Event Sources

Each log in the Eventlog key contains subkeys called event sources. The event source is the name of the software that logs the event. It is often the name of the application or the name of a subcomponent of the application if the application is large. You can add a maximum of 16,384 event sources to the registry. The Security log is for system use only. Device drivers should add their names to the System log. Applications and services should add their names to the Application log or create a custom log.

The structure of the event sources is as follows:


You cannot use a source name that has already been used as a log name. In addition, source names cannot be hierarchical; that is, they cannot contain the backslash character ("\").

Each event source contains information (such as a message file) specific to the software that will be logging the events, as shown in the following table.

Registry Value Description
CategoryCount Number of event categories supported. This value is of type REG_DWORD.
CategoryMessageFile Path to the category message file. A category message file contains language-dependent strings that describe the categories. This value can be of type REG_SZ or REG_EXPAND_SZ.
EventMessageFile Path to one or more event message files; use a semicolon to delimit multiple files. An event message file contains language-dependent strings that describe the events. This value can be of type REG_SZ or REG_EXPAND_SZ.
ParameterMessageFile Path to the parameter message file. A parameter message file contains language-independent strings that are to be inserted into the event description strings. This value can be of type REG_SZ or REG_EXPAND_SZ.
TypesSupported Bitmask of supported types. This value is of type REG_DWORD. It can be one or more of the following values:


When an application uses the RegisterEventSource or OpenEventLog function to get a handle to an event log, the event logging service searches for the specified event source in the registry. For example, the Application log might contain event sources for Microsoft SQL Server and Microsoft Excel. If an application uses RegisterEventSource or OpenEventLog with a source name of Application, SQL, or Excel, the event logging service returns a handle to the Application log.

An application can use the Application log without adding a new event source to the registry. If the application calls RegisterEventSource and passes a source name that cannot be found in the registry, the event-logging service uses the Application log by default. However, because there are no message files, the Event Viewer cannot map any event identifiers or event categories to a description string, and will display an error. For this reason, you should add a unique event source to the registry for your application and specify a message file.