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

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

     Standard C	Library	(libc, -lc)

     #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 * restrict from, socklen_t * restrict fromlen);

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

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

     The recvfrom(), recvmsg(),	and recvmmsg() system calls are	used to	re-
     ceive 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 ad-
     dress 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 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 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

     The select(2) system call may be used to determine	when more data ar-

     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_TRUNC	       return real packet or datagram length
	   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_TRUNC flag causes the receive operation to return the
     full length of the	packet or datagram even	if larger than provided	buf-
     fer. The flag is supported	on SOCK_DGRAM sockets for AF_INET , AF_INET6
     and AF_UNIX families.  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	dis-
     connect 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 ANSI X3.159-1989 ("ANSI C89") or
     ISO/IEC 9899:1999 ("ISO 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 source address if the socket is
     unconnected; msg_name may be given	as a null pointer if no	names are de-
     sired 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 ancillary data.  The mes-
     sages 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, the	SO_TIMESTAMP socket option returns a reception time-
     stamp for UDP packets.

     With AF_UNIX domain sockets, ancillary data can be	used to	pass file de-
     scriptors 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 re-
     ceived message.

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

     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 op-
			eration	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.

     fcntl(2), getsockopt(2), read(2), select(2), socket(2), CMSG_DATA(3),

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

FreeBSD	13.0			August 19, 2018			  FreeBSD 13.0


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