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 ØMQ 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_value 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_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 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

int64_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 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

int64_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 ØMQ infastructure (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 can 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_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

uint64_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

uint64_t

Option value unit

seconds

Default value

10

Applicable socket types

all, when using multicast transports

ZMQ_MCAST_LOOP: Control multicast loopback

The ZMQ_MCAST_LOOP option controls whether data sent via multicast transports can also be received by the sending host via loopback. A value of zero indicates that the loopback functionality is disabled, while the default value of 1 indicates that the loopback functionality is enabled. Leaving multicast loopback 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

uint64_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

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 ØMQ context associated with the specified socket was terminated.

EXAMPLE

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

SEE ALSO

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

AUTHORS

The ØMQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <mato@kotelna.sk>.