriented protocol
		 and has not been connected (see connect(2) and accept(2)).

     [ENOTSOCK]	 The argument s does not refer to a socket.

     [EAGAIN]	 The socket is marked non-blocking, and the receive operation
		 would block, or a receive timeout had been set, and the time-

		 out expired before data were received.

     [EINTR]	 The receive was interrupted by delivery of a signal before
		 any data were available.

     [EFAULT]	 The receive buffer pointer(s) point outside the process's ad-
		 dress space.

     fcntl(2),	read(2),  select(2),  getsockopt(2),  socket(2)

     The recv function call appeared in 4.2BSD.

4.3-Reno Berkeley Distribution February 21, 1994			     3

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