Sending Commands

You use DeviceIoControl to send commands to a VxD. You must specify the previously opened device handle, control code, and input and output parameters for the call. The device handle identifies the VxD, and the control code specifies an action for the VxD to perform. In the following example, the DIOC_GETVERSION control code directs the given VxD to return version information.

HANDLE hDevice
BYTE bOutput[4];
DWORD cb;

fResult = DeviceIoControl(
    hDevice,          // device handle
    DIOC_GETVERSION,  // control code
    NULL, 0,          // input parameters
    bOutput, 4, &cb,  // output parameters
    0);
 

The input and output parameters of DeviceIoControl include the addresses and sizes of any buffers needed to pass data into or out of the VxD. Whether you use these parameters depends on how the VxD processes the control code. You supply an input buffer if the VxD requires that you pass it data for processing, and you supply an output buffer if the VxD returns the results of processing. In the previous example, only the output parameters are supplied. These include the address of the output buffer; the size, in bytes, of the buffer; and the address of the variable to receive the count of bytes actually copied to the buffer by the VxD.

Although the Win32 header files define a set of standard control codes, Windows 95 does not support these standard codes. Instead, the meaning and value of control codes in Windows 95 are specific to each VxD. Different VxDs may support different control codes.

Some VxDs support the DIOC_GETVERSION control code, which directs the VxD to return version information in the output buffer. Although the version information can have any format that helps the application determine the version of the VxD, keeping the information to 4 bytes or less is recommended. The VxD returns the version information only if you supply a buffer and specify a nonzero size for the buffer.

If you opened the VxD using the FILE_FLAG_OVERLAPPED value, you must also provide an OVERLAPPED structure when calling DeviceIoControl. This structure contains information that the VxD uses to process the control code asynchronously.