Client

The client implementation allows remote access to all OPC UA services. For convenience, some functionality has been wrapped in high-level abstractions.

However: At this time, the client does not yet contain its own thread or event-driven main-loop, meaning that the client will not perform any actions automatically in the background. This is especially relevant for connection/session management and subscriptions. The user will have to periodically call UA_Client_run_iterate to ensure that asynchronous events are handled, including keeping a secure connection established. See more about asynchronicity and subscriptions.

Client Configuration

The client configuration is used for setting connection parameters and additional settings used by the client. The configuration should not be modified after it is passed to a client. Currently, only one client can use a configuration at a time.

Examples for configurations are provided in the /plugins folder. The usual usage is as follows:

  1. Create a client configuration with default settings as a starting point
  2. Modifiy the configuration, e.g. modifying the timeout
  3. Instantiate a client with it
  4. After shutdown of the client, clean up the configuration (free memory)

The Tutorials provide a good starting point for this.

typedef struct {
    /* Basic client configuration */
    void *clientContext; /* User-defined data attached to the client */
    UA_Logger logger;   /* Logger used by the client */
    UA_UInt32 timeout;  /* Response timeout in ms */

    /* The description must be internally consistent.
     * - The ApplicationUri set in the ApplicationDescription must match the
     *   URI set in the server certificate */
    UA_ApplicationDescription clientDescription;

    /* Basic connection configuration */
    UA_ExtensionObject userIdentityToken; /* Configured User-Identity Token */
    UA_MessageSecurityMode securityMode;  /* None, Sign, SignAndEncrypt. The
                                           * default is invalid. This indicates
                                           * the client to select any matching
                                           * endpoint. */
    UA_String securityPolicyUri; /* SecurityPolicy for the SecureChannel. An
                                  * empty string indicates the client to select
                                  * any matching SecurityPolicy. */

    /* Advanced connection configuration
     *
     * If either endpoint or userTokenPolicy has been set (at least one non-zero
     * byte in either structure), then the selected Endpoint and UserTokenPolicy
     * overwrite the settings in the basic connection configuration. The
     * userTokenPolicy array in the EndpointDescription is ignored. The selected
     * userTokenPolicy is set in the dedicated configuration field.
     *
     * If the advanced configuration is not set, the client will write to it the
     * selected Endpoint and UserTokenPolicy during GetEndpoints.
     *
     * The information in the advanced configuration is used during reconnect
     * when the SecureChannel was broken. */
    UA_EndpointDescription endpoint;
    UA_UserTokenPolicy userTokenPolicy;

    /* Advanced client configuration */

    UA_UInt32 secureChannelLifeTime; /* Lifetime in ms (then the channel needs
                                        to be renewed) */
    UA_UInt32 requestedSessionTimeout; /* Session timeout in ms */
    UA_ConnectionConfig localConnectionConfig;
    UA_UInt32 connectivityCheckInterval;     /* Connectivity check interval in ms.
                                              * 0 = background task disabled */
    const UA_DataTypeArray *customDataTypes; /* Custom DataTypes. Attention!
                                              * Custom datatypes are not cleaned
                                              * up together with the
                                              * configuration. So it is possible
                                              * to allocate them on ROM. */

    /* Available SecurityPolicies */
    size_t securityPoliciesSize;
    UA_SecurityPolicy *securityPolicies;

    /* Certificate Verification Plugin */
    UA_CertificateVerification certificateVerification;

    /* Callbacks for async connection handshakes */
    UA_ConnectClientConnection initConnectionFunc;
    UA_StatusCode (*pollConnectionFunc)(UA_Connection *connection,
                                        UA_UInt32 timeout,
                                        const UA_Logger *logger);

    /* Callback for state changes. The client state is differentated into the
     * SecureChannel state and the Session state. The connectStatus is set if
     * the client connection (including reconnects) has failed and the client
     * has to "give up". If the connectStatus is not set, the client still has
     * hope to connect or recover. */
    void (*stateCallback)(UA_Client *client,
                          UA_SecureChannelState channelState,
                          UA_SessionState sessionState,
                          UA_StatusCode connectStatus);

    /* When connectivityCheckInterval is greater than 0, every
     * connectivityCheckInterval (in ms), a async read request is performed on
     * the server. inactivityCallback is called when the client receive no
     * response for this read request The connection can be closed, this in an
     * attempt to recreate a healthy connection. */
    void (*inactivityCallback)(UA_Client *client);

