Exception when calling a C++ DLL from Dephi XE - c++

Question

I'm trying to call a C++ DLL function that is defined like this:
int read_record (filep *fptr, int key, char *contents, int *status)
This is a widely used DLL, so I'm pretty certain the problem I'm having is how I'm calling the function.
The DLL docs have this example of how to call it
TFILETYPE *fptr; /* file pointer
char contents[80];
int status = 0;
int single = key;
if (read_record(fptr, key, card, &status)) break;
printf("%s\n", card);
Here's what I think should work, and almost does:
type
TCharArray = Array[1..100] of AnsiChar; // Function returns an array less than 100 char
var
read_record : function( var fptr: TFILETYPE;
Key: Integer;
var Contents: TCharArray; // function fills this in
var Status: Integer): Integer cdecl stdcall;
Procedure Test;
var
Contents: TCharArray;
Status: Integer;
Key: Integer;
begin
#read_record:= GetProcAddress(DLLHandle, 'read_record');
for Key := 1 to 10 do
begin
Contents[1] := #0; // shouldn't be necessary
if (read_record( fptr^, Key, Contents, Status) <> 0) OR (Status <> 0) then
ShowMessage('Error')
else
ShowMessage(Contents); // This shows the expected proper string on all 10 calls
...Other calls at this point to other functions in the DLL result in
an Exception writing to x01a.
end;
Multiple calls from Delphi XE work fine. But after that, when I call different function in the DLL that has always worked in the past, I get an exception writing to x0000001a, which I suspect means I've trashed memory or the stack.
The *fptr pointer datatype I'm using in calls to other functions in the dll, so I don't think that's the problem.
This is the first time I've tried to call a function that returns a string, so I suspect I'm not understanding something with call by reference of string arrays.
Any suggestions on how I should call this function differently to avoid what appears to be trashing of memory?

Answer 1

You have a couple of problems here.
First, the function declaration in Delphi is wrong. First, it's declared as both cdecl and stdcallat the same time. It needs to be one or the other; it can't be both simultaneously.
Also, you have an extra level of dereferencing on the fptr variable.
The declaration indicates it's a pointer:
filep *fptr
You've said in your Delphi declaration that it's a pointer:
var fptr: TFileType
But you're passing a pointer to a pointer:
fptr^
Change your call to the DLL function to
if (read_record( fptr, Key, Contents, Status) <> 0) OR (Status <> 0)
(I think your test of the results should actually be <> 0) AND (Status <> 0), but without the docs I'm not sure.)
If you do in fact need to initialize Contents before passing it to the DLL, you should use FillChar(Contents, SizeOf(Contents), #0) (or ZeroMemory) to do so, BTW.
As an additional suggestion, you can simplify your code somewhat (I've chosen stdcall as the calling convention):
var
read_record : function( var fptr: TFILETYPE;
Key: Integer;
Contents: PAnsiChar; // function fills this in
var Status: Integer): Integer stdcall;
var
Contents: AnsiString;
...
begin
SetLength(Contents, 100);
if (read_record(fptr, Key, PAnsiChar(Contents), Status)...
....
end;