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zmq_msg_send - send a message part on a socket


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


The zmq_msg_send() function is identical to zmq_sendmsg(3), which shall be deprecated in future versions. zmq_msg_send() is more consistent with other message manipulation functions.

The zmq_msg_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:


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_msg_send() function shall fail with errno set to EAGAIN.


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.

The zmq_msg_t structure passed to zmq_msg_send() is nullified on a successful call. If you want to send the same message to multiple sockets you have to copy it (e.g. using zmq_msg_copy()). If the call fails, the zmq_msg_t structure stays intact, and must be consumed by another call to zmq_msg_send() on the same or another socket, or released using zmq_msg_close() to avoid a memory leak.


A successful invocation of zmq_msg_send() 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. You do not need to call zmq_msg_close() after a successful zmq_msg_send().

Multi-part messages

A 0MQ message is composed of 1 or more message parts. Each message part is an independent zmq_msg_t in its own right. 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.


The zmq_msg_send() function shall return number of bytes in the message if successful (if number of bytes is higher than MAX_INT, the function will return MAX_INT). 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_msg_send() operation is not supported by this socket type.


The sender tried to send multipart data, which the socket type does not allow.


The zmq_msg_send() 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 0MQ 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.


The message cannot be routed.


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_msg_send (&msg, socket, 0);
assert (rc == 6);

Sending a multi-part message.

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


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


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10/09/2023 0MQ 4.3.5