#ifdef UA_ENABLE_SUBSCRIPTIONS
    /* Number of PublishResponse queued up in the server */
    UA_UInt16 outStandingPublishRequests;

    /* If the client does not receive a PublishResponse after the defined delay
     * of ``(sub->publishingInterval * sub->maxKeepAliveCount) +
     * client->config.timeout)``, then subscriptionInactivityCallback is called
     * for the subscription.. */
    void (*subscriptionInactivityCallback)(UA_Client *client,
                                           UA_UInt32 subscriptionId,
                                           void *subContext);
#endif
} UA_ClientConfig;

Client Lifecycle

/* The method UA_Client_new is defined in client_config_default.h. So default
 * plugins outside of the core library (for logging, etc) are already available
 * during the initialization.
 *
 * UA_Client * UA_Client_new(void);
 */

/* Creates a new client. Moves the config into the client with a shallow copy.
 * The config content is cleared together with the client. */
UA_Client *
UA_Client_newWithConfig(const UA_ClientConfig *config);

/* Returns the current state. All arguments except ``client`` can be NULL. */
void
UA_Client_getState(UA_Client *client,
                   UA_SecureChannelState *channelState,
                   UA_SessionState *sessionState,
                   UA_StatusCode *connectStatus);

/* Get the client configuration */
UA_ClientConfig *
UA_Client_getConfig(UA_Client *client);

/* Get the client context */
static UA_INLINE void *
UA_Client_getContext(UA_Client *client) {
    return UA_Client_getConfig(client)->clientContext; /* Cannot fail */
}

/* (Disconnect and) delete the client */
void
UA_Client_delete(UA_Client *client);

Connect to a Server

Once a client is connected to an endpointUrl, it is not possible to switch to another server. A new client has to be created for that.

Once a connection is established, the client keeps the connection open and reconnects if necessary.

If the connection fails unrecoverably (state->connectStatus is set to an error), the client is no longer usable. Create a new client if required.

/* Connect to the server. First a SecureChannel is opened, then a Session. The
 * client configuration restricts the SecureChannel selection and contains the
 * UserIdentityToken for the Session.
 *
 * @param client to use
 * @param endpointURL to connect (for example "opc.tcp://localhost:4840")
 * @return Indicates whether the operation succeeded or returns an error code */
UA_StatusCode
UA_Client_connect(UA_Client *client, const char *endpointUrl);

/* Connect async (non-blocking) to the server. After initiating the connection,
 * call UA_Client_run_iterate repeatedly until the connection is fully
 * established. You can set a callback to client->config.stateCallback to be
 * notified when the connection status changes. Or use UA_Client_getState to get
 * the state manually. */
UA_StatusCode
UA_Client_connectAsync(UA_Client *client, const char *endpointUrl);

/* Connect to the server without creating a session
 *
 * @param client to use
 * @param endpointURL to connect (for example "opc.tcp://localhost:4840")
 * @return Indicates whether the operation succeeded or returns an error code */
UA_StatusCode
UA_Client_connectSecureChannel(UA_Client *client, const char *endpointUrl);

/* Connect async (non-blocking) only the SecureChannel */
UA_StatusCode
UA_Client_connectSecureChannelAsync(UA_Client *client, const char *endpointUrl);

/* Connect to the server and create+activate a Session with the given username
 * and password. This first set the UserIdentityToken in the client config and
 * then calls the regular connect method. */
static UA_INLINE UA_StatusCode
UA_Client_connectUsername(UA_Client *client, const char *endpointUrl,
                          const char *username, const char *password) {
    UA_UserNameIdentityToken* identityToken = UA_UserNameIdentityToken_new();
    if(!identityToken)
        return UA_STATUSCODE_BADOUTOFMEMORY;
    identityToken->userName = UA_STRING_ALLOC(username);
    identityToken->password = UA_STRING_ALLOC(password);
    UA_ClientConfig *cc = UA_Client_getConfig(client);
    UA_ExtensionObject_clear(&cc->userIdentityToken);
    cc->userIdentityToken.encoding = UA_EXTENSIONOBJECT_DECODED;
    cc->userIdentityToken.content.decoded.type = &UA_TYPES[UA_TYPES_USERNAMEIDENTITYTOKEN];
    cc->userIdentityToken.content.decoded.data = identityToken;
    return UA_Client_connect(client, endpointUrl);
}

