Log Engine Tutorial Overview

This overview provides the information to perform various tasks that clients perform with Log Engine.


The log engine stores events generated by operating system components, particularly telephony and messaging. This document explains how to use the log engine to create a record of events and how to create views of the events for display.


The log engine uses a client-server architecture. The log server is a permanent process maintaining event data and the log clients attach to it to add and retrieve data. When writing an application which uses the log engine you create a log client wrapped in a customized component. This component has two purposes. Firstly it causes the server to log events of a specified kind. Secondly it uses log filter and log view classes to extract and present the data appropriately.

The log engine is designed to abstract from the details of how events are notified, stored and displayed. You do not need to know exactly how this functionality is implemented.

The log engine is an interface to a database holding records of communication events. You do not need to know the implementation of the database, merely that the log engine has functionality to add, read, modify and delete records.

Not all devices are equipped with logging functionality. You need to determine whether your application will only run on platforms with logging functionality guaranteed or else write the application so as to fail gracefully. For this reason the class CLogClient and its parent class CLogBase are often wrapped in a class CLogWrapper which handles cases where logging functionality is absent.

Logging calls are asynchronous. The application developer is responsible for maintaining the integrity of the log client session. A typical way of doing this is to maintain a queue of log events.

The log engine provides limited configuration functionality. If you are involved in device creation and want to tune performance you need to know how to manage memory and configure the logging component explicitly: the topic is not covered in this document.

The log engine models standard types of communication event by default. If your application involves non-standard functionality you need to know how to create user defined data types to represent non-standard events.

The log engine uses the Symbian platform capabilities model of security. This means that your application must police its own security and you must therefore know the security capabilities required by each item of functionality.

The purpose of the log engine is to provide data for a viewer application which displays information such as missed calls to the end user of the device. You do not need to know anything about the viewer, merely that it requires data views to be provided in certain data structures.

The log engine gets updated by an observer component which notifies it of new communication events affecting either the data tables or the views created from them. You do not need to know the details of the interprocess communication, merely that an observer component exists and performs notifications.

Log clients and log views

The log engine API consists of two types of classes, the client classes and the view classes. The client classes exist to create logs and maintain and modify them. They are CLogClient, CLogBase and CLogWrapper. It is CLogClient which contains the actual functionality: the purpose of CLogBase and CLogWrapper is to provide default functionality when logging functionality is not available. The view classes exist to filter the log data and prepare it for presentation in a viewer application. They are CLogViewEvent and CLogViewRecent, which inherit from a class CLogView which cannot be instantiated.

Events and event types

The log engine is designed to log events having to do with communications: messaging, telephony, email and so on. The class CLogEvent represents events of this kind: its member objects mainly have to do with types of communication. The class CLogEventType contains information about the event type which a particular event belongs to.

Log events

The CLogEvent class encapsulates data about communication events.

  • The ID is the unique identifier of the event.

  • The remote party is the device originating or receiving the communication.

  • The direction indicates whether the communication is incoming or outgoing.

  • The time of the event is the time at which it began.

  • The duration type is one of three constants: KLogDurationNone, KLogDurationData or KLogDurationValid. You typically use them where the event being represented either has no duration, or has a duration of no relevance (such as a data transfer), or has a valid duration (such as a voice call).

  • The duration is the duration in seconds from the start of the event to the end.

  • The delivery status is one of a set of flags indicating whether the communication was successfully delivered.

  • The subject is a string to be displayed in the subject line of the viewer application.

  • The log link is the ID of the event in the calling application, as distinct from its ID in the log.

  • The description is a human readable description of the event type.

  • There is only one flag, KLogEventRead, which is set to true if the event has been read by a client. It can be cleared either by passing the constant KLogFlagsMask to the function SetFlags() or by calling the function ClearFlags().

Log event types

The log engine API defines constants of type TUid representing the standard types of communication event which can be logged.

  • KLogCallEventType: voice call

  • KLogDataEventType: data call

  • KLogFaxEventType: fax call

  • KLogShortMessageEventType: SMS call

  • KLogMailEventType: email

  • KLogTaskSchedulerEventType: task scheduler event

The log engine API provides the class CLogEventType which acts on all events of a specified type. Its functions are:

  • Uid() and SetUid() get and set the UID identifying the event type.

  • Description() and SetDescription() get and set the description in words of the event type.

  • LoggingEnabled() and SetLoggingEnabled() get and set a flag which enables events of this type to be logged.

  • Copy() copies an existing event type.

  • InternaliseL() and ExternaliseL() implement streaming input and output of an event type object.

You may need an event type not supplied with the log engine API, for instance if you are developing a new kind of messaging application. A new event type requires a new unique Id which must be allocated by Symbian Signed. Once you have that Id you can create a CLogEventType and pass the Id to SetUid() just like the standard event type constants.

The main use of CLogEventType is to enable or disable logging for all events of a particular type. You do this by creating a CLogEventType object with its UID set to the relevant event type and calling its SetLoggingEnabled() function with ETrue or EFalse as argument.

It is important to know that only an explicit call to the SetLoggingEnabled() function of a CLogEventType object will cause logging to take place. For instance if you create a new event type myEventType and want such events to be logged, you have to call its SetLoggingEnabled() function before logging will occur.

    CLogEventType* myEventType = CLogEventType::NewL();myEventType->SetLoggingEnabled(ETrue);

Log engine and platform security

The Symbian platform uses a capabilities model of security in which an application being called verifies that the calling application has the required security policies before returning data. This is important in connection with message logging which involves the end user's confidential information. When writing an application which incorporates logging functionality you must ensure that it has the capabilities to call the log engine APIs.

To call a log client, a calling application must have the capabilities required by the event type of the event being accessed. This means that a call to CLogClient::GetEvent() must have the capability specified in the read policy of the event type, while calls to AddEvent(), ChangeEvent() and DeleteEvent() must have the capability specified in the write policy. To modify the event type by calling AddEventType(), ChangeEventType() and DeleteEventType() a calling application requires the WriteDeviceData capability. No capability is required to call GetEventType().

No security capability is required to access the view classes.

Using filters when displaying a log

An event view typically contains only a selection of items from the main log which satisfy the constraints of a filter. A CLogFilter has members representing the same information as a CLogEvent object but is used to select on data not modify it. (In the database implementation, a filter corresponds to the WHERE clause of a SELECT query.) An event view is empty when created. To initialize it you first create and initialize a CLogFilter object and then pass this as a parameter of the SetFilterL() function of the event view. You may also want to initialize a view to satisfy any one of several filters, in which case you pass in a CLogFilterList: this is an array of several CLogFilter objects. You would do this, for instance, if you wanted to create a view to display events of several different event types such as voice mail, SMS and email, since a single CLogFilter only has one event type.

  1. Configuring Log Engine

    The clients must configure the Log Engine before using it.

  2. Setting Up A Log Engine Client

    After configuring the log engine, a log engine client must be developed.

  3. Maintaining Log Events

    The client applications can then add event, event types and delete if necessary.

  4. Requesting Log Engine Notifications

    Client applications can receive notification from log engine to get events recorded by other applications.

  5. Displaying Log Events

    Finally the client applications can then display the log events to the user.