The DRVM_MAPPER_PREFERRED_GET message retrieves the device ID of the preferred audio device.
DWORD xxxMessage( UINT uDeviceID, UINT uMsg, DWORD_PTR dwParam1, DWORD_PTR dwParam2 );
Specifies the ID of the target device. See the following Remarks section for more information about how to cast this value for use with the appropriate function.
Caller sets this parameter to DRVM_MAPPER_PREFERRED_GET when it calls xxxMessage to process this device message.
Pointer to device ID. This parameter points to a DWORD variable into which the function writes the device ID of the current preferred device. The function writes the value (-1) if no device is available that qualifies as a preferred device.
Pointer to status flags. This parameter points to a DWORD variable into which the function writes the device-status flags. Only one flag bit is currently defined (for waveInMessage and waveOutMessage calls only): DRVM_MAPPER_PREFERRED_FLAGS_PREFERREDONLY. For more information, see the following Remarks section.
The xxxMessage function returns MMSYSERR_NOERROR if the message is handled successfully. Otherwise, it returns an appropriate error code.
This message is valid only for the waveInMessage, waveOutMessage and midiOutMessage functions. When the caller calls these functions with the DRVM_MAPPER_PREFERRED_GET message, the caller must first specify the device ID as WAVE_MAPPER (for waveInMessage or waveOutMessage) or MIDI_MAPPER (for midiOutMessage), and then cast this value to the appropriate handle type. For the waveInMessage, waveOutMessage, or midiOutMessage functions, the caller must cast the device ID to a handle type HWAVEIN, HWAVEOUT or HMIDIOUT, respectively. Note that if the caller supplies a valid handle instead of a device ID for this parameter, the function fails and returns error code MMSYSERR_NOSUPPORT.
The system intercepts this message and returns the appropriate value without sending the message to the device driver. For general information about system-intercepted xxxMessage functions, see System-Intercepted Device Messages.
This message provides a way to determine which device is preferred for audio functions in general, in contrast to the DRVM_MAPPER_CONSOLEVOICECOM_GET message, which determines which device is preferred specifically for voice communications.
When the DRVM_MAPPER_PREFERRED_FLAGS_PREFERREDONLY flag bit is set in the DWORD location pointed to by dwParam2, the waveIn and waveOut APIs use only the current preferred device and do not search for other available devices if the preferred device is unavailable. Note that the midiOutMessage function does not output this flag--the midiOut API always uses only the preferred device. The flag that is output by either the waveInMessage or waveOutMessage call applies to the preferred device for both the waveIn and waveOut APIs, regardless of whether the call is made to waveInMessage or waveOutMessage.
The xxxMessage functions accept this value in place of a valid device handle in order to allow an application to determine the default device ID without first having to open a device. For more information, see Accessing the Preferred Device ID.
Supported in Microsoft Windows Me and Windows 2000 and later operating systems.
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