/* Disconnect and close a connection to the selected server. Disconnection is
 * always performed async (without blocking). */
UA_StatusCode
UA_Client_disconnect(UA_Client *client);

/* Disconnect async. Run UA_Client_run_iterate until the callback notifies that
 * all connections are closed. */
UA_StatusCode
UA_Client_disconnectAsync(UA_Client *client);

/* Disconnect the SecureChannel but keep the Session intact (if it exists).
 * This is always an async (non-blocking) operation. */
UA_StatusCode
UA_Client_disconnectSecureChannel(UA_Client *client);

Discovery

/* Gets a list of endpoints of a server
 *
 * @param client to use. Must be connected to the same endpoint given in
 *        serverUrl or otherwise in disconnected state.
 * @param serverUrl url to connect (for example "opc.tcp://localhost:4840")
 * @param endpointDescriptionsSize size of the array of endpoint descriptions
 * @param endpointDescriptions array of endpoint descriptions that is allocated
 *        by the function (you need to free manually)
 * @return Indicates whether the operation succeeded or returns an error code */
UA_StatusCode
UA_Client_getEndpoints(UA_Client *client, const char *serverUrl,
                       size_t* endpointDescriptionsSize,
                       UA_EndpointDescription** endpointDescriptions);

/* Gets a list of all registered servers at the given server.
 *
 * You can pass an optional filter for serverUris. If the given server is not registered,
 * an empty array will be returned. If the server is registered, only that application
 * description will be returned.
 *
 * Additionally you can optionally indicate which locale you want for the server name
 * in the returned application description. The array indicates the order of preference.
 * A server may have localized names.
 *
 * @param client to use. Must be connected to the same endpoint given in
 *        serverUrl or otherwise in disconnected state.
 * @param serverUrl url to connect (for example "opc.tcp://localhost:4840")
 * @param serverUrisSize Optional filter for specific server uris
 * @param serverUris Optional filter for specific server uris
 * @param localeIdsSize Optional indication which locale you prefer
 * @param localeIds Optional indication which locale you prefer
 * @param registeredServersSize size of returned array, i.e., number of found/registered servers
 * @param registeredServers array containing found/registered servers
 * @return Indicates whether the operation succeeded or returns an error code */
UA_StatusCode
UA_Client_findServers(UA_Client *client, const char *serverUrl,
                      size_t serverUrisSize, UA_String *serverUris,
                      size_t localeIdsSize, UA_String *localeIds,
                      size_t *registeredServersSize,
                      UA_ApplicationDescription **registeredServers);

#ifdef UA_ENABLE_DISCOVERY
/* Get a list of all known server in the network. Only supported by LDS servers.
 *
 * @param client to use. Must be connected to the same endpoint given in
 * serverUrl or otherwise in disconnected state.
 * @param serverUrl url to connect (for example "opc.tcp://localhost:4840")
 * @param startingRecordId optional. Only return the records with an ID higher
 *        or equal the given. Can be used for pagination to only get a subset of
 *        the full list
 * @param maxRecordsToReturn optional. Only return this number of records

 * @param serverCapabilityFilterSize optional. Filter the returned list to only
 *        get servers with given capabilities, e.g. "LDS"
 * @param serverCapabilityFilter optional. Filter the returned list to only get
 *        servers with given capabilities, e.g. "LDS"
 * @param serverOnNetworkSize size of returned array, i.e., number of
 *        known/registered servers
 * @param serverOnNetwork array containing known/registered servers
 * @return Indicates whether the operation succeeded or returns an error code */
UA_StatusCode
UA_Client_findServersOnNetwork(UA_Client *client, const char *serverUrl,
                               UA_UInt32 startingRecordId, UA_UInt32 maxRecordsToReturn,
                               size_t serverCapabilityFilterSize, UA_String *serverCapabilityFilter,
                               size_t *serverOnNetworkSize, UA_ServerOnNetwork **serverOnNetwork);
#endif

Services

The raw OPC UA services are exposed to the client. But most of them time, it is better to use the convenience functions from ua_client_highlevel.h that wrap the raw services.

