CGAL users discussion list

[Sylvain Pion <>]

 wrote:
> hi,
>
> i am using CGAL with exact arithmetic withGMP/MPFR.
>
> I am running MSVC2005. To install CGAL i used CGAL-3.4-Setup.exe.
> I compile with Multithreaded DLL (debug) runtime libs.
>
> My app binds agains (for debug version) among other libs: 
>
> gmp-vc80-mt-gd.lib 
> mpfr-vc80-mt-gd.lib 
> CGAL_Core-vc80-mt-gd.lib 
> CGAL-vc80-mt-gd.lib 
>
> To speed up computation i changed my algorithm to use more than one thread.
>
> i found out, that copying a Polygon_2 as an input for the threads is causing 
> my app to crash. it looks like:
> Polygon_2 a;
>
> // ... init a ...
>
> Polygon_2 b_thread1;
> Polygon_2 b_thread2;
> Polygon_2 b_thread3;
> Polygon_2 b_thread4;
> b_thread1 = a;
> b_thread2 = a;
> b_thread3 = a;
> b_thread4 = a;
>
> // ... start 4 threads each using b_threadX
>
> // CRASH
>
> when i do a workaround like this to copy "a":
>
> b_trheadX.push_back( Point_2( toDouble( a[0].x() ), toDouble( a[0].y() ) ) );
>
> there is no crash.
>
> It looks like assigining:
> b_threadX = a;
> does not realy do a seperate copy in memory and when using multiple threads
> there happen evil things.
>
> is there a clean way to do a copy of a Polygon_2  without that workaround? i
> use exact numbers and toDouble() is rounding.
>
> may it be a problem with CGAL-3.4-Setup.exe? maybe they are compiled not for
> multithreading? any idea?

Which kernel are you using ?

Cartesian_kernel and Exact_predicates_exact_constructions_kernel both
use a reference counting mechanism which is not thread-safe.
In comparison, Simple_cartesian and 
Exact_predicates_inexact_constructions_kernel
should both be fine.

--
Sylvain Pion
INRIA Sophia-Antipolis
Geometrica Project-Team
CGAL, http://cgal.org/