# Get and Set Options for a Socket, Context, Stream, Listener or Dialer Get and set the value of options for a Socket, Context, Stream, Listener or Dialer. ```r opt(object, name) opt(object, name) <- value ``` ## Arguments - `object`: a Socket, Context, Stream, Listener or Dialer. - `name`: name of option, e.g. 'recv-buffer', as a character string. See below options details. - `value`: value of option. Supply character type for 'string' options, integer or double for 'int', 'duration', 'size' and 'uint64', and logical for 'bool'. ## Returns The value of the option (logical for type 'bool', integer for 'int', 'duration' and 'size', character for 'string', and double for 'uint64'). ## Details Note: once a dialer or listener has started, it is not generally possible to change its configuration. Hence create the dialer or listener with `autostart = FALSE` if configuration needs to be set. To get or set options on a Listener or Dialer attached to a Socket or nano object, pass in the objects directly via for example `$listener[[1]]` for the first Listener. Some options are only meaningful or supported in certain contexts; for example there is no single meaningful address for a socket, since sockets can have multiple dialers and endpoints associated with them. For an authoritative guide please refer to the online documentation for the NNG library at [https://nng.nanomsg.org/man/](https://nng.nanomsg.org/man/). ## Serialization Apart from the NNG options documented below, there is the following special option: * 'serial' (type list) For Sockets only. This accepts a configuration created by `serial_config()`. Setting a new configuration replaces any already set. To remove entirely, supply an empty list. Note: this option is write-only and can be set but not retrieved. ## Global Options * 'reconnect-time-min' (type 'ms') This is the minimum amount of time (milliseconds) to wait before attempting to establish a connection after a previous attempt has failed. This can be set on a socket, but it can also be overridden on an individual dialer. The option is irrelevant for listeners. * 'reconnect-time-max' (type 'ms') This is the maximum amount of time (milliseconds) to wait before attempting to establish a connection after a previous attempt has failed. If this is non-zero, then the time between successive connection attempts will start at the value of 'reconnect-time-min', and grow exponentially, until it reaches this value. If this value is zero, then no exponential back-off between connection attempts is done, and each attempt will wait the time specified by 'reconnect-time-min'. This can be set on a socket, but it can also be overridden on an individual dialer. The option is irrelevant for listeners. * 'recv-size-max' (type 'size') This is the maximum message size that the will be accepted from a remote peer. If a peer attempts to send a message larger than this, then the message will be discarded. If the value of this is zero, then no limit on message sizes is enforced. This option exists to prevent certain kinds of denial-of-service attacks, where a malicious agent can claim to want to send an extraordinarily large message, without sending any data. This option can be set for the socket, but may be overridden for on a per-dialer or per-listener basis. NOTE: Applications on hostile networks should set this to a non-zero value to prevent denial-of-service attacks. NOTE: Some transports may have further message size restrictions. * 'recv-buffer' (type 'int') This is the depth of the socket’s receive buffer as a number of messages. Messages received by a transport may be buffered until the application has accepted them for delivery. This value must be an integer between 0 and 8192, inclusive. NOTE: Not all protocols support buffering received messages. For example req can only deal with a single reply at a time. * 'recv-timeout' (type 'ms') This is the socket receive timeout in milliseconds. When no message is available for receiving at the socket for this period of time, receive operations will fail with a return value of 5L ('timed out'). * 'send-buffer' (type 'int') This is the depth of the socket send buffer as a number of messages. Messages sent by an application may be buffered by the socket until a transport is ready to accept them for delivery. This value must be an integer between 0 and 8192, inclusive. NOTE: Not all protocols support buffering sent messages; generally multicast protocols like pub will simply discard messages when they cannot be delivered immediately. * 'send-timeout' (type 'ms') This is the socket send timeout in milliseconds. When a message cannot be queued for delivery by the socket for this period of time (such as if send buffers are full), the operation will fail with a return value of 5L ('timed out'). * 'recv-fd' (type 'int') This is the socket receive file descriptor. For supported protocols, this will become readable when a message is available for receiving on the socket. Attempts should not be made to read or write to the returned file descriptor, but it is suitable for use with poll(), select(), or WSAPoll() on Windows, and similar functions. * 'send-fd' (type 'int') This is the socket send file descriptor. Attempts should not be made to read or write to the returned file descriptor, but it is suitable for use with poll(), select(), or WSAPoll() on Windows, and similar functions. * 'socket-name' (type 'string') This is the socket name. By default this is a string corresponding to the value of the socket. The string must fit within 64-bytes, including the terminating NUL byte. The value is intended for application use, and is not used for anything in the library itself. * 'url' (type 'string') This read-only option is