/* Don't use this function. Use the type versions below instead. */
void
__UA_Client_Service(UA_Client *client, const void *request,
                    const UA_DataType *requestType, void *response,
                    const UA_DataType *responseType);

/*
 * Attribute Service Set
 * ^^^^^^^^^^^^^^^^^^^^^ */
static UA_INLINE UA_ReadResponse
UA_Client_Service_read(UA_Client *client, const UA_ReadRequest request) {
    UA_ReadResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_READREQUEST],
                        &response, &UA_TYPES[UA_TYPES_READRESPONSE]);
    return response;
}

static UA_INLINE UA_WriteResponse
UA_Client_Service_write(UA_Client *client, const UA_WriteRequest request) {
    UA_WriteResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_WRITEREQUEST],
                        &response, &UA_TYPES[UA_TYPES_WRITERESPONSE]);
    return response;
}

/*
* Historical Access Service Set
* ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ */
#ifdef UA_ENABLE_HISTORIZING
static UA_INLINE UA_HistoryReadResponse
UA_Client_Service_historyRead(UA_Client *client, const UA_HistoryReadRequest request) {
    UA_HistoryReadResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_HISTORYREADREQUEST],
        &response, &UA_TYPES[UA_TYPES_HISTORYREADRESPONSE]);
    return response;
}

static UA_INLINE UA_HistoryUpdateResponse
UA_Client_Service_historyUpdate(UA_Client *client, const UA_HistoryUpdateRequest request) {
    UA_HistoryUpdateResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_HISTORYUPDATEREQUEST],
        &response, &UA_TYPES[UA_TYPES_HISTORYUPDATERESPONSE]);
    return response;
}
#endif

/*
 * Method Service Set
 * ^^^^^^^^^^^^^^^^^^ */
#ifdef UA_ENABLE_METHODCALLS
static UA_INLINE UA_CallResponse
UA_Client_Service_call(UA_Client *client, const UA_CallRequest request) {
    UA_CallResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_CALLREQUEST],
                        &response, &UA_TYPES[UA_TYPES_CALLRESPONSE]);
    return response;
}
#endif

/*
 * NodeManagement Service Set
 * ^^^^^^^^^^^^^^^^^^^^^^^^^^ */
static UA_INLINE UA_AddNodesResponse
UA_Client_Service_addNodes(UA_Client *client, const UA_AddNodesRequest request) {
    UA_AddNodesResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_ADDNODESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_ADDNODESRESPONSE]);
    return response;
}

static UA_INLINE UA_AddReferencesResponse
UA_Client_Service_addReferences(UA_Client *client,
                                const UA_AddReferencesRequest request) {
    UA_AddReferencesResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_ADDREFERENCESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_ADDREFERENCESRESPONSE]);
    return response;
}

static UA_INLINE UA_DeleteNodesResponse
UA_Client_Service_deleteNodes(UA_Client *client,
                              const UA_DeleteNodesRequest request) {
    UA_DeleteNodesResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_DELETENODESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_DELETENODESRESPONSE]);
    return response;
}

static UA_INLINE UA_DeleteReferencesResponse
UA_Client_Service_deleteReferences(UA_Client *client,
                                   const UA_DeleteReferencesRequest request) {
    UA_DeleteReferencesResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_DELETEREFERENCESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_DELETEREFERENCESRESPONSE]);
    return response;
}

/*
 * View Service Set
 * ^^^^^^^^^^^^^^^^ */
static UA_INLINE UA_BrowseResponse
UA_Client_Service_browse(UA_Client *client, const UA_BrowseRequest request) {
    UA_BrowseResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_BROWSEREQUEST],
                        &response, &UA_TYPES[UA_TYPES_BROWSERESPONSE]);
    return response;
}

static UA_INLINE UA_BrowseNextResponse
UA_Client_Service_browseNext(UA_Client *client,
                             const UA_BrowseNextRequest request) {
    UA_BrowseNextResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_BROWSENEXTREQUEST],
                        &response, &UA_TYPES[UA_TYPES_BROWSENEXTRESPONSE]);
    return response;
}

