Re: [Caml-list] POSIX Threads: kill

[Xavier Leroy <xavier.leroy@inria.fr>]

> in file ocam-3.06/otherlibs/systhreads/thread_posix is this comment:
> 
> (* Thread.kill is currently not implemented due to problems with
>    cleanup handlers on several platforms *)
> 
> Is anybody working on it?

The problem is as follows.  

POSIX threads have a notion of cleanup handlers, which are functions
that are called when the thread dies, either voluntarily (via
pthread_exit) or by being cancelled by another thread (via
pthread_cancel).

On some platforms (Tru64 Unix for sure, perhaps Solaris as well),
cleanup handlers are implemented via C++ exceptions.  (It is true that
POSIX threads is a C interface, however the Tru64 C compiler
understands C++ exceptions even when compiling pure C sources.)
Namely, a cleanup handler is mapped to a try... finally construct that
just does the right thing.

The problem is that C++ exception handling is based on unwinding stack
frames one by one till a matching exception handler is found.  This
requires stack frames to adhere strictly to a particular format, and
be equipped with stack descriptors that the C++ stack unwind mechanism
understands.  But of course the stack frames used ocamlopt-generated
code do not adhere to this format, and do not come with C++ stack
descriptors.  Hence, if the "systhreads" library was using
pthread_exit and pthread_cancel, the C/C++ runtime system would try to
unwind Caml stack frames, and just crash the whole program.

> Is there a solution for linux-i386?

LinuxThreads on Linux doesn't rely on C++ exceptions, so it doesn't
suffer from the problem above.  However, LinuxThreads is being
replaced by NPTL, another, better threading library for Linux, and I
don't know how NPTL implements cleanup handlers.

The general solution is to avoid using Thread.kill.  Terminating another
thread at arbitrary times is an inherently unsafe operation: the
killed thread may be holding mutexes, for instance.  There's a good
explanation of the problems in the Java documentation:

http://java.sun.com/j2se/1.4.2/docs/guide/misc/threadPrimitiveDeprecation.html

explaining why they "deprecated" their equivalent of Thread.kill.

- Xavier Leroy

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