[fpc-pascal] TThread.Suspend under Linux

[Jonas Maebe]

On 02 Jun 2007, at 10:10, Graeme Geldenhuys wrote:

> The GUI in frozen after that. Unresponsive, doesn't repaint, etc...  I
> have to kill it with 'xkill' and click on the form.

No, you can also type "fg" to let the process continue.

> But as soon as I
> Suspend a thread, it returns me to the prompt!  Why???

Because sending pthread_kill(SIGSTOP) to a particular thread  
apparently has to freeze the entire the process on posix-compliant  
systems, and not just the target thread. Early Linuxthreads  
implementations did not implement this properly and only stopped the  
targeted thread, but this has been fixed in NPTL.

There is no portable way to suspend a thread at an arbitrary point  
under *nix, see
http://pauillac.inria.fr/~xleroy/linuxthreads/faq.html#E

(question E.4)

And the reason for that is probably the other thing that's mentioned  
there:

"Notice that thr_suspend() is inherently dangerous and prone to race  
conditions. For one thing, there is no control on where the target  
thread stops: it can very well be stopped in the middle of a critical  
section, while holding mutexes. Also, there is no guarantee on when  
the target thread will actually stop. For these reasons, you'd be  
much better off using mutexes and conditions instead. The only  
situations that really require the ability to suspend a thread are  
debuggers and some kind of garbage collectors."

Of course, since Windows provides such functionality, people  
nevertheless depend on this. But when creating cross-platform code,  
it's clearly a very bad idea (and several of the caveats mentioned  
above can of course also hold true under Windows).

I will remove the pthread_kill(SIGSTOP) from the RTL, since it  
obviously does not do what people expect it to do.


Jonas