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

NAME

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

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #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

     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 connec-
     tion-oriented.

     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
     parameter, initialized to the size of the buffer associated with from, and modified 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 completion.  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.  If no data is available and the remote peer was shut
     down, 0 is returned.  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) 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 values:

	   MSG_CMSG_CLOEXEC    set the close on exec property for passed file descriptors
	   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 oper-
     ation 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 returned.

     The recvmsg() call uses a msghdr structure to minimize the number of directly 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 */
	     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 desired or required.  If the socket is connected, msg_name and msg_namelen are ignored.  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 mes-
     sages 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[]; */
     };
     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() call.

     Open file descriptors are now passed as ancillary data for AF_LOCAL domain 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.  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 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.

RETURN VALUES

     These calls return the number of bytes received, or -1 if an error occurred.  For connected sockets whose remote peer was shut down, 0 is
     returned when no more data is available.

ERRORS

     The calls fail if:

     [EAGAIN]		The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the time-
			out expired before data were received.

     [EBADF]		The argument s is an invalid descriptor.

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

     [EINTR]		The receive was interrupted by delivery of a signal before any data were available.

     [EINVAL]		The total length of the I/O is more than can be expressed by the ssize_t return value.

     [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.

     recvmsg() will also fail if:

     [EMSGSIZE] 	The msg_iovlen member of the msg structure is less than or equal to 0 or is greater than {IOV_MAX}.

SEE ALSO

     fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2)

HISTORY

     The recv() function call appeared in 4.2BSD.

BSD
								 February 1, 2012							       BSD