# zmq_setsockopt(3) Manual Page

## NAME

zmq_setsockopt - set 0MQ socket options

## SYNOPSIS

int zmq_setsockopt (void *socket, int option_name, const void *option_value, size_t option_len);

Caution: All options, with the exception of ZMQ_SUBSCRIBE, ZMQ_UNSUBSCRIBE and ZMQ_LINGER, only take effect for subsequent socket bind/connects.

## DESCRIPTION

The zmq_setsockopt() function shall set the option specified by the option_name argument to the value pointed to by the option_value argument for the ØMQ socket pointed to by the socket argument. The option_len argument is the size of the option value in bytes.

The following socket options can be set with the zmq_setsockopt() function:

### ZMQ_HWM: Set high water mark

The ZMQ_HWM option shall set the high water mark for the specified socket. The high water mark is a hard limit on the maximum number of outstanding messages ØMQ shall queue in memory for any single peer that the specified socket is communicating with.

If this limit has been reached the socket shall enter an exceptional state and depending on the socket type, ØMQ shall take appropriate action such as blocking or dropping sent messages. Refer to the individual socket descriptions in zmq_socket(3) for details on the exact action taken for each socket type.

The default ZMQ_HWM value of zero means "no limit".

Option value type | uint64_t
Option value unit | messages
Default value | 0
Applicable socket types | all

### ZMQ_SWAP: Set disk offload size

The ZMQ_SWAP option shall set the disk offload (swap) size for the specified socket. A socket which has ZMQ_SWAP set to a non-zero value may exceed its high water mark; in this case outstanding messages shall be offloaded to storage on disk rather than held in memory.

The value of ZMQ_SWAP defines the maximum size of the swap space in bytes.

Option value type | int64_t
Option value unit | bytes
Default value | 0
Applicable socket types | all

### ZMQ_AFFINITY: Set I/O thread affinity

The ZMQ_AFFINITY option shall set the I/O thread affinity for newly created connections on the specified socket.

Affinity determines which threads from the ØMQ I/O thread pool associated with the socket’s context shall handle newly created connections. A value of zero specifies no affinity, meaning that work shall be distributed fairly among all ØMQ I/O threads in the thread pool. For non-zero values, the lowest bit corresponds to thread 1, second lowest bit to thread 2 and so on. For example, a value of 3 specifies that subsequent connections on socket shall be handled exclusively by I/O threads 1 and 2.

See also zmq_init(3) for details on allocating the number of I/O threads for a specific context.

Option value type | uint64_t
Option value unit | N/A (bitmap)
Default value | 0
Applicable socket types | N/A

### ZMQ_IDENTITY: Set socket identity

The ZMQ_IDENTITY option shall set the identity of the specified socket. Socket identity determines if existing ØMQ infrastructure (message queues, forwarding devices) shall be identified with a specific application and persist across multiple runs of the application.

If the socket has no identity, each run of an application is completely separate from other runs. However, with identity set the socket shall re-use any existing ØMQ infrastructure configured by the previous run(s). Thus the application may receive messages that were sent in the meantime, message queue limits shall be shared with previous run(s) and so on.

Identity should be at least one byte and at most 255 bytes long. Identities starting with binary zero are reserved for use by ØMQ infrastructure.

Option value type | binary data
Option value unit | N/A
Default value | NULL
Applicable socket types | all

### ZMQ_SUBSCRIBE: Establish message filter

The ZMQ_SUBSCRIBE option shall establish a new message filter on a ZMQ_SUB socket. Newly created ZMQ_SUB sockets shall filter out all incoming messages, therefore you should call this option to establish an initial message filter.

An empty option_value of length zero shall subscribe to all incoming messages. A non-empty option_value shall subscribe to all messages beginning with the specified prefix. Multiple filters may be attached to a single ZMQ_SUB socket, in which case a message shall be accepted if it matches at least one filter.

