Provided by: libzmq-dev_2.2.0+dfsg-7_amd64 
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_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_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.
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.
ENOTSOCK
The provided socket was invalid.
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 manual page was written by the 0MQ community.