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ZMQ_SEND(3) 0MQ Manual ZMQ_SEND(3)


zmq_send - send a message part on a socket


int zmq_send (void *socket, const void *buf, size_t len, int flags);


The zmq_send() function shall queue a message created from the buffer referenced by the buf and len arguments. 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_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.


A successful invocation of zmq_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.

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.


The zmq_send() 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_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_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.


The message cannot be routed.


Sending a multi-part message.

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


zmq_send_const(3) zmq_recv(3) zmq_socket(7) zmq(7)


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10/09/2023 0MQ 4.3.5