# zmq_getsockopt(3) Manual Page

## 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 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_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 data parts to follow

The ZMQ_RCVMORE option shall return True (1) if the message part last received from the socket was a data part with more parts to follow. If there are no data parts to follow, this option shall return False (0).

Refer to zmq_send(3) and zmq_recv(3) for a detailed description of multi-part messages.

Option value type | int
Option value unit | boolean
Default value | N/A
Applicable socket types | all

### ZMQ_SNDHWM: Retrieves high water mark for outbound messages

The ZMQ_SNDHWM option shall return the high water mark for outbound messages on 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. A value of zero means no limit.

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.

Option value type | int
Option value unit | messages
Default value | 1000
Applicable socket types | all

### ZMQ_RCVHWM: Retrieve high water mark for inbound messages

The ZMQ_RCVHWM option shall return the high water mark for inbound messages on 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. A value of zero means no limit.

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.

Option value type | int
Option value unit | messages
Default value | 1000
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 | 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 is used only by request/reply pattern. Namely, it can be used in tandem with ROUTER socket to route messages to the peer with specific identity.

Identity should 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 | ZMQ_REP, ZMQ_REQ, ZMQ_ROUTER, ZMQ_DEALER.

### 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 | int
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 milliseconds that a receiver can be absent from a multicast group before unrecoverable data loss will occur.

Option value type | int
Option value unit | milliseconds
Default value | 10000
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 | int
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 | int
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 ØMQ shall wait between attempts to reconnect disconnected peers when using connection-oriented transports. The value -1 means no reconnection.

Note | The reconnection interval may be randomized by ØMQ 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 ØMQ 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_MAXMSGSIZE: Maximum acceptable inbound message size

The option shall retrieve limit for the inbound messages. If a peer sends a message larger than ZMQ_MAXMSGSIZE it is disconnected. Value of -1 means no limit.

Option value type | int64_t
Option value unit | bytes
Default value | -1
Applicable socket types | all

### ZMQ_MULTICAST_HOPS: Maximum network hops for multicast packets

The option shall retrieve time-to-live used for outbound multicast packets. The default of 1 means that the multicast packets don’t leave the local network.

Option value type | int
Option value unit | network hops
Default value | 1
Applicable socket types | all, when using multicast transports

### 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_IPV6: Retrieve IPv6 socket status

Retrieve the IPv6 option for the socket. A value of `1` means IPv6 is enabled on the socket, while `0` means the socket will use only IPv4. When IPv6 is enabled the socket will connect to, or accept connections from, both IPv4 and IPv6 hosts.

Option value type | int
Option value unit | boolean
Default value | 0 (false)
Applicable socket types | all, when using TCP transports.

### ZMQ_IPV4ONLY: Retrieve IPv4-only socket override status

Retrieve the IPv4-only option for the socket. This option is deprecated. Please use the ZMQ_IPV6 option.

Option value type | int
Option value unit | boolean
Default value | 1 (true)
Applicable socket types | all, when using TCP transports.

### ZMQ_IMMEDIATE: Retrieve attach-on-connect value

Retrieve the state of the attach on connect value. If set to `1`, will delay the attachment of a pipe on connect until the underlying connection has completed. This will cause the socket to block if there are no other connections, but will prevent queues from filling on pipes awaiting connection.

Option value type | int
Option value unit | boolean
Default value | 0 (false)
Applicable socket types | all, primarily when using TCP/IPC 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 ØMQ 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.

Note | The returned file descriptor is also used internally by the zmq_send and zmq_recv functions. As the descriptor is edge triggered, applications must update the state of ZMQ_EVENTS after each invocation of zmq_send or zmq_recv.To be more explicit: after calling zmq_send the socket may become readable (and vice versa) without triggering a read event on the file descriptor.

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 | int
Option value unit | N/A (flags)
Default value | N/A
Applicable socket types | all

### ZMQ_LAST_ENDPOINT: Retrieve the last endpoint set

The ZMQ_LAST_ENDPOINT option shall retrieve the last endpoint bound for TCP and IPC transports. The returned value will be a string in the form of a ZMQ DSN. Note that if the TCP host is INADDR_ANY, indicated by a *, then the returned address will be 0.0.0.0 (for IPv4).

