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Re: pthreads VCE: problem with destructor

> > Recommended approach when using C++ in conjunction with pthreads is to
> > NEVER use pthread_exit... you should have your code return to your
> > thread mainline (use exceptions if you choose) and simply "return"
> > from the thread mainline.

How about pthread_cancel? Are you saying that I should
never use it in a threaded C++ program for the same
reason(s) as pthread_exit?

Personally, I disagree strongly with this approach.


"Several Threads Library features and most Threads Library
 identifiers are provided as C language macros. As such,
 their definitions may not be available in other languages.
 Developers working in other languages will have to provide
 their own declarations of functions and constants, and
 features such as TRY/CATCH exception handling and POSIX
 push/pop cleanup handlers may be completely unavailable
 (although it may be possible to provide similar
 functionality using native exception handling facilities).
 Note that in this context, C++ is a non-C language;
 while the C language macros may compile successfully
 under C++, TRY/CATCH and push/pop cleanup handlers
 are not supported for C++ code. C++ code should use
 object descructors and C++ exception handlers. "

"Exception         Definition
 ================  ===================================
 pthread_cancel_e  Thread cancelation in progress
 pthread_exit_e    Thread exiting using pthread_exit()"


Ross Johnson <> on 12/19/2001
12:37:59 PM

Please respond to

Sent by:

To:   "Bossom, John" <John.Bossom@Cognos.COM>
cc:   "'Gardian, Milan'" <>,
Subject:  Re: pthreads VCE: problem with destructor

I sense a rising and ruthless desire to deal with the problem of
the exception-based versions of the library. It would certainly
be a lot easier if they weren't there, and there are some
hacks in pthread.h supporting them that really are nasty.

So ... what to do about them?

I will firstly put John's warning in the README file
and the README.NONPORTABLE file, and on the Web page.

Secondly, there is a standard C version of the library courtesy
of Thomas Pfaff's contributions. It uses setjmp/longjmp.
Does this need to be built differently to work with C++
applications (assuming they are written as John suggests they
should be)? I can try putting it through the VCE run of the
test suite as soon as I reinstall my corrupted Windows 98 machine.

Thirdly, if possible, consider phasing out all but the VC and GC
versions of the library (currently the only standard C versions).
That is, phase out the VCE, VSE, and GCE versions.

Does anyone wan't to jump up and shout - NO!!


