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 parts to follow

The ZMQ_RCVMORE option shall return a boolean value indicating if the multi-part message currently being read from the specified socket has more message parts to follow. If there are no message parts to follow or if the message currently being read is not a multi-part message a value of zero shall be returned. Otherwise, a value of 1 shall be returned.

Refer to zmq_send(3) and zmq_recv(3) for a detailed description of sending/receiving multi-part messages.
Option value type | int64_t
Option value unit | boolean
Default value | N/A
Applicable socket types | all

  ZMQ_HWM: Retrieve high water mark

The ZMQ_HWM option shall retrieve the high water mark for 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.

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: Retrieve disk offload size

The ZMQ_SWAP option shall retrieve the disk offload (swap) size for the specified socket. A socket which has ZMQ_SWAP set to a non-zero value may exceed it’s 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: 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: Retrieve socket identity

The ZMQ_IDENTITY option shall retrieve the identity of the specified socket. Socket identity determines if existing 0MQ 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 0MQ 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 can 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 | int64_t
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 seconds that a receiver can be absent from a multicast group before unrecoverable data loss will occur.
Option value type | int64_t
Option value unit | seconds
Default value | 10
Applicable socket types | all, when using multicast transports

  ZMQ_RECOVERY_IVL_MSEC: Get multicast recovery interval in milliseconds

The ZMQ_RECOVERY_IVL’_MSEC option shall retrieve the recovery interval, in milliseconds, 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.

For backward compatibility, the default value of ZMQ_RECOVERY_IVL_MSEC is -1 indicating that the recovery interval should be obtained from the ZMQ_RECOVERY_IVL option. However, if the ZMQ_RECOVERY_IVL_MSEC value is not zero, then it will take precedence, and be used.
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 controls whether data sent via multicast transports can also be received by the sending host via loop-back. A value of zero indicates that the loop-back functionality is disabled, while the default value of 1 indicates that the loop-back functionality is enabled. 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: 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 | uint64_t
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 | uint64_t
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_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 | uint32_t
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.

EFAULT
    The provided socket was not valid (NULL).

EINTR
    The operation was interrupted by delivery of a signal.

EXAMPLE

Retrieving the high water mark.

    /* Retrieve high water mark into hwm */
    int64_t hwm;
    size_t hwm_size = sizeof (hwm);
    rc = zmq_getsockopt (socket, ZMQ_HWM, &hwm, &hwm_size);
    assert (rc == 0);

SEE ALSO

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

AUTHORS

This 0MQ manual page was written by Martin Sustrik <sustrik@250bpm.com[1]> and Martin Lucina <mato@kotelna.sk[2]>.

NOTES

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

2.  mato@kotelna.sk
    mailto:mato@kotelna.sk