# 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, ZMQ_LINGER, ZMQ_ROUTER_MANDATORY, ZMQ_PROBE_ROUTER, ZMQ_XPUB_VERBOSE, ZMQ_REQ_CORRELATE, and ZMQ_REQ_RELAXED, only take effect for subsequent socket bind/connects.

Specifically, security options take effect for subsequent bind/connect calls, and can be changed at any time to affect subsequent binds and/or 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_SNDHWM: Set high water mark for outbound messages

The ZMQ_SNDHWM option shall set the high water mark for outbound messages on 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. A value of zero means no limit.

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.

Note | ØMQ does not guarantee that the socket will accept as many as ZMQ_SNDHWM messages, and the actual limit may be as much as 60-70% lower depending on the flow of messages on the socket.

Option value type | int
Option value unit | messages
Default value | 1000
Applicable socket types | all

### ZMQ_RCVHWM: Set high water mark for inbound messages

The ZMQ_RCVHWM option shall set the high water mark for inbound messages on 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. A value of zero means no limit.

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.

Option value type | int
Option value unit | messages
Default value | 1000
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_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_IDENTITY: Set socket identity

The ZMQ_IDENTITY option shall set the identity of the specified socket. Socket identity is used only by request/reply pattern. Namely, it can be used in tandem with ROUTER socket to route messages to the peer with specific identity.

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.

If two peers use the same identity when connecting to a third peer, the results shall be undefined.

Option value type | binary data
Option value unit | N/A
Default value | NULL
Applicable socket types | ZMQ_REQ, ZMQ_REP, ZMQ_ROUTER, ZMQ_DEALER.

### 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 | int
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 milliseconds 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 | int
Option value unit | milliseconds
Default value | 10000
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 | int
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 | int
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. The value -1 means no reconnection.

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.

### ZMQ_MAXMSGSIZE: Maximum acceptable inbound message size

Limits the size of the inbound message. If a peer sends a message larger than ZMQ_MAXMSGSIZE it is disconnected. Value of -1 means no limit.

Option value type | int64_t
Option value unit | bytes
Default value | -1
Applicable socket types | all

### ZMQ_MULTICAST_HOPS: Maximum network hops for multicast packets

Sets the time-to-live field in every multicast packet sent from this socket. The default is 1 which means that the multicast packets don’t leave the local network.

Option value type | int
Option value unit | network hops
Default value | 1
Applicable socket types | all, when using multicast transports

### ZMQ_RCVTIMEO: Maximum time before a recv operation returns with EAGAIN

Sets the timeout for receive operation on the socket. If the value is `0`, zmq_recv(3) will return immediately, with a EAGAIN error if there is no message to receive. If the value is `-1`, it will block until a message is available. For all other values, it will wait for a message for that amount of time before returning with an EAGAIN error.

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

### ZMQ_SNDTIMEO: Maximum time before a send operation returns with EAGAIN

Sets the timeout for send operation on the socket. If the value is `0`, zmq_send(3) will return immediately, with a EAGAIN error if the message cannot be sent. If the value is `-1`, it will block until the message is sent. For all other values, it will try to send the message for that amount of time before returning with an EAGAIN error.

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

### ZMQ_IPV6: Enable IPv6 on socket

Set the IPv6 option for the socket. A value of `1` means IPv6 is enabled on the socket, while `0` means the socket will use only IPv4. When IPv6 is enabled the socket will connect to, or accept connections from, both IPv4 and IPv6 hosts.

Option value type | int
Option value unit | boolean
Default value | 0 (false)
Applicable socket types | all, when using TCP transports.

### ZMQ_IPV4ONLY: Use IPv4-only on socket

Set the IPv4-only option for the socket. This option is deprecated. Please use the ZMQ_IPV6 option.

Option value type | int
Option value unit | boolean
Default value | 1 (true)
Applicable socket types | all, when using TCP transports.

### ZMQ_IMMEDIATE: Queue messages only to completed connections

By default queues will fill on outgoing connections even if the connection has not completed. This can lead to "lost" messages on sockets with round-robin routing (REQ, PUSH, DEALER). If this option is set to `1`, messages shall be queued only to completed connections. This will cause the socket to block if there are no other connections, but will prevent queues from filling on pipes awaiting connection.

