Client ====== `pvxs::client::Context` represents a PVA protocol client. :: #include namespace pvxs { namespace client { ... } } Configuration ------------- The recommended starting point is creating new context configured from $PVA_* environment variables. Use `pvxs::server::Config::from_env` and then `pvxs::server::Config::build`. EPICS_PVA_ADDR_LIST A list of destination addresses to which UDP search messages will be sent. May contain unicast and/or broadcast addresses. EPICS_PVA_AUTO_ADDR_LIST If "YES" then all local broadcast addresses will be implicitly appended to $EPICS_PVA_ADDR_LIST "YES" if unset. EPICS_PVA_BROADCAST_PORT Default UDP port to which UDP searches will be sent. 5076 if unset. .. code-block:: c++ using namespace pvxs; // Context configured from process environment client::Context ctxt = client::Config::from_env().build(); Programatic configuration can be accomplished by explicitly filling in a `pvxs::server::Config`. Making Requests --------------- A `pvxs::client::Context` instance is the entry point for all client network operations. Begin by calling one of the info(), get(), put(), rpc(), or monitor() methods. Each of these methods returns a \*Builder object which can be used to provide additional configuration in what are in effected named arguments. .. doxygenclass:: pvxs::client::Context :members: Get/Info ^^^^^^^^ `pvxs::client::Context::info` and `pvxs::client::Context::get` return a `pvxs::client::GetBuilder` to prepare either a get() or info() (GET_FIELD) operation. The practical difference being that info() yields a Value which will never have any fields marked. .. doxygenclass:: pvxs::client::GetBuilder :members: Put ^^^ `pvxs::client::Context::put` returns a `pvxs::client::PutBuilder` to prepare a put() operation. In the generic form of put(), the field values to sent have to be passed to the builder callback. This is necessary as the server mandated PV type definition is not known when an Put operation is initiated. Additionally, a put operation will by default first fetch the present value of the PV and provide it to the builder callback. This allows eg. to perform string to index lookup when writing to an NTEnum. .. doxygenclass:: pvxs::client::PutBuilder :members: RPC ^^^ `pvxs::client::Context::rpc` returns a `pvxs::client::RPCBuilder` to prepare an rpc() operation. rpc() differs from put() in that the call determines the type definition by providing a Value directly, so no builder callback is needed. .. doxygenclass:: pvxs::client::RPCBuilder :members: Operation and Result ^^^^^^^^^^^^^^^^^^^^ The exec() method of the \*Builder objects returns a shared_ptr to an `pvxs::client::Operation` handle, which represents the in-progress network operation. The caller **must** retain this handle until completion, or the operation will be implicitly cancelled. When an Operation completes, a `pvxs::client::Result` is passed to the result() callback. This object holds either a `pvxs::Value` if the operation succeeded, or an exception. .. doxygenstruct:: pvxs::client::Operation :members: .. doxygenclass:: pvxs::client::Result :members: Monitor ^^^^^^^ `pvxs::client::Context::monitor` returns a `pvxs::client::MonitorBuilder` to prepare a MONITOR operation. The result of this preparation is a `pvxs::client::Subscription` which represents the in-progress network operation. The caller **must** retain this handle or the operation will be implicitly cancelled. Until cancelled, a Subscription will attempt to (re)connect to the requested PV. A Subscription object allows access to an queue of data updates as Value and events/errors as exceptions. The `pvxs::client::Subscription::pop` method will remove an entry from the queue, or return an empty/invalid Value. Data updates are returned as a valid Value. Events/errors are thrown as exceptions. The special exceptions `pvxs::client::Connected`, `pvxs::client::Disconnect`, and `pvxs::client::Finished` have specific meaning for a Subscription. Connected Depending on `pvxs::client::MonitorBuilder::maskConnected` (default true). Queued when a Subscription becomes connected. The Connected object include the server host:port as well as a (client) time of connection. Disconnect Depending on `pvxs::client::MonitorBuilder::maskDisconnected` (default false). Queued when a Subscription becomes disconnected. Finished Depending on `pvxs::client::MonitorBuilder::maskDisconnected` (default false). Queued when a the server indicates that Subscription will receive no more date updates as a normal completion. Finished is a sub-class of Disconnect. There are several aspects of a Subscription which may be selected through the MonitorBuilder. The special `pvxs::client::Connected` and `pvxs::client::Disconnect` "errors" may appear in the event queue .. doxygenclass:: pvxs::client::MonitorBuilder :members: .. doxygenstruct:: pvxs::client::Subscription :members: Threading ^^^^^^^^^ A client Context will invoke user callback functions from one or more internal worker threads. However, it is guaranteed that callbacks relating to a given Channel (PV name + priority) will never be executed concurrently. This implies that callbacks for a single operation will also never be executed concurrently. User code must avoid doing unnecessary work from within a callback function as this will prevent other callbacks from be executed. Ownership ^^^^^^^^^ User provided callbacks are in the form of std::function which may, directly or indirectly, store shared_ptr<> instances. The returned Operation and Subscription instances may be treated as storing the std::function instance(s) and thus any shared_ptr<> captured in them. Therefore, in order to avoid a resource leak, it is advisable to consider whether a returned Operation or Subscription may participate in a reference loop. For example, the following creates a reference loop between the Operation instance and the "mystruct" instance. .. code-block:: c++ struct mystruct { std::shared_ptr op; // <-- Danger! }; auto myptr = std::make_shared(); Context ctxt(...); myptr->op = ctxt.get("pv:name") .result([ctxt](Result&& result) { }) .exec(); While such loops can be explicitly broken (eg. by NULLing 'myptr->op') it is strongly recommended to avoid such situations as unexpected (exceptional) conditions can easily lead to resource leaks which are quite difficult to detect and isolate. Where possible it is recommended to capture weak_ptr<> instances. pvRequest --------- All operations except info() (GET_FIELD) take a Value which servers may use to modify or qualify the operation. Conventionally, the two ways this may be done is to provide a mask to limit the (sub)fields for which data is returned. Secondly, to provide certain well-known options to modify the operation. the pvRequest conditions may be specified in three ways through the methods of `pvxs::client::detail::CommonBuilder` exposed through the individual \*Builder types. Programatic The field() and record() methods. Textual The pvRequest() method accepts a string which is parsed into calls to the field() and record() methods. These two approaches may be intermixed. Fallback The rawRequest() method accepts an externally assembled Value which is sent without modification. .. doxygenclass:: pvxs::client::detail::CommonBuilder :members: Syntax ^^^^^^ The parser byhind `pvxs::client::detail::CommonBuilder::pvRequest` understands the following grammar. .. productionlist:: pvRequest : | entry | pvRequest entry entry : field | record | field_name field : "field" "(" field_list ")" record : "record" "[" option_list "]" field_list : | field_name | field_list "," field_name option_list : | option | option_list option option : key "=" value For examples: * "field()" * "field(value)" * "value" * "field(value,alarm)" * "field(value)field(alarm)" * "record[wait=true]" * "field()record[wait=true]" * "field(value)record[wait=true]" Misc ---- .. doxygenstruct:: pvxs::client::Config :members: .. doxygenstruct:: pvxs::client::Connected :members: .. doxygenstruct:: pvxs::client::Disconnect :members: .. doxygenstruct:: pvxs::client::Finished :members: .. doxygenstruct:: pvxs::client::RemoteError :members: