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.

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.

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".

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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);

AUTHORS

This manual page was written by the 0MQ community.