Passing VB Callback function to C dll - noob is stuck.
- by WaveyDavey
Callbacks in VB (from C dll).
I need to pass a vb function as a callback to a c function in a dll. I know I need to use
addressof for the function but I'm getting more and more confused as to how to do it.
Details:
The function in the dll that I'm passing the address of a callback to is defined in C as :
PaError Pa_OpenStream( PaStream** stream,
const PaStreamParameters *inputParameters,
const PaStreamParameters *outputParameters,
double sampleRate,
unsigned long framesPerBuffer,
PaStreamFlags streamFlags,
PaStreamCallback *streamCallback,
void *userData );
where the function is parameter 7, *streamCallback. The type PaStreamCallback is defines thusly:
typedef int PaStreamCallback(
const void *input, void *output,
unsigned long frameCount,
const PaStreamCallbackTimeInfo* timeInfo,
PaStreamCallbackFlags statusFlags,
void *userData );
In my vb project I have:
Private Declare Function Pa_OpenStream Lib "portaudio_x86.dll" _
( ByVal stream As IntPtr _
, ByVal inputParameters As IntPtr _
, ByVal outputParameters As PaStreamParameters _
, ByVal samprate As Double _
, ByVal fpb As Double _
, ByVal paClipoff As Long _
, ByVal patestCallBack As IntPtr _
, ByVal data As IntPtr) As Integer
(don't worry if I've mistyped some of the other parameters, I'll get to them later! Let's
concentrate on the callback for now.)
In module1.vb I have defined the callback function:
Function MyCallback( ByVal inp As Byte, _
ByVal outp As Byte, _
ByVal framecount As Long, _
ByVal pastreamcallbacktimeinfo As Byte, _
ByVal pastreamcallbackflags As Byte, _
ByVal userdata As Byte)
As Integer
' do clever things here
End Function
The external function in the dll is called with
err = Pa_OpenStream( ptr, _
nulthing, _
outputParameters, _
SAMPLE_RATE, _
FRAMES_PER_BUFFER, _
clipoff, _
AddressOf MyCallback, _
dataptr)
This is broken in the declaration of the external function - it doesn't like the type IntPtr
as a function pointer for AddressOf.
Can anyone show me how to implement passing this callback function please ?
Many thanks
David
