ØMQ Manual - ØMQ/3.0.2


zmq_sendmsg - send a message part on a socket


int zmq_sendmsg (void *socket, zmq_msg_t *msg, int flags);


The zmq_send() function shall queue the message referenced by the msg argument to be sent to the socket referenced by the socket argument. The flags argument is a combination of the flags defined below:

Specifies that the operation should be performed in non-blocking mode. If the message cannot be queued on the socket, the zmq_sendmsg() function shall fail with errno set to EAGAIN.
Specifies that the message part being sent is an address label, and that further message parts are to follow. Refer to zmq_socket(3) for the semantics of address labels in each socket pattern.
Specifies that the message being sent is a multi-part message, and that further message data parts are to follow. Message data parts always follow labels, if any.

The zmq_msg_t structure passed to zmq_sendmsg() is nullified during the call. If you want to send the same message to multiple sockets you have to copy it using (e.g. using zmq_msg_copy()).

A successful invocation of zmq_sendmsg() 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, starting with zero or more address label parts, followed by 1 or more data parts. Each message part is an independent zmq_msg_t in its own right. 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 multipart messages must use the ZMQ_SNDMORE flag when sending each data part except the final one. An application that sends address labels must use ZMQ_SNDLABEL when sending each label.

Return value

The zmq_sendmsg() 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.


Non-blocking mode was requested and the message cannot be sent at the moment.
The zmq_sendmsg() operation is not supported by this socket type.
The zmq_sendmsg() 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.
The ØMQ context associated with the specified socket was terminated.
The provided socket was invalid.
The operation was interrupted by delivery of a signal before the message was sent.
Invalid message.
Message cannot be routed to the destination specified as the peer is either dead or disconnected. This error makes sense only with ZMQ_ROUTER socket.


Filling in a message and sending it to a socket

/* Create a new message, allocating 6 bytes for message content */
zmq_msg_t msg;
int rc = zmq_msg_init_size (&msg, 6);
assert (rc == 0);
/* Fill in message content with 'AAAAAA' */
memset (zmq_msg_data (&msg), 'A', 6);
/* Send the message to the socket */
rc = zmq_sendmsg (socket, &msg, 0); assert (rc == 6);

Sending a multi-part message

/* Send a multi-part message consisting of three parts to socket */
rc = zmq_sendmsg (socket, &part1, ZMQ_SNDMORE);
rc = zmq_sendmsg (socket, &part2, ZMQ_SNDMORE);
/* Final part; no more parts to follow */ rc = zmq_sendmsg (socket, &part3, 0);

See also

zmq_recv(3) zmq_recv(3) zmq_recvmsg(3) zmq_socket(7) zmq(7)


This man page was written by Martin Sustrik <moc.mpb052|kirtsus#moc.mpb052|kirtsus>, Martin Lucina <ks.anletok|otam#ks.anletok|otam> and Pieter Hintjens <moc.xitami|hp#moc.xitami|hp>.