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OpenDarwin 7.2.1 - man page for recvmsg (opendarwin section 2)

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RECV(2) 			     BSD System Calls Manual				  RECV(2)

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

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

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

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

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

DESCRIPTION
     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-oriented.

     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 parameter, initialized to the size of the
     buffer associated with from, and modified 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 identi-
     cal to recvfrom() with a nil from parameter.  As it is redundant, it may not be supported in
     future releases.

     On successful completion, all three routines return the number of message bytes read.  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 received 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'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 nor-
     mal 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 returned.

     The recvmsg() call uses a msghdr structure to minimize the number of directly 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 source 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 form:

     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 requesting a recvmsg with no data buffer pro-
     vided immediately after an accept() call.

     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 SCM_RIGHTS.

     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 datagram was larger than the buffer supplied.	MSG_CTRUNC indicates that some
     control data were discarded due to lack of space in the buffer for ancillary data.  MSG_OOB
     is returned to indicate that expedited or out-of-band data were received.

RETURN VALUES
     These calls return the number of bytes received, or -1 if an error occurred.

ERRORS
     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 timeout 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 address space.

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

HISTORY
     The recv() function call appeared in 4.2BSD.

4.3-Reno Berkeley Distribution		February 21, 1994	   4.3-Reno Berkeley Distribution
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