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sendmsg(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
     returned, and the message is not transmitted.

     No indication of failure to deliver is implicit in a send().  Locally
     detected 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	     0x1   /* process out-of-band data */
     #define MSG_PEEK	     0x2   /* peek at incoming message */
     #define MSG_DONTROUTE   0x4   /* bypass routing, use direct interface */
     #define MSG_EOR	     0x8   /* data completes record */
     #define MSG_EOF	     0x100 /* data completes transaction */

     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 protocols 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 sockets in the PF_INET protocol family, and is used to
     implement Transaction TCP (see ttcp(4)).  MSG_DONTROUTE is usually used
     only by diagnostic or routing programs.


     [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
			argument.

     [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
			operation would block.

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

     [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-
			gestion.

     [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
			port.

     [EHOSTDOWN]	The remote host was down.

     [ENETDOWN] 	The remote network was down.

     [EPERM]		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.

BUGS

     Because sendmsg() doesn't 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
     descriptor.  It is left to the application to implement an acknowledgment
     mechanism to prevent this from happening.

SEE ALSO

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