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

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

SYNOPSIS
     #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,
	 socklen_t *fromlen);

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

DESCRIPTION
     recvfrom()	and recvmsg() are used to receive messages from	a socket, s,
     and may be	used to	receive	data on	a socket whether or not	it is connec-
     tion-oriented.

     If	from is	non-null 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 null from parameter.

     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, ex-
     cess 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) or poll(2) system calls may be used to determine when more
     data arrive.

     The flags argument	is the bitwise OR of zero or more of the following
     values:

	   MSG_OOB	       process out-of-band data
	   MSG_PEEK	       peek at incoming	message
	   MSG_WAITALL	       wait for	full request or	error
	   MSG_DONTWAIT	       don't block
	   MSG_CMSG_CLOEXEC    set the close-on-exec flag on received file de-
			       scriptors

     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 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_CMSG_CLOEXEC requests that any	file descriptors received as ancillary
     data with recvmsg() (see below) have their	close-on-exec flag set.

     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 */
	     unsigned 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.	 msg_iov and msg_iovlen	describe scatter gather	loca-
     tions, 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[]; */
     };

     See CMSG_DATA(3) for how these messages are constructed and decomposed.

     Open file descriptors are now passed as ancillary data for	AF_UNIX	domain
     and socketpair(2) 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.
     It	will contain zero or more of the following values:

	   MSG_OOB     Returned	to indicate that expedited or out-of-band data
		       was 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 buf-
		       fer supplied.
	   MSG_CTRUNC  Indicates that some control data	were discarded due to
		       lack of space in	the buffer for ancillary data.
	   MSG_BCAST   Indicates that the packet was received as broadcast.
	   MSG_MCAST   Indicates that the packet was received as multicast.

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

ERRORS
     recv(), recvfrom(), and recvmsg() fail if:

     [EBADF]	     The argument s is an invalid descriptor.

     [ENOTCONN]	     The socket	is associated with a connection-oriented pro-
		     tocol 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	opera-
		     tion 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 be-
		     fore any data were	available.

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

     [EHOSTUNREACH]  A socket operation	was attempted to an unreachable	host.

     [EHOSTDOWN]     A socket operation	failed because the destination host
		     was down.

     [ENETDOWN]	     A socket operation	encountered a dead network.

     [ECONNREFUSED]  The socket	is associated with a connection-oriented pro-
		     tocol and the connection was forcefully rejected (see
		     connect(2)).

     In	addition, recv() and recvfrom()	may return the following error:

     [EINVAL]		len was	larger than SSIZE_MAX.

     And recvmsg() may return one of the following errors:

     [EINVAL]		The sum	of the iov_len values in the msg_iov array
			overflowed an ssize_t.

     [EMSGSIZE]		The msg_iovlen member of msg was less than 0 or	larger
			than IOV_MAX.

     [EMSGSIZE]		The receiving program did not have sufficient free
			file descriptor	slots.	The descriptors	are closed and
			any pending data can be	returned by another call to
			recvmsg().

SEE ALSO
     connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2),
     socket(2),	socketpair(2), CMSG_DATA(3), sockatmark(3)

STANDARDS
     The recv(), recvfrom(), and recvmsg() functions conform to	IEEE Std
     1003.1-2008 ("POSIX.1").  The MSG_DONTWAIT, MSG_BCAST, and	MSG_MCAST
     flags are extensions to that specification.

HISTORY
     The recv()	function call appeared in 4.1cBSD.

CAVEATS
     Calling recvmsg() with a control message having no	or an empty scat-
     ter/gather	array exposes variations in implementations.  To avoid these,
     always use	an iovec with at least a one-byte buffer and set msg_iov and
     an	msg_iovlen to use this vector.

FreeBSD	13.0		       January 11, 2019			  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | CAVEATS

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