[fpc-pascal] Mozilla XPCOM

[Jonas Maebe]

On 06 Jun 2010, at 19:28, José Mejuto wrote:

> I had found the problem, after a lot of debuging and trying to
> understand the underlayed COM interface the bug is in the mozilla
> XPCOM or in the fpc undertanding of COM interfaces in Linux. Aside the
> fact that under safecall in linux fpc does not call
> "fpc_safecallcheck" which I think it should be done to test the EAX
> register for errors,

safecall is not supported (= ignored) on non-Windows platforms,  
because it is completely Windows-specific. Afaik, GCC does not support  
any safecall calling convention on Linux either.

> fpc uses the same exact code under Windows and
> Linux but mozilla's XPCOM and I think that finally GCC uses a hybird
> calling convention, it is the same as in Linux but the stack must be
> cleaned by the caller, not the callee.

That's actually the standard C calling convention, not a hybrid  
calling convention (= cdecl, as you mention below).

> So the same code to work in
> Linux (XPCOM) and in Windows:
>
> asm
>    mov eax, [TheCOMInterface];
>    mov eax,[eax]
>    push Intf;
>    push IID;
>    push ContractID;
>    push TheCOMInterface;
>    Call [eax]+16;
> {$IFDEF LINUX}
>    add esp,16;
> {$ENDIF}
>    call fpc_safecallcheck;
> end;

Are you certain when using XPCOM with GCC, it automatically transforms  
every function so that
a) the result is passed as a hidden parameter by reference
b) the actual function results becomes an error code
c) a check is inserted after every call that checks this error code

?

> cdecl that fails (Linux):
> -----------------
>
> procedure GetServiceByContractID(const aContractID: PAnsiChar;
>                                 const aIID: TGUID;
>                                 out _result); cdecl;
>
> cdecl that works as expected (Linux):
> -------------------------------------
>
> procedure GetServiceByContractID(const aContractID: PAnsiChar;
>                                 const aIID: PGUID;
>                                 out _result); cdecl;
>
> Changing the TGUID to PGUID and in the call changing the IID by @IID
> it works fine.
>
> Could this be considered a bug in fpc interfaces ?

Yes, it is always wrong to assume that "const" enforces passing by  
reference (except when using the mwpascal calling convention).


Jonas