libzmq/doc/zmq_getsockopt.txt

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.

[horizontal]
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 linkzmq:zmq_send[3] and linkzmq:zmq_recv[3] for a detailed description
of sending/receiving multi-part messages.

[horizontal]
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 linkzmq:zmq_socket[3] for details on the exact action taken for each socket
type.

The default 'ZMQ_HWM' value of zero means "no limit".

[horizontal]
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.

[horizontal]
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 linkzmq:zmq_init[3] for details on allocating the number of I/O
threads for a specific _context_.

[horizontal]
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 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 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.

[horizontal]
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'.

[horizontal]
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.

[horizontal]
Option value type:: int64_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.

[horizontal]
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.

[horizontal]
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.

[horizontal]
Option value type:: uint64_t
Option value unit:: bytes
Default value:: 0
Applicable socket types:: all


ZMQ_FD: Retrieve file descriptor associated with the socket
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_FD' option shall retrieve file descriptor associated with the 0MQ
socket. The descriptor can be used to integrate 0MQ socket into an existing
event loop. It should never be used for anything else than polling -- such as
reading or writing. The descriptor signals edge-triggered IN event when
something has happened within the 0MQ socket. It does not necessarily mean that
the messages can be read or written. Check ZMQ_EVENTS option to find out whether
the 0MQ socket is readable or writeable.

[horizontal]
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: Check whether socket is readable and/or writeable
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'ZMQ_EVENTS' option shall retrieve event flags for the specified socket.
If a message can be read from the socket ZMQ_POLLIN flag is set. If message can
be written to the socket ZMQ_POLLOUT flag is set.

[horizontal]
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
--------
linkzmq:zmq_setsockopt[3]
linkzmq:zmq_socket[3]
linkzmq:zmq[7]


AUTHORS
-------
The 0MQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and
Martin Lucina <mato@kotelna.sk>.