PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       send — send a message on a socket

SYNOPSIS

       #include <sys/socket.h>

       ssize_t send(int socket, const void *buffer, size_t length, int flags);

DESCRIPTION

       The  send() function shall initiate transmission of a message from the specified socket to
       its peer. The send() function shall send a message only when the socket is  connected.  If
       the socket is a connectionless-mode socket, the message shall be sent to the pre-specified
       peer address.

       The send() function takes the following arguments:

       socket      Specifies the socket file descriptor.

       buffer      Points to the buffer containing the message to send.

       length      Specifies the length of the message in bytes.

       flags       Specifies the type of message transmission. Values of this argument are formed
                   by logically OR'ing zero or more of the following flags:

                   MSG_EOR       Terminates a record (if supported by the protocol).

                   MSG_OOB       Sends  out-of-band  data  on  sockets  that  support out-of-band
                                 communications. The significance and  semantics  of  out-of-band
                                 data are protocol-specific.

                   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.
                                 The [EPIPE] error shall still be returned.

       The  length  of the message to be sent is specified by the length argument. If the message
       is too long to pass through the underlying protocol, send() shall fail and no  data  shall
       be transmitted.

       Successful  completion  of  a call to send() does not guarantee delivery of the message. A
       return value of -1 indicates only locally-detected errors.

       If space is not available at the sending socket to hold the message to be transmitted, and
       the socket file descriptor does not have O_NONBLOCK set, send() shall block until space is
       available. If space is not available at the sending socket  to  hold  the  message  to  be
       transmitted,  and  the socket file descriptor does have O_NONBLOCK set, send() shall fail.
       The select() and poll() functions can be used to determine when it  is  possible  to  send
       more data.

       The socket in use may require the process to have appropriate privileges to use the send()
       function.

RETURN VALUE

       Upon successful completion, send() shall return the number of bytes  sent.  Otherwise,  -1
       shall be returned and errno set to indicate the error.

ERRORS

       The send() function shall fail if:

       EAGAIN or EWOULDBLOCK
              The socket's file descriptor is marked O_NONBLOCK and the requested operation would
              block.

       EBADF  The socket argument is not a valid file descriptor.

       ECONNRESET
              A connection was forcibly closed by a peer.

       EDESTADDRREQ
              The socket is not connection-mode and no peer address is set.

       EINTR  A signal interrupted send() before any data was transmitted.

       EMSGSIZE
              The message is too large to be sent all at once, as the socket requires.

       ENOTCONN
              The socket is not connected.

       ENOTSOCK
              The socket argument does not refer to a socket.

       EOPNOTSUPP
              The socket argument is associated with a socket that does not support one  or  more
              of the values set in flags.

       EPIPE  The  socket  is  shut  down for writing, or the socket is connection-mode and is no
              longer connected. In the latter case, and if the socket is of type  SOCK_STREAM  or
              SOCK_SEQPACKET  and  the  MSG_NOSIGNAL  flag  is  not  set,  the  SIGPIPE signal is
              generated to the calling thread.

       The send() function may fail if:

       EACCES The calling process does not have appropriate privileges.

       EIO    An I/O error occurred while reading from or writing to the file system.

       ENETDOWN
              The local network interface used to reach the destination is down.

       ENETUNREACH
              No route to the network is present.

       ENOBUFS
              Insufficient resources were available in the system to perform the operation.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       If the socket argument  refers  to  a  connection-mode  socket,  the  send()  function  is
       equivalent  to  sendto() (with any value for the dest_addr and dest_len arguments, as they
       are ignored in this case). If the socket  argument  refers  to  a  socket  and  the  flags
       argument is 0, the send() function is equivalent to write().

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       connect(),  getsockopt(),  poll(),  pselect(),  recv(),  recvfrom(), recvmsg(), sendmsg(),
       sendto(), setsockopt(), shutdown(), socket(), write()

       The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1-2017,  Standard  for  Information Technology -- Portable Operating System Interface
       (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C)  2018  by
       the  Institute  of  Electrical  and Electronics Engineers, Inc and The Open Group.  In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most  likely  to  have
       been  introduced  during  the conversion of the source files to man page format. To report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .