# zmq_send_const(3) Manual Page

## 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 ØMQ has assumed responsibility for the message.

### Multi-part messages

A ØMQ message is composed of 1 or more message parts. ØMQ 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 zmq_socket(3) for more information.

ETERM

  The ØMQ 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

zmq_send(3) zmq_recv(3) zmq_socket(7) zmq(7)

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.