Before playing or recording audio, you must determine the capabilities of the audio hardware present in the system. Audio capability can vary from one multimedia computer to the next; applications should not make assumptions about the audio hardware present in a given system.
The Multimedia extensions provide the following functions to determine how many devices of a certain type are available in a given system:
auxGetNumDevs
Retrieves the number of auxiliary audio devices present in the system.
midiInGetNumDevs
Retrieves the number of MIDI input devices present in the system.
midiOutGetNumDevs
Retrieves the number of MIDI output devices present in the system.
waveInGetNumDevs
Retrieves the number of waveform input devices present in the system.
waveOutGetNumDevs
Retrieves the number of waveform output devices present in the system.
Audio devices are identified by a device identifier (device ID). The device ID is determined implicitly from the number of devices present in a given system. Device IDs range from 0 to 1 less than the number of devices present. For example, if there are 2 MIDI output devices in a system, valid device IDs are 0 and 1.
Once you determine how many devices of a certain type are present in a system, you can inquire about the capabilities of each device. The Multimedia extensions provide the following functions to determine the capabilities of audio devices:
auxGetDevCaps
Retrieves the capabilities of a given auxiliary audio device.
midiInGetDevCaps
Retrieves the capabilities of a given MIDI input device.
midiOutGetDevCaps
Retrieves the capabilities of a given MIDI output device.
waveInGetDevCaps
Retrieves the capabilities of a given waveform input device.
waveOutGetDevCaps
Retrieves the capabilities of a given waveform output device.
Summary: Device-Capability Data Structures
Each of these functions takes a far pointer to a data structure the function fills with information on the capabilities of a specified device. The following are the data structures that correspond to each of the device-inquiry functions:
| Function | Data Structure |
| auxGetDevCaps | AUXCAPS |
| midiInGetDevCaps | MIDIINCAPS |
| midiOutGetDevCaps | MIDIOUTCAPS |
| waveInGetDevCaps | WAVEINCAPS |
| waveOutGetDevCaps | WAVEOUTCAPS |
All of the device capabilities data structures have the following fields:
| Field | Description |
| wMid | Specifies a manufacturer ID for the author of the device driver. |
| wPid | Specifies a product ID for the device. |
| szPname | Specifies an array of characters containing the name of the device in a null-terminated string. |
| vDriverVersion | Specifies the version number of the device driver. |
Microsoft will assign manufacturer IDs and product IDs specified by the wMid and wPid fields. The MMSYSTEM.H file contains constants for currently defined IDs.
The szPname field points to a null-terminated string containing the product name. You should use the product name to identify devices to users.
The vDriverVersion field specifies a version number for the device driver. The high-order of this field is the major version number; the low-order byte is the minor version number.