Option value type | binary data
Option value unit | N/A
Default value | N/A
Applicable socket types | ZMQ_SUB

### ZMQ_UNSUBSCRIBE: Remove message filter

The ZMQ_UNSUBSCRIBE option shall remove an existing message filter on a ZMQ_SUB socket. The filter specified must match an existing filter previously established with the ZMQ_SUBSCRIBE option. If the socket has several instances of the same filter attached the ZMQ_UNSUBSCRIBE option shall remove only one instance, leaving the rest in place and functional.

Option value type | binary data
Option value unit | N/A
Default value | N/A
Applicable socket types | ZMQ_SUB

### ZMQ_RATE: Set multicast data rate

The ZMQ_RATE option shall set the maximum send or receive data rate for multicast transports such as zmq_pgm(7) using the specified socket.

Option value type | int64_t
Option value unit | kilobits per second
Default value | 100
Applicable socket types | all, when using multicast transports

### ZMQ_RECOVERY_IVL: Set multicast recovery interval

The ZMQ_RECOVERY_IVL option shall set the recovery interval for multicast transports using the specified socket. The recovery interval determines the maximum time in seconds that a receiver can be absent from a multicast group before unrecoverable data loss will occur.

Caution | Exercise care when setting large recovery intervals as the data needed for recovery will be held in memory. For example, a 1 minute recovery interval at a data rate of 1Gbps requires a 7GB in-memory buffer.

Option value type | int64_t
Option value unit | seconds
Default value | 10
Applicable socket types | all, when using multicast transports

### ZMQ_RECOVERY_IVL_MSEC: Set multicast recovery interval in milliseconds

The ZMQ_RECOVERY_IVL_MSEC option shall set the recovery interval, specified in milliseconds (ms) for multicast transports using the specified socket. The recovery interval determines the maximum time in milliseconds that a receiver can be absent from a multicast group before unrecoverable data loss will occur.

A non-zero value of the ZMQ_RECOVERY_IVL_MSEC option will take precedence over the ZMQ_RECOVERY_IVL option, but since the default for the ZMQ_RECOVERY_IVL_MSEC is -1, the default is to use the ZMQ_RECOVERY_IVL option value.

Caution | Exercise care when setting large recovery intervals as the data needed for recovery will be held in memory. For example, a 1 minute recovery interval at a data rate of 1Gbps requires a 7GB in-memory buffer.

Option value type | int64_t
Option value unit | milliseconds
Default value | -1
Applicable socket types | all, when using multicast transports

### ZMQ_MCAST_LOOP: Control multicast loop-back

The ZMQ_MCAST_LOOP option shall control whether data sent via multicast transports using the specified socket can also be received by the sending host via loop-back. A value of zero disables the loop-back functionality, while the default value of 1 enables the loop-back functionality. Leaving multicast loop-back enabled when it is not required can have a negative impact on performance. Where possible, disable ZMQ_MCAST_LOOP in production environments.

Option value type | int64_t
Option value unit | boolean
Default value | 1
Applicable socket types | all, when using multicast transports

### ZMQ_SNDBUF: Set kernel transmit buffer size

The ZMQ_SNDBUF option shall set the underlying kernel transmit buffer size for the socket to the specified size in bytes. A value of zero means leave the OS default unchanged. For details please refer to your operating system documentation for the SO_SNDBUF socket option.

Option value type | uint64_t
Option value unit | bytes
Default value | 0
Applicable socket types | all

### ZMQ_RCVBUF: Set kernel receive buffer size

The ZMQ_RCVBUF option shall set the underlying kernel receive buffer size for the socket to the specified size in bytes. A value of zero means leave the OS default unchanged. For details refer to your operating system documentation for the SO_RCVBUF socket option.

Option value type | uint64_t
Option value unit | bytes
Default value | 0
Applicable socket types | all

### ZMQ_LINGER: Set linger period for socket shutdown

The ZMQ_LINGER option shall set the linger period for the specified socket. The linger period determines how long pending messages which have yet to be sent to a peer shall linger in memory after a socket is closed with zmq_close(3), and further affects the termination of the socket’s context with zmq_term(3). The following outlines the different behaviours:

- The default value of -1 specifies an infinite linger period. Pending messages shall not be discarded after a call to zmq_close(); attempting to terminate the socket’s context with zmq_term() shall block until all pending messages have been sent to a peer.

