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     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 connect (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
     underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

     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

     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         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 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.  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 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 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 structure.


     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 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, 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 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.

     [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.

     [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

     [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 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.