[fpc-pascal] writeln: FPC runtime race conditions in external C threads

[Michael Van Canneyt]

Basically, any heap memory access and standard pascal I/O are forbidden in
threads started outside FPC, because the necessary thread structures are not
initialized correctly.

Michael.

On Mon, 12 Jul 2010, Alexander Grau wrote:

> Hello,
>
> While using an external C framework (Mac OS X, discrecording OS framework, 
> FPC 2.4), I noticed that my FPC console app crashes when I use writeln's in 
> both C threads (callbacks).
>
> Example:  Two FPC functions that are called from an external C library 
> (c_callback1 and c_callback2) - callback2 is called within callback1 in 
> another C thread (other than the FPC main thread).
>
>
> // called from external C framework within the FPC main thread
> c_callback1; cdecl;
> begin
> writeln('callback1begin');
> // c_ callback2 is called here from another C thread
> writeln('callback1end');   <==   my FPC console app crashes here ( 
> "EInOutError - Disk Full")
> end;
>
>
> // called from external C framework in another C thread
> c_callback2; cdecl;
> begin
> writeln('callback2begin');
> writeln('callback2end');
> end;
>
> An unhandled exception occurred at $000669E3 :
> EInOutError : Disk Full
> $000669E3  CALLBACK1,  line xxx
>
>
> Question:    Why isn't it safe to call writeln's in non-FPC threads? Are 
> there any other FPC runtime functions I should not call within non-FPC 
> threads?
>
> I'm going to make an FPC wiki page ('Threading Pitfalls')  that will discuss 
> these and other issues related to threading and I welcome all explanations on 
> this so I can better understand this problem! :-)
>
> Thanks,
> Alexander
>
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