Can someone help me with an example for an existing wrapper DLL which uses function pointer with callback function

[Neon_Light]

40 minutes ago, ShaunR said:

I think he's alluding to memory alignment/packing but getting confused with unions.

Yes I think that was it, I could not find the correct words for it straight away...

[Rolf Kalbermatter]

2 hours ago, Neon_Light said:

So I can give the adres of handle to testDataBlock.dataRecords, like this:

 

and thats it ? 

 

when I try the code above I get:

a value of type "DataArrayHdl*" cannot be assigned to an antity of type "DataArrayHdl"

in the line: 

testDataBlock.DataRecords = &handle;

Can anyone tell me what I might be doing wrong ?

Please, follow a C programming course first and work through it until you get to the advanced topics including memory pointers! This is not LabVIEW but C programming.

You are missing very fundamental pointer knowledge!

testDataBlock.DataRecords = (DataArrayHdl)handle;

Alternatively you can write the code directly like this.

    size_t len = offsetof(DataArrayRec, elms) + size * sizeof(DataRecord)
    if (!testDataBlock.dataRecords)
    {
        testDataBlock.dataRecords = (DataArrayHdl)DSNewHandle(len);
        if (!testDataBlock.dataRecords)
            err = mFullErr;
    }
    else
    {
        err = DSSetHandleSize((UHandle)testDataBlock.dataRecords, len);
    }

And you need to do that every time again.

testDataBlock is on the stack, this stack gets invalidated every time you leave your function. That is why we do a DSDisposeHandle() before leaving the function, to deallocate the previously allocated handle before the stack is invalidated and the handle lost forever.