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

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

     Standard C	Library	(libc, -lc)

     #include <sys/types.h>
     #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);

     The recvfrom() and	recvmsg() 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 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.

     All three routines	return the length of the message on successful comple-
     tion.  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 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) 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_WAITALL	       wait for	full request or	error
	   MSG_DONTWAIT	       do not block
	   MSG_CMSG_CLOEXEC    set received fds	close-on-exec

     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 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 de-
     scribe 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.

     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 close-on-exec flag on received descriptors is set	ac-
     cording to	the MSG_CMSG_CLOEXEC flag passed to recvmsg().

     Process credentials can also be passed as ancillary data for AF_UNIX do-
     main sockets using	a cmsg_type of SCM_CREDS.  In this case, cmsg_data
     should be a structure of type cmsgcred, which is defined in
     <sys/socket.h> as follows:

     struct cmsgcred {
	     pid_t   cmcred_pid;	     /*	PID of sending process */
	     uid_t   cmcred_uid;	     /*	real UID of sending process */
	     uid_t   cmcred_euid;	     /*	effective UID of sending process */
	     gid_t   cmcred_gid;	     /*	real GID of sending process */
	     short   cmcred_ngroups;	     /*	number or groups */
	     gid_t   cmcred_groups[CMGROUP_MAX];     /*	groups */

     If	a sender supplies ancillary data with enough space for the above
     struct tagged as SCM_CREDS	control	message	type to	the sendmsg() system
     call, then	kernel will fill in the	credential information of the sending
     process and deliver it to the receiver.  Since receiver usually has no
     control over a sender, this method	of retrieving credential information
     isn't reliable.  For reliable retrieval of	remote side credentials	it is
     advised to	use the	LOCAL_CREDS socket option on the receiving socket.
     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.

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

     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 re-

     [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), unix(4)

     The recv()	function appeared in 4.2BSD.

BSD				March 19, 2013				   BSD


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