static UA_INLINE UA_TranslateBrowsePathsToNodeIdsResponse
UA_Client_Service_translateBrowsePathsToNodeIds(UA_Client *client,
                        const UA_TranslateBrowsePathsToNodeIdsRequest request) {
    UA_TranslateBrowsePathsToNodeIdsResponse response;
    __UA_Client_Service(client, &request,
                        &UA_TYPES[UA_TYPES_TRANSLATEBROWSEPATHSTONODEIDSREQUEST],
                        &response,
                        &UA_TYPES[UA_TYPES_TRANSLATEBROWSEPATHSTONODEIDSRESPONSE]);
    return response;
}

static UA_INLINE UA_RegisterNodesResponse
UA_Client_Service_registerNodes(UA_Client *client,
                                const UA_RegisterNodesRequest request) {
    UA_RegisterNodesResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_REGISTERNODESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_REGISTERNODESRESPONSE]);
    return response;
}

static UA_INLINE UA_UnregisterNodesResponse
UA_Client_Service_unregisterNodes(UA_Client *client,
                                  const UA_UnregisterNodesRequest request) {
    UA_UnregisterNodesResponse response;
    __UA_Client_Service(client, &request,
                        &UA_TYPES[UA_TYPES_UNREGISTERNODESREQUEST],
                        &response, &UA_TYPES[UA_TYPES_UNREGISTERNODESRESPONSE]);
    return response;
}

/*
 * Query Service Set
 * ^^^^^^^^^^^^^^^^^ */
#ifdef UA_ENABLE_QUERY

static UA_INLINE UA_QueryFirstResponse
UA_Client_Service_queryFirst(UA_Client *client,
                             const UA_QueryFirstRequest request) {
    UA_QueryFirstResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_QUERYFIRSTREQUEST],
                        &response, &UA_TYPES[UA_TYPES_QUERYFIRSTRESPONSE]);
    return response;
}

static UA_INLINE UA_QueryNextResponse
UA_Client_Service_queryNext(UA_Client *client,
                            const UA_QueryNextRequest request) {
    UA_QueryNextResponse response;
    __UA_Client_Service(client, &request, &UA_TYPES[UA_TYPES_QUERYFIRSTREQUEST],
                        &response, &UA_TYPES[UA_TYPES_QUERYFIRSTRESPONSE]);
    return response;
}

#endif

Asynchronous Services

All OPC UA services are asynchronous in nature. So several service calls can be made without waiting for the individual responses. Depending on the server’s priorities responses may come in a different ordering than sent.

As noted in the client overview currently no means of handling asynchronous events automatically is provided. However, some synchronous function calls will trigger handling, but to ensure this happens a client should periodically call UA_Client_run_iterate explicitly.

Connection and session management are also performed in UA_Client_run_iterate, so to keep a connection healthy any client need to consider how and when it is appropriate to do the call. This is especially true for the periodic renewal of a SecureChannel’s SecurityToken which is designed to have a limited lifetime and will invalidate the connection if not renewed.

/* Use the type versions of this method. See below. However, the general
 * mechanism of async service calls is explained here.
 *
 * We say that an async service call has been dispatched once this method
 * returns UA_STATUSCODE_GOOD. If there is an error after an async service has
 * been dispatched, the callback is called with an "empty" response where the
 * statusCode has been set accordingly. This is also done if the client is
 * shutting down and the list of dispatched async services is emptied.
 *
 * The statusCode received when the client is shutting down is
 * UA_STATUSCODE_BADSHUTDOWN.
 *
 * The statusCode received when the client don't receive response
 * after specified config->timeout (in ms) is
 * UA_STATUSCODE_BADTIMEOUT.
 *
 * Instead, you can use __UA_Client_AsyncServiceEx to specify
 * a custom timeout
 *
 * The userdata and requestId arguments can be NULL. */

typedef void (*UA_ClientAsyncServiceCallback)(UA_Client *client, void *userdata,
        UA_UInt32 requestId, void *response);

UA_StatusCode
__UA_Client_AsyncService(UA_Client *client, const void *request,
                         const UA_DataType *requestType,
                         UA_ClientAsyncServiceCallback callback,
                         const UA_DataType *responseType,
                         void *userdata, UA_UInt32 *requestId);

UA_StatusCode
UA_Client_sendAsyncRequest(UA_Client *client, const void *request,
        const UA_DataType *requestType, UA_ClientAsyncServiceCallback callback,
        const UA_DataType *responseType, void *userdata, UA_UInt32 *requestId);

