= zmq_send_const(3)


== NAME
zmq_send_const - send a constant-memory message part on a socket


== SYNOPSIS
*int zmq_send_const (void '*socket', const void '*buf', size_t 'len', int 'flags');*


== DESCRIPTION
The _zmq_send_const()_ function shall queue a message created from the buffer
referenced by the 'buf' and 'len' arguments. The message buffer is assumed
to be constant-memory and will therefore not be copied or deallocated
in any way. The 'flags' argument is a combination of the flags defined below:

*ZMQ_DONTWAIT*::
For socket types (DEALER, PUSH) that block (either with ZMQ_IMMEDIATE option set
and no peer available, or all peers having full high-water mark), specifies that
the operation should be performed in non-blocking mode. If the message cannot be
queued on the 'socket', the _zmq_send_const()_ function shall fail with 'errno' set
to EAGAIN.

*ZMQ_SNDMORE*::
Specifies that the message being sent is a multi-part message, and that further
message parts are to follow. Refer to the section regarding multi-part messages
below for a detailed description.

NOTE: A successful invocation of _zmq_send_const()_ does not indicate that the
message has been transmitted to the network, only that it has been queued on
the 'socket' and 0MQ has assumed responsibility for the message.


Multi-part messages
~~~~~~~~~~~~~~~~~~~
A 0MQ message is composed of 1 or more message parts. 0MQ ensures atomic
delivery of messages: peers shall receive either all _message parts_ of a
message or none at all. The total number of message parts is unlimited except
by available memory.

An application that sends multi-part messages must use the _ZMQ_SNDMORE_ flag
when sending each message part except the final one.


== RETURN VALUE
The _zmq_send_const()_ function shall return number of bytes in the message
if successful. Otherwise it shall return `-1` and set 'errno' to one of the
values defined below.


== ERRORS
*EAGAIN*::
Non-blocking mode was requested and the message cannot be sent at the moment.
*ENOTSUP*::
The _zmq_send_const()_ operation is not supported by this socket type.
*EFSM*::
The _zmq_send_const()_ operation cannot be performed on this socket at the moment
due to the socket not being in the appropriate state.  This error may occur with
socket types that switch between several states, such as ZMQ_REP.  See the
_messaging patterns_ section of xref:zmq_socket.adoc[zmq_socket] for more information.
*ETERM*::
The 0MQ 'context' associated with the specified 'socket' was terminated.
*ENOTSOCK*::
The provided 'socket' was invalid.
*EINTR*::
The operation was interrupted by delivery of a signal before the message was
sent.
*EHOSTUNREACH*::
The message cannot be routed.


== EXAMPLE
.Sending a multi-part message
----
/* Send a multi-part message consisting of three parts to socket */
rc = zmq_send_const (socket, "ABC", 3, ZMQ_SNDMORE);
assert (rc == 3);
rc = zmq_send_const (socket, "DEFGH", 5, ZMQ_SNDMORE);
assert (rc == 5);
/* Final part; no more parts to follow */
rc = zmq_send_const (socket, "JK", 2, 0);
assert (rc == 2);
----

== SEE ALSO
* xref:zmq_send.adoc[zmq_send]
* xref:zmq_recv.adoc[zmq_recv]
* xref:zmq_socket.adoc[zmq_socket]
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
