A strange problem

[Curt Carpenter]

I have a unit that looks like this:

========================================
Unit WaveinSupport;

Interface

Uses Windows, MMSystem, SysUtils;

Const MAX_BUFFERS = 2;

  TWaveIn = class(TObject)
    Constructor Create(newFormHandle:HWnd; BfSize, newTotalBuffers : Integer);
    Destructor  Destroy;      Override;

    private
      pvpWaveHeader: Array [0..MAX_BUFFERS] of PWAVEHDR;

     ...

========================================

In the body of the code, the only reference to pvpWaveHeader looks like this:

for index = 0 to 1 do pvpWaveHeader [index] := ...

The upper limit on index here is always 1 regardless of the value of MAX_BUFFERS.

With the constant MAX_BUFFERS = 2, this unit works fine.  But if I change it to MAX_BUFFERS = 1, I get an error

'External: SIGSEGV', at address 72D245EC

I realize this is a very limited description of the problem, but does anyone have any idea what might be going on here?  I'm using Lazarus 1.0.2 with FPC 2.6.0 under Windows XP.

[Blaazen]

Indexing seems to be OK here.
What is the "PWAVEHDR"?
Is is object? --> Is created?
OR
Is it a pointer to memory? --> Is the memory allocated (getmem)?

EDIT: You can use View -> Debug Windows -> Call Stack
It will help you to track the bug.

[Curt Carpenter]

Hello Blaazen.

PWAVEHDR is a pointer to a record defined in MMSYSTEM.   pvpWaveHeader is an array of PWAVEHDR pointers.

In my code with MAX_BUFFERS = 1, I allocate memory for pvpWaveHeader[0] and pvpWaveHeader[1] with no problem.  The error occurs when I try to access pvpWaveHeader[1] later in my code. 

Let me try to write a short application that demonstrates the problem -- if I can re-create it that way  .

[User137]

Code: [Select]

Const MAX_BUFFERS = 2;
...
pvpWaveHeader: Array [0..MAX_BUFFERS] of PWAVEHDR;I hope you do realize, that here pvpWaveHeader has length of 3. It has indexes 0, 1, 2. Maybe you meant to write it like:

Code: [Select]

[0..MAX_BUFFERS-1]
Pascal array indexing is different from C. This:

Code: [Select]

pvpWaveHeader: Array [10..11] of PWAVEHDR;would mean that pvpWaveHeader has indexes 10 and 11. Not that it starts from 10 and has 11 starting from it.