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(libc.info)Sending Data

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Sending Data
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   The `send' function is declared in the header file `sys/socket.h'.
If your FLAGS argument is zero, you can just as well use `write'
instead of `send'; see Note: I/O Primitives.  If the socket was
connected but the connection has broken, you get a `SIGPIPE' signal for
any use of `send' or `write' (Note: Miscellaneous Signals).

 - Function: int send (int SOCKET, void *BUFFER, size_t SIZE, int FLAGS)
     The `send' function is like `write', but with the additional flags
     FLAGS.  The possible values of FLAGS are described in Note: Socket
     Data Options.

     This function returns the number of bytes transmitted, or `-1' on
     failure.  If the socket is nonblocking, then `send' (like `write')
     can return after sending just part of the data.  Note: File Status
     Flags, for information about nonblocking mode.

     Note, however, that a successful return value merely indicates that
     the message has been sent without error, not necessarily that it
     has been received without error.

     The following `errno' error conditions are defined for this
     function:

    `EBADF'
          The SOCKET argument is not a valid file descriptor.

    `EINTR'
          The operation was interrupted by a signal before any data was
          sent.  Note: Interrupted Primitives.

    `ENOTSOCK'
          The descriptor SOCKET is not a socket.

    `EMSGSIZE'
          The socket type requires that the message be sent atomically,
          but the message is too large for this to be possible.

    `EWOULDBLOCK'
          Nonblocking mode has been set on the socket, and the write
          operation would block.  (Normally `send' blocks until the
          operation can be completed.)

    `ENOBUFS'
          There is not enough internal buffer space available.

    `ENOTCONN'
          You never connected this socket.

    `EPIPE'
          This socket was connected but the connection is now broken.
          In this case, `send' generates a `SIGPIPE' signal first; if
          that signal is ignored or blocked, or if its handler returns,
          then `send' fails with `EPIPE'.

     This function is defined as a cancellation point in multi-threaded
     programs, so one has to be prepared for this and make sure that
     allocated resources (like memory, files descriptors, semaphores or
     whatever) are freed even if the thread is canceled.