ZMQ_RECVMSG

Section: 0MQ Manual (3)
Updated: 01/30/2021
Page Index

NAME

zmq_recvmsg - receive a message part from a socket

SYNOPSIS

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

DESCRIPTION

The zmq_recvmsg() function shall receive a message part from the socket referenced by the socket argument and store it in the message referenced by the msg argument. Any content previously stored in msg shall be properly deallocated. If there are no message parts available on the specified socket the zmq_recvmsg() function shall block until the request can be satisfied. The flags argument is a combination of the flags defined below:

ZMQ_DONTWAIT

: Specifies that the operation should be performed in non-blocking mode. If there are no messages available on the specified socket, the zmq_recvmsg() function shall fail with errno set to EAGAIN.

Note

this API method is deprecated in favor of zmq_msg_recv(3).

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 processes multi-part messages must use the ZMQ_RCVMORE zmq_getsockopt(3) option after calling zmq_recvmsg() to determine if there are further parts to receive.

RETURN VALUE

The zmq_recvmsg() 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

: Either the timeout set via the socket-option ZMQ_RCVTIMEO (see zmq_setsockopt(3)) has been reached (flag ZMQ_DONTWAIT not set) without being able to read a message from the socket or there are no messages available at the moment (flag ZMQ_DONTWAIT set) and the operation would block.

ENOTSUP

: The zmq_recvmsg() operation is not supported by this socket type.

EFSM

: The zmq_recvmsg() 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 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 a message was available.

EFAULT

: The message passed to the function was invalid.

EXAMPLE

Receiving a message from a socket.

/* Create an empty 0MQ message */
zmq_msg_t msg;
int rc = zmq_msg_init (&msg);
assert (rc == 0);
/* Block until a message is available to be received from socket */
rc = zmq_recvmsg (socket, &msg, 0);
assert (rc != -1);
/* Release message */
zmq_msg_close (&msg);

Receiving a multi-part message.

int more;
size_t more_size = sizeof (more);
do {
    /* Create an empty 0MQ message to hold the message part */
    zmq_msg_t part;
    int rc = zmq_msg_init (&part);
    assert (rc == 0);
    /* Block until a message is available to be received from socket */
    rc = zmq_recvmsg (socket, &part, 0);
    assert (rc != -1);
    /* Determine if more message parts are to follow */
    rc = zmq_getsockopt (socket, ZMQ_RCVMORE, &more, &more_size);
    assert (rc == 0);
    zmq_msg_close (&part);
} while (more);

AUTHORS

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