Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
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 *from,
	 socklen_t *fromlen);

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

     Recvfrom()	and recvmsg() are used to receive messages from	a socket, and
     may be used to receive data on a socket whether or	not it is connection-

     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 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.

     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 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) call	may be used to determine when more data	arrive.

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

	   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-

     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 {
	     caddr_t msg_name;	     /*	optional address */
	     u_int   msg_namelen;    /*	size of	address	*/
	     struct  iovec *msg_iov; /*	scatter/gather array */
	     u_int   msg_iovlen;     /*	# elements in msg_iov */
	     caddr_t msg_control;    /*	ancillary data,	see below */
	     u_int   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.  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

     struct cmsghdr {
	     u_int   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()

     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

     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 */

     The kernel	will fill in the credential information	of the sending process
     and deliver it to the receiver.

     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.

     [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.

     [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)

     The recv()	function call appeared in 4.2BSD.

BSD			       February	21, 1994			   BSD


Want to link to this manual page? Use this URL:

home | help