DragonFly On-Line Manual Pages

SEND(2)                  DragonFly 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 *msgbuf, size_t len, int flags);

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

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

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

     The socket file descriptor is given by s.  msgbuf points to a buffer
     containing the message.  msg points to a msghdr structure.  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) call may be used to
     determine when it is possible to send more data.

     The flags parameter 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 */
     #define MSG_NOSIGNAL    0x400 /* No SIGPIPE to unconnected socket stream */

     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.  MSG_DONTROUTE is
     usually used only by diagnostic or routing programs.  MSG_NOSIGNAL
     requests not to send the SIGPIPE signal if an attempt to send is made on
     a stream-oriented socket that is no longer connected.

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

RETURN VALUES
     Upon successful completion the number of characters which were sent is
     returned.  Otherwise -1 is returned and the global variable errno is set
     to indicate the error.

ERRORS
     Send(), sendto(), and sendmsg() 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 a
                        parameter.

     [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
                        available.

     [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
                        congestion.

     [EHOSTUNREACH]     The remote host was unreachable.

     [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.

     [EPIPE]            The socket is unable to send any more data
                        (SS_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 call appeared in 4.2BSD.

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
     actually 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.

DragonFly 6.3-DEVELOPMENT       October 6, 2010      DragonFly 6.3-DEVELOPMENT