boost_1_45_0/libs/math/doc/sf_and_dist/thread_safety.qbk

[section:threads Thread Safety]

The library is fully thread safe and re-entrant provided the function
and class templates in the library are instantiated with
built-in floating point types: i.e. the types `float`, `double` 
and `long double`.  

However, the library [*is not thread safe] when
used with user-defined (i.e. class type) numeric types.

The reason for the latter limitation is the need to
initialise symbolic constants using constructs such as:

   static const T coefficient_array = { ... list of values ... };

Which is always thread safe when T is a built-in floating point type, 
but not when T is a user defined type: as in this case there 
is a need for T's constructors to be run, leading to potential
race conditions.

This limitation may be addressed in a future release.

[endsect] [/section:threads Thread Safety]

[/ 
  Copyright 2006 John Maddock and Paul A. Bristow.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]