Option value type | int
Option value unit | boolean
Default value | 0 (false)
Applicable socket types | all, only for connection-oriented transports.

### ZMQ_ROUTER_MANDATORY: accept only routable messages on ROUTER sockets

Sets the ROUTER socket behavior when an unroutable message is encountered. A value of `0` is the default and discards the message silently when it cannot be routed. A value of `1` returns an EHOSTUNREACH error code if the message cannot be routed.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_ROUTER

### ZMQ_ROUTER_RAW: switch ROUTER socket to raw mode

Sets the raw mode on the ROUTER, when set to 1. When the ROUTER socket is in raw mode, and when using the tcp:// transport, it will read and write TCP data without ØMQ framing. This lets ØMQ applications talk to non-ØMQ applications. When using raw mode, you cannot set explicit identities, and the ZMQ_MSGMORE flag is ignored when sending data messages. In raw mode you can close a specific connection by sending it a zero-length message (following the identity frame).

Note | This option is deprecated, please use ZMQ_STREAM sockets instead.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_ROUTER

### ZMQ_PROBE_ROUTER: bootstrap connections to ROUTER sockets

When set to 1, the socket will automatically send an empty message when a new connection is made or accepted. You may set this on REQ, DEALER, or ROUTER sockets connected to a ROUTER socket. The application must filter such empty messages. The ZMQ_PROBE_ROUTER option in effect provides the ROUTER application with an event signaling the arrival of a new peer.

Note | do not set this option on a socket that talks to any other socket types: the results are undefined.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_ROUTER, ZMQ_DEALER, ZMQ_REQ

### ZMQ_XPUB_VERBOSE: provide all subscription messages on XPUB sockets

Sets the XPUB socket behavior on new subscriptions and unsubscriptions. A value of 0 is the default and passes only new subscription messages to upstream. A value of 1 passes all subscription messages upstream.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_XPUB

### ZMQ_REQ_CORRELATE: match replies with requests

The default behavior of REQ sockets is to rely on the ordering of messages to match requests and responses and that is usually sufficient. When this option is set to 1, the REQ socket will prefix outgoing messages with an extra frame containing a request id. That means the full message is (request id, 0, user frames…). The REQ socket will discard all incoming messages that don’t begin with these two frames.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_REQ

### ZMQ_REQ_RELAXED: relax strict alternation between request and reply

By default, a REQ socket does not allow initiating a new request with zmq_send(3) until the reply to the previous one has been received. When set to 1, sending another message is allowed and has the effect of disconnecting the underlying connection to the peer from which the reply was expected, triggering a reconnection attempt on transports that support it. The request-reply state machine is reset and a new request is sent to the next available peer.

If set to 1, also enable ZMQ_REQ_CORRELATE to ensure correct matching of requests and replies. Otherwise a late reply to an aborted request can be reported as the reply to the superseding request.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | ZMQ_REQ

### ZMQ_TCP_KEEPALIVE: Override SO_KEEPALIVE socket option

Override SO_KEEPALIVE socket option (where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,0,1
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_IDLE: Override TCP_KEEPCNT (or TCP_KEEPALIVE on some OS)

Override TCP_KEEPCNT(or TCP_KEEPALIVE on some OS) socket option (where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_CNT: Override TCP_KEEPCNT socket option

Override TCP_KEEPCNT socket option (where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_INTVL: Override TCP_KEEPINTVL socket option

Override TCP_KEEPINTVL socket option(where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_ACCEPT_FILTER: Assign filters to allow new TCP connections

Assign an arbitrary number of filters that will be applied for each new TCP transport connection on a listening socket. If no filters are applied, then the TCP transport allows connections from any IP address. If at least one filter is applied then new connection source ip should be matched. To clear all filters call zmq_setsockopt(socket, ZMQ_TCP_ACCEPT_FILTER, NULL, 0). Filter is a null-terminated string with ipv6 or ipv4 CIDR.

Option value type | binary data
Option value unit | N/A
Default value | no filters (allow from all)
Applicable socket types | all listening sockets, when using TCP transports.