Option value type | NULL-terminated character string
Option value unit | N/A
Default value | NULL
Applicable socket types | all, when binding TCP or IPC transports

### ZMQ_TCP_KEEPALIVE: Override SO_KEEPALIVE socket option

Override SO_KEEPALIVE socket option(where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,0,1
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_IDLE: Override TCP_KEEPCNT(or TCP_KEEPALIVE on some OS)

Override TCP_KEEPCNT(or TCP_KEEPALIVE on some OS) socket option (where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_CNT: Override TCP_KEEPCNT socket option

Override TCP_KEEPCNT socket option(where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_TCP_KEEPALIVE_INTVL: Override TCP_KEEPINTVL socket option

Override TCP_KEEPINTVL socket option(where supported by OS). The default value of `-1` means to skip any overrides and leave it to OS default.

Option value type | int
Option value unit | -1,>0
Default value | -1 (leave to OS default)
Applicable socket types | all, when using TCP transports.

### ZMQ_MECHANISM: Retrieve current security mechanism

The ZMQ_MECHANISM option shall retrieve the current security mechanism for the socket.

Option value type | int
Option value unit | ZMQ_NULL, ZMQ_PLAIN, or ZMQ_CURVE
Default value | ZMQ_NULL
Applicable socket types | all, when using TCP or IPC transports

### ZMQ_PLAIN_SERVER: Retrieve current PLAIN server role

Returns the ZMQ_PLAIN_SERVER option, if any, previously set on the socket.

Option value type | int
Option value unit | 0, 1
Default value | int
Applicable socket types | all, when using TCP or IPC transports

### ZMQ_PLAIN_USERNAME: Retrieve current PLAIN username

The ZMQ_PLAIN_USERNAME option shall retrieve the last username set for the PLAIN security mechanism. The returned value shall be a NULL-terminated string and MAY be empty. The returned size SHALL include the terminating null byte.

Option value type | NULL-terminated character string
Option value unit | N/A
Default value | null string
Applicable socket types | all, when using TCP or IPC transports

### ZMQ_PLAIN_PASSWORD: Retrieve current password

The ZMQ_PLAIN_PASSWORD option shall retrieve the last password set for the PLAIN security mechanism. The returned value shall be a NULL-terminated string and MAY be empty. The returned size SHALL include the terminating null byte.

Option value type | NULL-terminated character string
Option value unit | N/A
Default value | null string
Applicable socket types | all, when using TCP or IPC transports

### ZMQ_CURVE_PUBLICKEY: Retrieve current CURVE public key

Retrieves the current long term public key for the socket. You can provide either a 32 byte buffer, to retrieve the binary key value, or a 41 byte buffer, to retrieve the key in a printable Z85 format. NOTE: to fetch a printable key, the buffer must be 41 bytes large to hold the 40-char key value and one null byte.

Option value type | binary data or Z85 text string
Option value size | 32 or 41
Default value | null
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_SECRETKEY: Retrieve current CURVE secret key

Retrieves the current long term secret key for the socket. You can provide either a 32 byte buffer, to retrieve the binary key value, or a 41 byte buffer, to retrieve the key in a printable Z85 format.

Option value type | binary data or Z85 text string
Option value size | 32 or 41
Default value | null
Applicable socket types | all, when using TCP transport

### ZMQ_CURVE_SERVERKEY: Retrieve current CURVE server key

Retrieves the current server key for the client socket. You can provide either a 32 byte buffer, to retrieve the binary key value, or a 40 byte buffer, to retrieve the key in a printable Z85 format.

Option value type | binary data or Z85 text string
Option value size | 32 or 41
Default value | null
Applicable socket types | all, when using TCP transport

### ZMQ_ZAP_DOMAIN: Retrieve RFC 27 authentication domain

The ZMQ_ZAP_DOMAIN option shall retrieve the last ZAP domain set for the socket. The returned value shall be a NULL-terminated string and MAY be empty. The returned size SHALL include the terminating null byte.

Option value type | character string
Option value unit | N/A
Default value | not set
Applicable socket types | all, when using TCP transport

## 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.

ENOTSOCK

  The provided socket was invalid.

EINTR

  The operation was interrupted by delivery of a signal.

## EXAMPLE

Retrieving the high water mark for outgoing messages

    /* Retrieve high water mark into sndhwm */
    int sndhwm;
    size_t sndhwm_size = sizeof (sndhwm);
    rc = zmq_getsockopt (socket, ZMQ_SNDHWM, &sndhwm, &sndhwm_size);
    assert (rc == 0);

## SEE ALSO

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

## AUTHORS

This page was written by the ØMQ community. To make a change please read the ØMQ Contribution Policy at http://www.zeromq.org/docs:contributing.