/* Listen on the network and process arriving asynchronous responses in the
 * background. Internal housekeeping, renewal of SecureChannels and subscription
 * management is done as well. */
UA_StatusCode
UA_Client_run_iterate(UA_Client *client, UA_UInt32 timeout);

/* Force the manual renewal of the SecureChannel. This is useful to renew the
 * SecureChannel during a downtime when no time-critical operations are
 * performed. This method is asynchronous. The renewal is triggered (the OPN
 * message is sent) but not completed. The OPN response is handled with
 * ``UA_Client_run_iterate`` or a synchronous servica-call operation.
 *
 * @return The return value is UA_STATUSCODE_GOODCALLAGAIN if the SecureChannel
 *         has not elapsed at least 75% of its lifetime. Otherwise the
 *         ``connectStatus`` is returned. */
UA_StatusCode
UA_Client_renewSecureChannel(UA_Client *client);

/* Use the type versions of this method. See below. However, the general
 * mechanism of async service calls is explained here.
 *
 * We say that an async service call has been dispatched once this method
 * returns UA_STATUSCODE_GOOD. If there is an error after an async service has
 * been dispatched, the callback is called with an "empty" response where the
 * statusCode has been set accordingly. This is also done if the client is
 * shutting down and the list of dispatched async services is emptied.
 *
 * The statusCode received when the client is shutting down is
 * UA_STATUSCODE_BADSHUTDOWN.
 *
 * The statusCode received when the client don't receive response
 * after specified timeout (in ms) is
 * UA_STATUSCODE_BADTIMEOUT.
 *
 * The timeout can be disabled by setting timeout to 0
 *
 * The userdata and requestId arguments can be NULL. */
UA_StatusCode
__UA_Client_AsyncServiceEx(UA_Client *client, const void *request,
                           const UA_DataType *requestType,
                           UA_ClientAsyncServiceCallback callback,
                           const UA_DataType *responseType,
                           void *userdata, UA_UInt32 *requestId,
                           UA_UInt32 timeout);

Timed Callbacks

Repeated callbacks can be attached to a client and will be executed in the defined interval.

typedef void (*UA_ClientCallback)(UA_Client *client, void *data);

/* Add a callback for execution at a specified time. If the indicated time lies
 * in the past, then the callback is executed at the next iteration of the
 * server's main loop.
 *
 * @param client The client object.
 * @param callback The callback that shall be added.
 * @param data Data that is forwarded to the callback.
 * @param date The timestamp for the execution time.
 * @param callbackId Set to the identifier of the repeated callback . This can
 *        be used to cancel the callback later on. If the pointer is null, the
 *        identifier is not set.
 * @return Upon success, UA_STATUSCODE_GOOD is returned. An error code
 *         otherwise. */
UA_StatusCode
UA_Client_addTimedCallback(UA_Client *client, UA_ClientCallback callback,
                           void *data, UA_DateTime date, UA_UInt64 *callbackId);

/* Add a callback for cyclic repetition to the client.
 *
 * @param client The client object.
 * @param callback The callback that shall be added.
 * @param data Data that is forwarded to the callback.
 * @param interval_ms The callback shall be repeatedly executed with the given
 *        interval (in ms). The interval must be positive. The first execution
 *        occurs at now() + interval at the latest.
 * @param callbackId Set to the identifier of the repeated callback . This can
 *        be used to cancel the callback later on. If the pointer is null, the
 *        identifier is not set.
 * @return Upon success, UA_STATUSCODE_GOOD is returned. An error code
 *         otherwise. */
UA_StatusCode
UA_Client_addRepeatedCallback(UA_Client *client, UA_ClientCallback callback,
                              void *data, UA_Double interval_ms,
                              UA_UInt64 *callbackId);

UA_StatusCode
UA_Client_changeRepeatedCallbackInterval(UA_Client *client,
                                         UA_UInt64 callbackId,
                                         UA_Double interval_ms);

void
UA_Client_removeCallback(UA_Client *client, UA_UInt64 callbackId);

Client Utility Functions

/* Lookup a datatype by its NodeId. Takes the custom types in the client
 * configuration into account. Return NULL if none found. */
const UA_DataType *
UA_Client_findDataType(UA_Client *client, const UA_NodeId *typeId);