used on a listener or dialer to obtain the URL with which it was configured. ## Protocol-specific Options * 'req:resend-time' (type 'ms') (Request protocol) When a new request is started, a timer of this duration is also started. If no reply is received before this timer expires, then the request will be resent. (Requests are also automatically resent if the peer to whom the original request was sent disconnects, or if a peer becomes available while the requester is waiting for an available peer.) * 'sub:subscribe' (type 'string') (Subscribe protocol) This option registers a topic that the subscriber is interested in. Each incoming message is checked against the list of subscribed topics. If the body begins with the entire set of bytes in the topic, then the message is accepted. If no topic matches, then the message is discarded. To receive all messages, set the topic to NULL. * 'sub:unsubscribe' (type 'string') (Subscribe protocol) This option removes a topic from the subscription list. Note that if the topic was not previously subscribed to with 'sub:subscribe' then an 'entry not found' error will result. * 'sub:prefnew' (type 'bool') (Subscribe protocol) This option specifies the behavior of the subscriber when the queue is full. When TRUE (the default), the subscriber will make room in the queue by removing the oldest message. When FALSE, the subscriber will reject messages if the message queue does not have room. * 'surveyor:survey-time' (type 'ms') (Surveyor protocol) Duration of surveys. When a new survey is started, a timer of this duration is also started. Any responses arriving after this time will be discarded. Attempts to receive after the timer expires with no other surveys started will result in an 'incorrect state' error. Attempts to receive when this timer expires will result in a 'timed out' error. ## Transport-specific Options * 'ipc:permissions' (type 'int') (IPC transport) This option may be applied to a listener to configure the permissions that are used on the UNIX domain socket created by that listener. This property is only supported on POSIX systems. The value is of type int, representing the normal permission bits on a file, such as 0600 (typically meaning read-write to the owner, and no permissions for anyone else.) The default is system-specific, most often 0644. * 'tcp-nodelay' (type 'bool') (TCP transport) This option is used to disable (or enable) the use of Nagle's algorithm for TCP connections. When TRUE (the default), messages are sent immediately by the underlying TCP stream without waiting to gather more data. When FALSE, Nagle’s algorithm is enabled, and the TCP stream may wait briefly in an attempt to coalesce messages. Nagle’s algorithm is useful on low-bandwidth connections to reduce overhead, but it comes at a cost to latency. When used on a dialer or a listener, the value affects how newly created connections will be configured. * 'tcp-keepalive' (type 'bool') (TCP transport) This option is used to enable the sending of keep-alive messages on the underlying TCP stream. This option is FALSE by default. When enabled, if no messages are seen for a period of time, then a zero length TCP message is sent with the ACK flag set in an attempt to tickle some traffic from the peer. If none is still seen (after some platform-specific number of retries and timeouts), then the remote peer is presumed dead, and the connection is closed. When used on a dialer or a listener, the value affects how newly created connections will be configured. This option has two purposes. First, it can be used to detect dead peers on an otherwise quiescent network. Second, it can be used to keep connection table entries in NAT and other middleware from expiring due to lack of activity. * 'tcp-bound-port' (type 'int') (TCP transport) Local TCP port number. This is used on a listener, and is intended to be used after starting the listener in combination with a wildcard (0) local port. This determines the actual ephemeral port that was selected and bound. The value is provided as an integer, but only the low order 16 bits will be set, and is in native byte order for convenience. * 'ws:request-headers' (type 'string') (WebSocket transport) Concatenation of multiple lines terminated by CRLF sequences, that can be used to add further headers to the HTTP request sent when connecting. This option can be set on dialers, and must be done before the transport is started. * 'ws:response-headers' (type 'string') (WebSocket transport) Concatenation of multiple lines terminated by CRLF sequences, that can be used to add further headers to the HTTP response sent when connecting. This option can be set on listeners, and must be done before the transport is started. * 'ws:request-uri' (type 'string') (WebSocket transport) For obtaining the URI sent by the client. This can be useful when a handler supports an entire directory tree. ## Examples ```r s <- socket("pair") opt(s, "send-buffer") close(s) s <- socket("req") ctx <- context(s) opt(ctx, "send-timeout") close(ctx) close(s) s <- socket("pair", dial = "inproc://nanonext", autostart = FALSE) opt(s$dialer[[1]], "reconnect-time-min") close(s) s <- socket("pair", listen = "inproc://nanonext", autostart = FALSE) opt(s$listener[[1]], "recv-size-max") close(s) s <- socket("pair") opt(s, "recv-timeout") <- 2000 close(s) s <- socket("req") ctx <- context(s) opt(ctx, "send-timeout") <- 2000 close(ctx) close(s) s <- socket("pair", dial = "inproc://nanonext", autostart = FALSE) opt(s$dialer[[1]], "reconnect-time-min") <- 2000 start(s$dialer[[1]]) close(s) s <- socket("pair", listen = "inproc://nanonext", autostart = FALSE) opt(s$listener[[1]], "recv-size-max") <- 1024 start(s$listener[[1]]) close(s) ```

opt function