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RECV(2) 		       System Calls Manual			 RECV(2)

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

LIBRARY
     Standard C Library (libc, -lc)

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

DESCRIPTION
     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  con-
     nect(2))  and  is identical to recvfrom() with a null pointer passed as its
     from argument.  As it is redundant, it may not be supported in  future  re-
     leases.

     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	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  re-
     quested;  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 arrive.

     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

     The MSG_OOB flag requests receipt of out-of-band data that would not be re-
     ceived  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 re-
     quest is satisfied.  However, the call may still return less data than  re-
     quested  if  a signal is caught, an error or disconnect 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 other-
     wise.  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 de-
     fined 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 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 messages are
     of the form:

     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 requesting  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.

     Process credentials can also be passed as ancillary data for AF_UNIX domain
     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 fol-
     lows:

     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 (gen-
     erally 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	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 re-
     ceived.

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

ERRORS
     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 pro-
			tocol and has not been connected (see connect(2) and ac-
			cept(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 the 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 oper-
			ation  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.

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

HISTORY
     The recv() function appeared in 4.2BSD.

FreeBSD 6.0			February 21, 1994			 RECV(2)
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