SEND(2)                      BSD 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 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 transmission.

     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

     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

     [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

     [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 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.

BSD                              April 27, 2020                              BSD