NAME

       send, sendto, sendmsg, sendmmsg — send message(s) from a socket

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #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);

       ssize_t
       sendmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags);

DESCRIPTION

       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, while sendto(), sendmsg() and sendmmsg() 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.

       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_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.  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.

RETURN VALUES

       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.

ERRORS

       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.

       [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 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 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), CMSG_DATA(3)

HISTORY

       The send() function appeared in 4.2BSD.  The sendmmsg() function appeared in FreeBSD 11.0.

BUGS

       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.

Debian                                           August 19, 2018                                         SEND(2)