"Bossom, John" wrote:
> <Yes - I'm not going to lurk over this one... Hi Ross - I'm still here)
> You are entering dark territory here. You are assuming that
> Operating System primitives actually cooperate with C++ primitives (i.e
> exception handling, implicit
> call of local destructors)
> This is not the case. At the time of implementation of pthread_win32
> (original version) pthread_exit on UNIX did NOT call the C++ destructors
> and perform graceful stack unwinding. pthread_exit basically did an end
> exit and locally called all registered pthread cleanup methods.
> Recommended approach when using C++ in conjunction with pthreads is to
> NEVER use pthread_exit... you should have your code return to your
> thread mainline (use exceptions if you choose) and simply "return"
> from the thread mainline.
> If you (Ross) implement this marrying of C++ stack-unwinding with
> pthread_exit, I suggest you
> document that it is not typical behavior and you can expect that if you
> your code back
> to various unix platforms that actually support pthreads natively, you
> will encounter exactly the same problem.
> -----Original Message-----
> From: Gardian, Milan []
> Sent: December 18, 2001 9:17 AM
> To: Pthreads-Win32@Sources.Redhat.Com
> Cc: Ross Johnson (E-mail)
> Subject: pthreads VCE: problem with destructor
> Hi,
> I came across a problem with pthreads VCE: destructors for objects in
> scope (automatic variables on stack) of the thread-function are not
> when using 'pthread_exit' to exit the thread (or when using any function
> that eventually throws a C++ exception (because of VCE build) using
> ptw32_throw internal function).
> To illustrate this point I created a new test case, 'exit4.c'. It uses a
> guard object to increment a shared reference count in constructor and
> decrement it in destructor. This object is then created on stack in both
> main thread (before creating the new thread) and in the thread-function
> (i.e. reference count is increased by 2).
> If we finish the thread-function by returning from it (falling off the
> destructor for both objects is called as expected (first the
> local object and then the main thread local object). This case is
> illustrated by commenting out the macro USE_PTHREAD_EXIT in exit4.c, line
> -> the test case passes.
> On the other hand if we finish the thread-function by using
> instead of falling off the end, the destructor for the thread-function
> object is NOT called (although the destructor for main thread local
> is called). This case is illustrated by compiling with the macro
> USE_PTHREAD_EXIT in exit4.c, line 11 -> the test case fails.
> C++ resource idiom (acquire in constructor, release in destructor) is
> throughout our threaded program. The described behaviour of pthreads is
> introducing possibility of a resource leak and potential deadlock. Does
> anybody have any ideas what is happening and how to resolve the problem?
> Thank you very much for your replies and help,
> Cheers,
>         Milan
> PS0: My config -> SMP 2 x PIII/500, W2k SP1, VC++ 6 SP4, Platform SDK
> June/2001, pthreads-latest-2001_12_17 (snaphost of the CVS taken on
> 17-DEC-2001 by Ross)
> PS1: Note that the reference counting need not be protected by a mutex as
> will never be accessed simultaneously by both threads due to
> using pthread_join.
> PS2: The exit4.c file
> ===================================
> /*
>  * Test for pthread_exit().
>  *
>  * Depends on API functions: pthread_create().
>  */
> #include "test.h"
> #ifdef __cplusplus
> static const int init_counter_value = 3;
> static void *ret_value = reinterpret_cast<void *>(1);
> static int counter = init_counter_value;
> class Guard
> {
>     const char * const _str;
>     int &_ref;
>     Guard &operator=(const Guard&);
>     Guard(const Guard&);
> public:
>     Guard(const char * const str, int &ref) : _str(str), _ref(ref) {
>         printf("Construct %s [%d->%d]\n", _str, _ref, _ref+++1);
>     };
>     ~Guard() {
>         printf("~Destruct %s [%d->%d]\n", _str, _ref, _ref---1);
>     };
> };
> void *
> func(void * arg)
> {
>     Guard g("func", counter);
>     pthread_exit(arg);
>     assert(0); //Never reached with pthread_exit
>     return ret_value;
> }
> #endif /*__cplusplus */
> int main(int, char **)
> {
> #ifndef __cplusplus
>     printf("Test requires C++. Skipped.\n");
> #else
>     {
>         void *ret = 0;
>         Guard g("main", counter);
>         pthread_t id;
>         assert(0 == pthread_create(&id, 0, func, ret_value));
>         assert(0 == pthread_join(id, &ret));
>         assert(ret == ret_value);
>     }
>     assert(counter == init_counter_value);
> #endif /*__cplusplus */
>     return 0;
> }
> ===================================
> PS3: Changes to the 'tests/Makefile' (diff file, use patch):
> ===================================
> --- ..\..\pthreads\tests\Makefile       Fri Oct 26 12:12:48 2001
> +++ Makefile    Tue Dec 18 14:16:00 2001
> @@ -41,7 +41,7 @@
>           mutex1.pass  mutex1n.pass  mutex1e.pass  mutex1r.pass
> mutex3.pass  \
>           condvar1.pass  condvar2.pass  condvar2_1.pass  \
>           exit1.pass  create1.pass  equal1.pass  \
> -         exit2.pass  exit3.pass  \
> +         exit2.pass  exit3.pass exit4.pass \
>           join0.pass  join1.pass  join2.pass  \
>           mutex4.pass  mutex6.pass  mutex6n.pass  mutex6e.pass
> \
>           count1.pass  once1.pass  tsd1.pass  \
> @@ -186,6 +186,7 @@
>  exit1.pass:
>  exit2.pass: create1.pass
>  exit3.pass: create1.pass
> +exit4.pass: create1.pass
>  eyal1.pass: tsd1.pass
>  inherit1.pass: join1.pass
>  join0.pass: create1.pass
> ===================================
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