### ZMQ_PLAIN_SERVER: Set PLAIN server role

Defines whether the socket will act as server for PLAIN security, see zmq_plain(7). A value of 1 means the socket will act as PLAIN server. A value of 0 means the socket will not act as PLAIN server, and its security role then depends on other option settings. Setting this to 0 shall reset the socket security to NULL.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | all, when using TCP transport

### ZMQ_PLAIN_USERNAME: Set PLAIN security username

Sets the username for outgoing connections over TCP or IPC. If you set this to a non-null value, the security mechanism used for connections shall be PLAIN, see zmq_plain(7). If you set this to a null value, the security mechanism used for connections shall be NULL, see zmq_null(3).

Option value type | character string
Option value unit | N/A
Default value | not set
Applicable socket types | all, when using TCP transport

### ZMQ_PLAIN_PASSWORD: Set PLAIN security password

Sets the password for outgoing connections over TCP or IPC. If you set this to a non-null value, the security mechanism used for connections shall be PLAIN, see zmq_plain(7). If you set this to a null value, the security mechanism used for connections shall be NULL, see zmq_null(3).

Option value type | character string
Option value unit | N/A
Default value | not set
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_SERVER: Set CURVE server role

Defines whether the socket will act as server for CURVE security, see zmq_curve(7). A value of 1 means the socket will act as CURVE server. A value of 0 means the socket will not act as CURVE server, and its security role then depends on other option settings. Setting this to 0 shall reset the socket security to NULL. When you set this you must also set the server’s secret key using the ZMQ_CURVE_SECRETKEY option. A server socket does not need to know its own public key.

Option value type | int
Option value unit | 0, 1
Default value | 0
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_PUBLICKEY: Set CURVE public key

Sets the socket’s long term public key. You must set this on CURVE client sockets, see zmq_curve(7). You can provide the key as 32 binary bytes, or as a 40-character string encoded in the Z85 encoding format. The public key must always be used with the matching secret key. To generate a public/secret key pair, use zmq_curve_keypair(3).

Option value type | binary data or Z85 text string
Option value size | 32 or 40
Default value | NULL
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_SECRETKEY: Set CURVE secret key

Sets the socket’s long term secret key. You must set this on both CURVE client and server sockets, see zmq_curve(7). You can provide the key as 32 binary bytes, or as a 40-character string encoded in the Z85 encoding format. To generate a public/secret key pair, use zmq_curve_keypair(3).

Option value type | binary data or Z85 text string
Option value size | 32 or 40
Default value | NULL
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_SERVERKEY: Set CURVE server key

Sets the socket’s long term server key. You must set this on CURVE client sockets, see zmq_curve(7). You can provide the key as 32 binary bytes, or as a 40-character string encoded in the Z85 encoding format. This key must have been generated together with the server’s secret key.

Option value type | binary data or Z85 text string
Option value size | 32 or 40
Default value | NULL
Applicable socket types | all, when using TCP transport

### ZMQ_ZAP_DOMAIN: Set RFC 27 authentication domain

Sets the domain for ZAP (ZMQ RFC 27) authentication. For NULL security (the default on all tcp:// connections), ZAP authentication only happens if you set a non-empty domain. For PLAIN and CURVE security, ZAP requests are always made, if there is a ZAP handler present. See http://rfc.zeromq.org/spec:27 for more details.

Option value type | character string
Option value unit | N/A
Default value | not set
Applicable socket types | all, when using TCP transport

### ZMQ_CONFLATE: Keep only last message

If set, a socket shall keep only one message in its inbound/outbound queue, this message being the last message received/the last message to be sent. Ignores ZMQ_RECVHWM and ZMQ_SENDHWM options. Does not supports multi-part messages, in particular, only one part of it is kept in the socket internal queue.

Option value type | int
Option value unit | boolean
Default value | 0 (false)
Applicable socket types | ZMQ_PULL, ZMQ_PUSH, ZMQ_SUB, ZMQ_PUB, ZMQ_DEALER

## 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_plain(7) zmq_curve(7) zmq(7)

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.