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

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

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/socket.h>

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

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

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

     int
     recvmmsg(int s, struct mmsghdr *mmsg, unsigned int	vlen,
	 unsigned int flags, struct timespec *timeout);

DESCRIPTION
     recvfrom(), recvmsg() and recvmmsg() 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 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 recvmmsg() call can be	used to	receive	multiple messages in the same
     call using	an array of mmsghdr elements with the following	form, as de-
     fined in <sys/socket.h>:

     struct mmsghdr {
	     struct msghdr   msg_hdr;	     /*	the message to be sent */
	     unsigned int    msg_len;	     /*	number of bytes	received */
     };

     The msg_len member	contains the number of bytes received for each msg_hdr
     member.  The array	has vlen elements, which is limited to 1024.  If there
     is	an error, a number fewer than vlen may be returned, and	the error may
     be	retrieved using	getsockopt(2) with SO_ERROR.  If the flag
     MSG_WAITFORONE is set in flags then the recvmmsg()	call will wait for one
     message, and set MSG_DONTWAIT for the rest.  If the timeout parameter is
     not NULL, then recvmmsg() will return if that time	is exceeded.

     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.

     recv(), recvfrom()	and recvmsg() routines return the length of the	mes-
     sage on successful	completion.  recvmmsg()	returns	the number of messages
     received.	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.	 If no data is available and the remote	peer was shut down, 0
     is	returned.  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) or poll(2) call may be used to determine when more data ar-
     rives.

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

	   MSG_CMSG_CLOEXEC    set the close on	exec property for passed file
			       descriptors
	   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-
     turned.

     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 {
	     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 source address if the socket is
     unconnected; msg_name may be given	as a null pointer if no	names are de-
     sired or required.	 If the	socket is connected, msg_name and msg_namelen
     are ignored.  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 {
	     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()
     call.

     Open file descriptors are now passed as ancillary data for	AF_LOCAL do-
     main 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 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.

RETURN VALUES
     The recv(), recvfrom() and	recvmsg() calls	return the number of bytes re-
     ceived, or	-1 if an error occurred.  For connected	sockets	whose remote
     peer was shut down, 0 is returned when no more data is available.	The
     recvmmsg()	call returns the number	of messages received, or -1 if an er-
     ror occurred.

ERRORS
     The calls fail if:

     [EAGAIN]		The socket is marked non-blocking, and the receive op-
			eration	would block, or	a receive timeout had been
			set, and the timeout expired before data were re-
			ceived.

     [EBADF]		The argument s is an invalid descriptor.

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

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

     [EINVAL]		The total length of the	I/O is more than can be	ex-
			pressed	by the ssize_t return value.

     [ENOBUFS]		A message was not delivered because it would have
			overflowed the buffer.

     [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.

     recvmsg() will also fail if:

     [EMSGSIZE]		The msg_iovlen member of the msg structure is less
			than or	equal to 0, or is greater than {IOV_MAX}.

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

HISTORY
     The recv()	function call appeared in 4.2BSD.  The recvmmsg() function
     call appeared in Linux 2.6.32 and NetBSD 7.0.

FreeBSD	13.0			March 19, 2018			  FreeBSD 13.0

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

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