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SEND(2)			  FreeBSD System Calls Manual		       SEND(2)

     send, sendto, sendmsg, sendmmsg --	send message(s)	from a socket

     Standard C	Library	(libc, -lc)

     #include <sys/socket.h>

     send(int s, const void *msg, size_t len, int flags);

     sendto(int	s, const void *msg, size_t len,	int flags,
	 const struct sockaddr *to, socklen_t tolen);

     sendmsg(int s, const struct msghdr	*msg, int flags);

     sendmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen,
	 int flags);

     The send()	and sendmmsg() functions, and sendto() and sendmsg() system
     calls are used to transmit	one or more messages (with the sendmmsg()
     call) to another socket.  The send() function may be used only when the
     socket is in a connected state.  The functions sendto(), sendmsg()	and
     sendmmsg()	may be used at any time	if the socket is connectionless-mode.
     If	the socket is connection-mode, the protocol must support implied con-
     nect (currently tcp(4) is the only	protocol with support) or the socket
     must be in	a connected state before use.

     The address of the	target is given	by to with tolen specifying its	size,
     or	the equivalent msg_name	and msg_namelen	in struct msghdr.  If the
     socket is in a connected state, the target	address	passed to sendto(),
     sendmsg() or sendmmsg() is	ignored.  The length of	the message is given
     by	len.  If the message is	too long to pass atomically through the	under-
     lying protocol, the error EMSGSIZE	is returned, and the message is	not

     The sendmmsg() function sends multiple messages at	a call.	 They are
     given by the msgvec vector	along with vlen	specifying the vector size.
     The number	of octets sent per each	message	is placed in the msg_len field
     of	each processed element of the vector after transmission.

     No	indication of failure to deliver is implicit in	a send().  Locally de-
     tected errors are indicated by a return value of -1.

     If	no messages space is available at the socket to	hold the message to be
     transmitted, then send() normally blocks, unless the socket has been
     placed in non-blocking I/O	mode.  The select(2) system call may be	used
     to	determine when it is possible to send more data.

     The flags argument	may include one	or more	of the following:

     #define MSG_OOB	     0x00001 /*	process	out-of-band data */
     #define MSG_DONTROUTE   0x00004 /*	bypass routing,	use direct interface */
     #define MSG_EOR	     0x00008 /*	data completes record */
     #define MSG_DONTWAIT    0x00080 /*	do not block */
     #define MSG_EOF	     0x00100 /*	data completes transaction */
     #define MSG_NOSIGNAL    0x20000 /*	do not generate	SIGPIPE	on EOF */

     The flag MSG_OOB is used to send "out-of-band" data on sockets that sup-
     port this notion (e.g. SOCK_STREAM); the underlying protocol must also
     support "out-of-band" data.  MSG_EOR is used to indicate a	record mark
     for protocols which support the concept.  The MSG_DONTWAIT	flag request
     the call to return	when it	would block otherwise.	MSG_EOF	requests that
     the sender	side of	a socket be shut down, and that	an appropriate indica-
     tion be sent at the end of	the specified data; this flag is only imple-
     mented for	SOCK_STREAM sockets in the PF_INET protocol family.
     MSG_DONTROUTE is usually used only	by diagnostic or routing programs.
     MSG_NOSIGNAL is used to prevent SIGPIPE generation	when writing a socket
     that may be closed.

     See recv(2) for a description of the msghdr structure and the mmsghdr

     The send(), sendto() and sendmsg()	calls return the number	of octets
     sent.  The	sendmmsg() call	returns	the number of messages sent.  If an
     error occurred a value of -1 is returned.

     The send()	and sendmmsg() functions and sendto() and sendmsg() system
     calls fail	if:

     [EBADF]		An invalid descriptor was specified.

     [EACCES]		The destination	address	is a broadcast address,	and
			SO_BROADCAST has not been set on the socket.

     [ENOTCONN]		The socket is connection-mode but is not connected.

     [ENOTSOCK]		The argument s is not a	socket.

     [EFAULT]		An invalid user	space address was specified for	an ar-

     [EMSGSIZE]		The socket requires that message be sent atomically,
			and the	size of	the message to be sent made this im-

     [EAGAIN]		The socket is marked non-blocking, or MSG_DONTWAIT is
			specified, and the requested operation would block.

     [ENOBUFS]		The system was unable to allocate an internal buffer.
			The operation may succeed when buffers become avail-

     [ENOBUFS]		The output queue for a network interface was full.
			This generally indicates that the interface has
			stopped	sending, but may be caused by transient	con-

     [EHOSTUNREACH]	The remote host	was unreachable.

     [EISCONN]		A destination address was specified and	the socket is
			already	connected.

     [ECONNREFUSED]	The socket received an ICMP destination	unreachable
			message	from the last message sent.  This typically
			means that the receiver	is not listening on the	remote

     [EHOSTDOWN]	The remote host	was down.

     [ENETDOWN]		The remote network was down.

     [EADDRNOTAVAIL]	The process using a SOCK_RAW socket was	jailed and the
			source address specified in the	IP header did not
			match the IP address bound to the prison.

     [EPIPE]		The socket is unable to	send anymore data
			(SBS_CANTSENDMORE has been set on the socket).	This
			typically means	that the socket	is not connected.

     connect(2), fcntl(2), getsockopt(2), recv(2), select(2), socket(2),
     write(2), CMSG_DATA(3)

     The send()	function appeared in 4.2BSD.  The sendmmsg() function appeared
     in	FreeBSD	11.0.

     Because sendmsg() does not	necessarily block until	the data has been
     transferred, it is	possible to transfer an	open file descriptor across an
     AF_UNIX domain socket (see	recv(2)), then close() it before it has	actu-
     ally been sent, the result	being that the receiver	gets a closed file de-
     scriptor.	It is left to the application to implement an acknowledgment
     mechanism to prevent this from happening.

FreeBSD	13.0			April 27, 2020			  FreeBSD 13.0


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