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RECV(2)			  FreeBSD System Calls Manual		       RECV(2)

NAME
     recv, recvfrom, recvmsg, recvmmsg -- receive message(s) from a socket

LIBRARY
     Standard C	Library	(libc, -lc)

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 * restrict from, socklen_t * restrict fromlen);

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

     ssize_t
     recvmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int	flags,
	 const struct timespec * restrict timeout);

DESCRIPTION
     The recvfrom(), recvmsg(),	and recvmmsg() system calls 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	not a null pointer and the socket is not connection-oriented,
     the source	address	of the message is filled in.  The fromlen argument is
     a value-result argument, 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()	function is normally used only on a connected socket (see
     connect(2)) and is	identical to recvfrom()	with a null pointer passed as
     its from argument.

     The recvmmsg() function is	used to	receive	multiple messages at a call.
     Their number is supplied by vlen.	The messages are placed	in the buffers
     described by msgvec vector, after reception.  The size of each received
     message is	placed in the msg_len field of each element of the vector.  If
     timeout is	NULL the call blocks until the data is available for each sup-
     plied message buffer.  Otherwise it waits for data	for the	specified
     amount of time.  If the timeout expired and there is no data received, a
     value 0 is	returned.  The ppoll(2)	system call is used to implement the
     timeout mechanism,	before first receive is	performed.

     The recv(), recvfrom() and	recvmsg() return the length of the message on
     successful	completion, whereas recvmmsg() returns the number of received
     messages.	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 non-blocking (see fcntl(2)) in
     which case	the value -1 is	returned and the global	variable errno is set
     to	EAGAIN.	 The receive calls except recvmmsg() 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
     recvmmsg()	function implements this behaviour for each message in the
     vector.

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

     The flags argument	to a recv() function 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
	   MSG_DONTWAIT	       do not block
	   MSG_CMSG_CLOEXEC    set received fds	close-on-exec
	   MSG_WAITFORONE      do not block after receiving the	first message
			       (only for recvmmsg() )

     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
     returned.	The MSG_DONTWAIT flag requests the call	to return when it
     would block otherwise.  If	no data	is available, errno is set to EAGAIN.
     This flag is not available	in strict ANSI or C99 compilation mode.	 The
     MSG_WAITFORONE flag sets MSG_DONTWAIT after the first message has been
     received.	This flag is only relevant for recvmmsg().

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

     struct msghdr {
	     void	     *msg_name;	     /*	optional address */
	     socklen_t	      msg_namelen;   /*	size of	address	*/
	     struct iovec    *msg_iov;	     /*	scatter/gather array */
	     int	      msg_iovlen;    /*	# elements in msg_iov */
	     void	     *msg_control;   /*	ancillary data,	see below */
	     socklen_t	      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.  The msg_iov and msg_iovlen	arguments
     describe scatter gather locations,	as discussed in	read(2).  The
     msg_control argument, which has length msg_controllen, points to a	buffer
     for other protocol	control	related	messages or other miscellaneous	ancil-
     lary data.	 The messages are of the form:

     struct cmsghdr {
	     socklen_t	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() system call.

     With AF_UNIX domain sockets, ancillary data can be	used to	pass file
     descriptors and process credentials.  See unix(4) for details.

     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 indi-
     cates 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.

     The recvmmsg() system call	uses the mmsghdr structure, defined as follows
     in	the <sys/socket.h> header :

     struct mmsghdr {
	     struct msghdr    msg_hdr;	     /*	message	header */
	     ssize_t	      msg_len;	     /*	message	length */
     };

     On	data reception the msg_len field is updated to the length of the
     received message.

RETURN VALUES
     These calls except	recvmmsg() return the number of	bytes received.
     recvmmsg()	returns	the number of messages received.  A value of -1	is
     returned if an error occurred.

ERRORS
     The calls fail if:

     [EBADF]		The argument s is an invalid descriptor.

     [ECONNRESET]	The remote socket end is forcibly closed.

     [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.

     [EMSGSIZE]		The recvmsg() system call was used to receive rights
			(file descriptors) that	were in	flight on the connec-
			tion.  However,	the receiving program did not have
			enough free file descriptor slots to accept them.  In
			this case the descriptors are closed, any pending data
			can be returned	by another call	to recvmsg().

     [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), getsockopt(2), read(2), select(2), socket(2), unix(4)

HISTORY
     The recv()	function appeared in 4.2BSD.  The recvmmsg() function appeared
     in	FreeBSD	11.0.

FreeBSD	11.1		       February	3, 2017			  FreeBSD 11.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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