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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.

     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 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 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 arrives.

     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 {
	     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 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 {
	     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() 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  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 9.0			December 28, 2006			 RECV(2)
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