xs_getsockopt(3)

# xs_getsockopt(3) Manual Page

## NAME

xs_getsockopt - get Crossroads socket option

## SYNOPSIS

int xs_getsockopt (void *socket, int option_name, void *option_value, size_t *option_len);

## DESCRIPTION

The xs_getsockopt() function shall retrieve the value for the option specified by the option_name argument for the Crossroads 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 xs_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 xs_getsockopt() function:

### XS_TYPE: Retrieve socket type

The XS_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

### XS_RCVMORE: More message data parts to follow

The XS_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 xs_send(3) and xs_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

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

The XS_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 the library 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, the library shall take appropriate action such as blocking or dropping sent messages. Refer to the individual socket descriptions in xs_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

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

The XS_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 the library 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, the library shall take appropriate action such as blocking or dropping sent messages. Refer to the individual socket descriptions in xs_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

### XS_AFFINITY: Retrieve I/O thread affinity

The XS_AFFINITY option shall retrieve the I/O thread affinity for newly created connections on the specified socket.

Affinity determines which threads from the Crossroads 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 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 xs_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

### XS_IDENTITY: Set socket identity

The XS_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 a XS_XREP 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 Crossroads infrastructure.

Option value type | binary data
Option value unit | N/A
Default value | NULL
Applicable socket types | all

### XS_RATE: Retrieve multicast data rate

The XS_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

### XS_RECOVERY_IVL: Get multicast recovery interval

The XS_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

### XS_SNDBUF: Retrieve kernel transmit buffer size

The XS_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

### XS_RCVBUF: Retrieve kernel receive buffer size

The XS_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

### XS_LINGER: Retrieve linger period for socket shutdown

The XS_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 xs_close(3), and further affects the termination of the socket’s context with xs_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 xs_close(); attempting to terminate the socket’s context with xs_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 xs_close().

- Positive values specify an upper bound for the linger period in milliseconds. Pending messages shall not be discarded after a call to xs_close(); attempting to terminate the socket’s context with xs_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

### XS_RECONNECT_IVL: Retrieve reconnection interval

The XS_RECONNECT_IVL option shall retrieve the initial reconnection interval for the specified socket. The reconnection interval is the period the library shall wait between attempts to reconnect disconnected peers when using connection-oriented transports.

Note | The reconnection interval may be randomized by the library 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

### XS_RECONNECT_IVL_MAX: Retrieve maximum reconnection interval

The XS_RECONNECT_IVL_MAX option shall retrieve the maximum reconnection interval for the specified socket. This is the maximum period the library shall wait between attempts to reconnect. On each reconnect attempt, the previous interval shall be doubled untill XS_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 XS_RECONNECT_IVL.

Note | Values less than XS_RECONNECT_IVL will be ignored.

Option value type | int
Option value unit | milliseconds
Default value | 0 (only use XS_RECONNECT_IVL)
Applicable socket types | all, only for connection-oriented transport

### XS_BACKLOG: Retrieve maximum length of the queue of outstanding connections

The XS_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

### XS_MAXMSGSIZE: Maximum acceptable inbound message size

The option shall retrieve limit for the inbound messages. If a peer sends a message larger than XS_MAXMSGSIZE it is disconnected.

Option value type | uint64_t
Option value unit | bytes
Default value | 2^64-1
Applicable socket types | all

### XS_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

### XS_RCVTIMEO: Maximum time before a socket operation returns with EAGAIN

Retrieve the timeout for recv operation on the socket. If the value is `0`, xs_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

### XS_SNDTIMEO: Maximum time before a socket operation returns with EAGAIN

Retrieve the timeout for send operation on the socket. If the value is `0`, xs_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

### XS_IPV4ONLY: Retrieve IPv4-only socket override status

Retrieve the underlying native socket type. A value of `1` will use IPv4 sockets, while the value of `0` will use IPv6 sockets. An IPv6 socket lets applications connect to and accept connections from both IPv4 and IPv6 hosts.

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

### XS_FD: Retrieve file descriptor associated with the socket

The XS_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 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 XS_EVENTS option.

Note | The returned file descriptor is also used internally by the xs_send and xs_recv functions. As the descriptor is edge triggered, applications must update the state of XS_EVENTS after each invocation of xs_send or xs_recv.To be more explicit: after calling xs_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

### XS_EVENTS: Retrieve socket event state

The XS_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:

XS_POLLIN

  Indicates that at least one message may be received from the specified socket without blocking.

XS_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 XS_FD option being ready for reading but no actual events returned by a subsequent retrieval of the XS_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

### XS_KEEPALIVE: Enable transport keepalives

When set to `1`, this option enables use of protocol keepalives on the socket, if supported by the underlying transport protocol.

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

### XS_SURVEY_TIMEOUT: Retrieve deadline for the survey

Specifies how long to wait for responses to the survey. After the interval expires, any firther calls to xs_recv() will return EAGAIN error. All the responses received later on will be silently discarded. Value of -1 means infinite.

Option value type | int
Option value unit | milliseconds
Default value | -1 (infinite)
Applicable socket types | XS_SURVEYOR

## RETURN VALUE

The xs_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 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 = xs_getsockopt (socket, XS_SNDHWM, &sndhwm, &sndhwm_size);
    assert (rc == 0);

## SEE ALSO

xs_setsockopt(3) xs_socket(3) xs(7)

## AUTHORS

The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <martin@lucina.net>.