ZMQ_GETSOCKOPT(3)

NAME
zmq_getsockopt - get 0MQ socket options

SYNOPSIS

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

DESCRIPTION

The zmq_getsockopt() function shall retrieve the value for the option specified by the option_name argument for the 0MQ socket pointed to by the socket argument, and store it in the buffer pointed to by the option_value argument. The option_len argument is the size in bytes of the buffer pointed to by option_value; upon successful completion zmq_getsockopt() shall modify the option_len argument to indicate the actual size of the option value stored in the buffer.

The following options can be retrieved with the zmq_getsockopt() function:

  ZMQ_TYPE: Retrieve socket type

The ZMQ_TYPE option shall retrieve the socket type for the specified socket. The socket type is specified at socket creation time and cannot be modified afterwards.
Option value type | int
Option value unit | N/A
Default value | N/A
Applicable socket types | all

  ZMQ_RCVMORE: More message data parts to follow

The ZMQ_RCVMORE option shall return True (1) if the message part last received from the socket was a data part with more parts to follow. If there are no data parts to follow, this option shall return False (0).

Refer to zmq_send(3) and zmq_recv(3) for a detailed description of multi-part messages.
Option value type | int
Option value unit | boolean
Default value | N/A
Applicable socket types | all

  ZMQ_SNDHWM: Retrieves high water mark for outbound messages

The ZMQ_SNDHWM option shall return 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 0MQ 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, 0MQ 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_RCVHWM: Retrieve high water mark for inbound messages

The ZMQ_RCVHWM option shall return 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 0MQ 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, 0MQ 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: Retrieve I/O thread affinity

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

Affinity determines which threads from the 0MQ 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 0MQ 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 retrieve 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 0MQ infrastructure.
Option value type | binary data
Option value unit | N/A
Default value | NULL
Applicable socket types | all

  ZMQ_RATE: Retrieve multicast data rate

The ZMQ_RATE option shall retrieve the maximum send or receive data rate for multicast transports 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: Get multicast recovery interval

The ZMQ_RECOVERY_IVL option shall retrieve 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.
Option value type | int
Option value unit | milliseconds
Default value | 10000
Applicable socket types | all, when using multicast transports

  ZMQ_SNDBUF: Retrieve kernel transmit buffer size

The ZMQ_SNDBUF option shall retrieve the underlying kernel transmit buffer size for the specified socket. A value of zero means that the OS default is in effect. For details 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: Retrieve kernel receive buffer size

The ZMQ_RCVBUF option shall retrieve the underlying kernel receive buffer size for the specified socket. A value of zero means that the OS default is in effect. 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: Retrieve linger period for socket shutdown

The ZMQ_LINGER option shall retrieve 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: Retrieve reconnection interval

The ZMQ_RECONNECT_IVL option shall retrieve the initial reconnection interval for the specified socket. The reconnection interval is the period 0MQ shall wait between attempts to reconnect disconnected peers when using connection-oriented transports.

    Note

    The reconnection interval may be randomized by 0MQ 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: Retrieve maximum reconnection interval

The ZMQ_RECONNECT_IVL_MAX option shall retrieve the maximum reconnection interval for the specified socket. This is the maximum period 0MQ 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 transport

  ZMQ_BACKLOG: Retrieve maximum length of the queue of outstanding connections

The ZMQ_BACKLOG option shall retrieve 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

The option shall retrieve limit for the inbound messages. 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

The option shall retrieve time-to-live used for outbound multicast packets. The default of 1 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 socket operation returns with EAGAIN

Retrieve the timeout for recv 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 socket operation returns with EAGAIN

Retrieve 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_IPV4ONLY: Retrieve IPv4-only socket override status

Retrieve the underlying native socket type. A value of 1 will use IPv4 sockets, while the value of 0 will use IPv6 sockets. An IPv6 socket lets applications connect to and accept connections from both IPv4 and IPv6 hosts.
Option value type | int
Option value unit | boolean
Default value | 1 (true)
Applicable socket types | all, when using TCP transports.

  ZMQ_FD: Retrieve file descriptor associated with the socket

The ZMQ_FD option shall retrieve the file descriptor associated with the specified socket. The returned file descriptor can be used to integrate the socket into an existing event loop; the 0MQ library shall signal any pending events on the socket in an edge-triggered fashion by making the file descriptor become ready for reading.

    Note

    The ability to read from the returned file descriptor does not necessarily indicate that messages are available to be read from, or can be written to, the underlying socket; applications must retrieve the actual event state with a subsequent retrieval of the ZMQ_EVENTS option.

    Caution

    The returned file descriptor is intended for use with a poll or similar system call only. Applications must never attempt to read or write data to it directly, neither should they try to close it.

Option value type | int on POSIX systems, SOCKET on Windows
Option value unit | N/A
Default value | N/A
Applicable socket types | all

  ZMQ_EVENTS: Retrieve socket event state

The ZMQ_EVENTS option shall retrieve the event state for the specified socket. The returned value is a bit mask constructed by OR’ing a combination of the following event flags:

ZMQ_POLLIN
    Indicates that at least one message may be received from the specified socket without blocking.

ZMQ_POLLOUT
    Indicates that at least one message may be sent to the specified socket without blocking.

The combination of a file descriptor returned by the ZMQ_FD option being ready for reading but no actual events returned by a subsequent retrieval of the ZMQ_EVENTS option is valid; applications should simply ignore this case and restart their polling operation/event loop.
Option value type | int
Option value unit | N/A (flags)
Default value | N/A
Applicable socket types | all

RETURN VALUE

The zmq_getsockopt() 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, or the size of the buffer pointed to by option_value, as specified by option_len, is insufficient for storing the option value.

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.

EXAMPLE

Retrieving the high water mark for outgoing messages.

    /* Retrieve high water mark into sndhwm */
    int sndhwm;
    size_t sndhwm_size = sizeof (sndhwm);
    rc = zmq_getsockopt (socket, ZMQ_SNDHWM, &sndhwm, &sndhwm_size);
    assert (rc == 0);

SEE ALSO

zmq_setsockopt(3) zmq_socket(3) zmq(7)

AUTHORS

The 0MQ documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and Martin Lucina <martin@lucina.net[2]>.

NOTES

1.  sustrik@250bpm.com
    mailto:sustrik@250bpm.com

2.  martin@lucina.net
    mailto:martin@lucina.net