Re: [Quickfix-developers] Confused by possible bug in Utility.cpp / thread_spawn

[K. F. <kfr...@gm...>]

Hi List!

Ping?

On its face this looks like a bug in the QuickFIX code.  I'm assuming
quickfix-developers is the right place to raise such an issue.  Is there
a better place?


Thanks.


K. Frank


On Thu, Jan 10, 2013 at 12:36 PM, K. Frank <kfr...@gm...> wrote:
> Hello List!
>
> Background:  I am trying to build QuickFIX with mingw-w64.  To do this,
> I am starting with the windows "version" of the code, i.e.,the code
> paths that are active when _MSC_VER is defined.
>
> I am confused by what looks to me like a bug, but I probably don't
> understand it correctly.  If the bug I'm imagining were actually
> present, it would seem that QuickFIX would neither compile nor
> run on windows.
>
> Specifically the function thread_spawn is defined in Utility.cpp.
>
> The function begins:
>
>    bool thread_spawn( THREAD_START_ROUTINE func, void* var, thread_id& thread )
>
> In the windows section is the line:
>
>    result = _beginthreadex( NULL, 0, &func, var, 0, &id );
>
> It appears to me that the '&' in "&func" is an error, and shouldn't
> be there.
>
> I reason as follows:
>
> THREAD_START_ROUTINE is a typedef in Utility.h. Its windows version is:
>
>    typedef unsigned int (_stdcall THREAD_START_ROUTINE)(void *);
>
> I read this as a pointer to a function that takes a void* and returns
> an unsigned.
>
> Compare with the unix version:
>
>    extern "C" { typedef void * (THREAD_START_ROUTINE)(void *); }
>
> which I also read as a pointer to a function, but now the function
> returns a void*.
>
> Because of the '&' in "&func", a pointer to a pointer to a function
> is being passed to _beginthreadex, rather than pointer to a function.
> (recall:   result = _beginthreadex( NULL, 0, &func, var, 0, &id );)
>
> From msdn, _beginthreadex has the prototype:
>
>    uintptr_t _beginthreadex(
>       void *security,
>       unsigned stack_size,
>       unsigned ( *start_address )( void * ),
>       void *arglist,
>       unsigned initflag,
>       unsigned *thrdaddr
>    );
>
> which I read as _beginthreadex expecting a pointer to a function.
>
> Compare with the unix version.  From Utility.cpp:
>
>    if( pthread_create( &result, 0, func, var ) != 0 ) return false;
>
> Here there is no '&' on func, so pthread_create is being passed a
> pointer to a function (as expected).
>
> From the pthreads documentation, pthread_create has the prototype:
>
>    int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
>       void *(*start_routine)(void*), void *arg);
>
> which I read as taking a pointer to a function that takes a void*
> and returns a void*.
>
> So it looks to me that the unix version is correct and there is
> no type mismatch in the function call to pthread_create, but in
> the windows version there is a type mismatch in the corresponding
> call to _beginthreadex, with a pointer-to-function being expected,
> but a pointer-to-pointer-to-function being passed, with the extra
> "pointer-to" coming from the '&' in "&func".
>
> Of course, the problem with my interpretation is that the windows
> version shouldn't compile, and if one were to shut the compiler
> up by casting away the type mismatch, the program should crash.
>
> My QuickFIX source is from quickfix-1.13.3.zip, downloaded on
> Jan. 2.
>
> (I have been tweaking the code to get it to build with mingw-w64,
> and after tweaking that doesn't change this part of the code,
> mingw-w64 does give the type-mismatch error described.)
>
>
> Any help understanding what is going on here would be appreciated.
>
>
> K. Frank