Sending Output to the Debugger
User-mode and kernel-mode drivers use different routines to send output to the debugger.
The OutputDebugString routine sends a null-terminated string to the debugger of the calling process. In a user-mode driver, OutputDebugString displays the string in the Debugger Command window. If a debugger is not running, this routine has no effect. OutputDebugString does not support the variable arguments of a printf formatted string.
The prototype for this routine is as follows:
VOID OutputDebugString( LPCTSTR lpOutputString );
This routine can be called by any user-mode driver that includes the windows.h header. For complete documentation of this routine and other user-mode routines useful for debugging, see the Microsoft Windows SDK.
The DbgPrint routine displays output in the debugger window. This routine supports the basic printf format parameters. Only kernel-mode drivers can call DbgPrint.
The DbgPrintEx routine is similar to DbgPrint, but it allows you to "tag" your messages. When running the debugger, you can permit only those messages with certain tags to be sent. This allows you to view only those messages that you are interested in. For details, see Reading and Filtering Debugging Messages.
The KdPrint and KdPrintEx macros are identical to DbgPrint and DbgPrintEx, respectively, when compiled in the checked build environment. When compiled in the free build environment, they have no effect.