You are linking to the static Boost.Thread library. This means that
you've got two distinct Boost.Thread libraries loaded into your process,
one into the executable and one in the DLL.

Thanks Lars, you solve my problem.

Effectively, I was linking with the static lib.

When I was looking how to build shared libs, I found in the boost documentation that shared build should be the default. http://www.boost.org/boost-build2/doc/html/bbv2/tutorial/linkage.html

I'm under Windows, and without add link=shared to the b2 command line, the build is static.

Next was the problem to link my app with boost. It was necessary to define BOOST_ALL_DYN_LINK in boost\config\user.hpp, while the comment just above says that it should not be needed for boost_thread : "Forces all libraries [...] to be linked as dll's rather than static libraries on Microsoft Windows. [...] Note that there may be some libraries that can only [...] be dynamically linked (Boost.Threads for example), in these cases this macro has no effect."

Is something I didn't well understand ?

Anyway, it now works,

Thank you again Lars,

Sebastien Gallou