home | help 
SEND(2) 		       System Calls Manual			 SEND(2)

NAME
     send, sendto, sendmsg -- send a message from a socket

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/socket.h>

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

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

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

DESCRIPTION
     The  send()  function,  and sendto() and sendmsg() system calls are used to
     transmit a message to another socket.  The send() function may be used only
     when the socket is in a connected state, while sendto() and  sendmsg()  may
     be used at any time.

     The  address  of  the target is given by to with tolen specifying its size.
     The length of the message is given by len.  If the message is too	long  to
     pass  atomically through the underlying protocol, the error EMSGSIZE is re-
     turned, and the message is not transmitted.

     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  deter-
     mine 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_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 support
     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  proto-
     cols which support the concept.  MSG_EOF requests that the sender side of a
     socket  be shut down, and that an appropriate indication be sent at the end
     of the specified data; this flag is only implemented for SOCK_STREAM  sock-
     ets  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.

RETURN VALUES
     The call returns the number of characters sent, or -1 if an error occurred.

ERRORS
     The send() function 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.

     [ENOTSOCK] 	The argument s is not a socket.

     [EFAULT]		An invalid user space address was specified for an argu-
			ment.

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

     [EAGAIN]		The socket is marked non-blocking and the requested  op-
			eration would block.

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

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

     [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 mes-
			sage  from  the last message sent.  This typically means
			that the receiver is not listening on the remote port.

     [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.

SEE ALSO
     fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2)

HISTORY
     The send() function appeared in 4.2BSD.

BUGS
     Because sendmsg() does not necessarily block until the data has been trans-
     ferred, it is possible to	transfer  an  open  file  descriptor  across  an
     AF_UNIX domain socket (see recv(2)), then close() it before it has actually
     been  sent,  the result being that the receiver gets a closed file descrip-
     tor.  It is left to the application to implement an  acknowledgment  mecha-
     nism to prevent this from happening.

FreeBSD 9.0			February 5, 2009			 SEND(2)
home | help