- The value of 0 specifies no linger period. Pending messages shall be discarded immediately when the socket is closed with zmq_close().

- Positive values specify an upper bound for the linger period in milliseconds. Pending messages shall not be discarded after a call to zmq_close(); attempting to terminate the socket’s context with zmq_term() shall block until either all pending messages have been sent to a peer, or the linger period expires, after which any pending messages shall be discarded.

  Option value type | int
  Option value unit | milliseconds
  Default value | -1 (infinite)
  Applicable socket types | all

### ZMQ_RECONNECT_IVL: Set reconnection interval

The ZMQ_RECONNECT_IVL option shall set the initial reconnection interval for the specified socket. The reconnection interval is the period ØMQ shall wait between attempts to reconnect disconnected peers when using connection-oriented transports.

Note | The reconnection interval may be randomized by ØMQ to prevent reconnection storms in topologies with a large number of peers per socket.

Option value type | int
Option value unit | milliseconds
Default value | 100
Applicable socket types | all, only for connection-oriented transports

### ZMQ_RECONNECT_IVL_MAX: Set maximum reconnection interval

The ZMQ_RECONNECT_IVL_MAX option shall set the maximum reconnection interval for the specified socket. This is the maximum period ØMQ shall wait between attempts to reconnect. On each reconnect attempt, the previous interval shall be doubled untill ZMQ_RECONNECT_IVL_MAX is reached. This allows for exponential backoff strategy. Default value means no exponential backoff is performed and reconnect interval calculations are only based on ZMQ_RECONNECT_IVL.

Note | Values less than ZMQ_RECONNECT_IVL will be ignored.

Option value type | int
Option value unit | milliseconds
Default value | 0 (only use ZMQ_RECONNECT_IVL)
Applicable socket types | all, only for connection-oriented transports

### ZMQ_BACKLOG: Set maximum length of the queue of outstanding connections

The ZMQ_BACKLOG option shall set the maximum length of the queue of outstanding peer connections for the specified socket; this only applies to connection-oriented transports. For details refer to your operating system documentation for the listen function.

Option value type | int
Option value unit | connections
Default value | 100
Applicable socket types | all, only for connection-oriented transports.

## RETURN VALUE

The zmq_setsockopt() function shall return zero if successful. Otherwise it shall return `-1` and set errno to one of the values defined below.

## ERRORS

EINVAL

  The requested option option_name is unknown, or the requested option_len or option_value is invalid.

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.

## EXAMPLE

Subscribing to messages on a ZMQ_SUB socket

    /* Subscribe to all messages */
    rc = zmq_setsockopt (socket, ZMQ_SUBSCRIBE, "", 0);
    assert (rc == 0);
    /* Subscribe to messages prefixed with "ANIMALS.CATS" */
    rc = zmq_setsockopt (socket, ZMQ_SUBSCRIBE, "ANIMALS.CATS", 12);

Setting I/O thread affinity

    int64_t affinity;
    /* Incoming connections on TCP port 5555 shall be handled by I/O thread 1 */
    affinity = 1;
    rc = zmq_setsockopt (socket, ZMQ_AFFINITY, &affinity, sizeof affinity);
    assert (rc);
    rc = zmq_bind (socket, "tcp://lo:5555");
    assert (rc);
    /* Incoming connections on TCP port 5556 shall be handled by I/O thread 2 */
    affinity = 2;
    rc = zmq_setsockopt (socket, ZMQ_AFFINITY, &affinity, sizeof affinity);
    assert (rc);
    rc = zmq_bind (socket, "tcp://lo:5556");
    assert (rc);

## SEE ALSO

zmq_getsockopt(3) zmq_socket(3) zmq(7)

## AUTHORS

This ØMQ manual page was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <mato@kotelna.sk>.