The WODM_OPEN message requests a waveform output driver to open an instance of a specified device.
Parameters
uDeviceId
Device identifier (0, 1, 2, and so on) for the target device.
uMsg
WODM_OPEN
dwUser
Pointer to location to receive device instance identifier.
dwParam1
Pointer to a WAVEOPENDESC structure, containing the client's device handle, notification target, and instance ID.
dwParam2
Contains flags. The following flags are defined:
| Flag | Definition |
| CALLBACK_EVENT | Indicates the dwCallback member of WAVEOPENDESC is an event handle. |
| CALLBACK_FUNCTION | Indicates the dwCallback member of WAVEOPENDESC is the address of a callback function. |
| CALLBACK_TASK | Indicates the dwCallback member of WAVEOPENDESC is a task handle. |
| CALLBACK_WINDOW | Indicates the dwCallback member of WAVEOPENDESC is a window handle. |
| WAVE_FORMAT_DIRECT | Data compression/decompression operations should take place in hardware. See Comments section below. |
| WAVE_FORMAT_QUERY | The driver should indicate whether or not it supports the specified format. See comments. |
| WAVE_ALLOWSYNC | Indicates the driver should allow opening of a synchronous device. Ignored by mmdrv.dll and drvlib.lib. |
Return Value
The driver should return MMSYSERR_NOERROR if the operation succeeds. Otherwise it should return one of the MMSYSERR or WAVERR error codes defined in mmsystem.h. See waveOutOpen return values in the Win32 SDK.
Comments
A client sends the WODM_OPEN message by calling the user-mode driver's wodMessage entry point, passing the specified parameters.
Typically, user-mode drivers connect to kernel-mode drivers by calling CreateFile, specifying the MS-DOS device name of one of the kernel-mode driver's devices.
The driver assigns a device instance identifier and returns it in the location pointed to by dwUser. The driver can expect to receive this value as the dwUser input argument to all other wodMessage messages.
The driver determines the number of clients it allows to use a particular device. If a device is opened by the maximum number of clients, it returns MMSYSERR_ALLOCATED for subsequent open requests.
The WAVE_FORMAT_DIRECT flag is meant for use with a wave mapper. If the flag is set in dwParam2, the driver should not call the Audio Compression Manager to handle compression/decompression operations; the caller wants the hardware to perform these operations directly. If the hardware is not capable of performing compression/decompression operations, the driver should return MMSYSERR_NOTSUPPORTED when WAVE_FORMAT_DIRECT is set.
If the WAVE_FORMAT_QUERY flag is set in dwParam2, the driver should not open the device, but should instead determine whether it supports the format specified by the WAVEOPENDESC structure's lpFormat member. If the driver supports the requested format, it should return MMSYSERR_NOERROR. Otherwise it should return WAVERR_BADFORMAT.
If the open operation succeeds, the driver should send the client a WOM_OPEN message by calling the DriverCallback function.
For additional information, see Transferring Waveform Output Data.