Re: [boost] boost::shared_ptr and atomic_count_linux.hpp

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-05-29 11:00:42

From: "Jerry Napoli" <jnapoli_at_[hidden]>
> Hi,
>
> I apologize in advance if I'm sending this question to the wrong list,
> but I already tried the user's list and got no response.

I see two responses on boost-users, one from Darin and another from Lars.

> I'm using boost version 1.27.0 on an x86 linux platform and have just
> upgraded my kernel to 2.4.18.  After the upgrade, boost::shared_ptr
> fails to compile because functions such as "atomic_inc_and_test",
> "atomic_dec_and_test", etc.. which were previously defined in "atomic.h"
> are no longer defined there.  I can get around the problem by using an
> older "atomic.h" or by using pthreads, but I was wondering why those
> functions were removed from the header, or what direction development
> will take to use atomicity without going through a thread library like
> pthreads.

The dependency on asm/atomic.h in 1.27.0 was a mistake. User programs should
not depend on kernel headers; they aren't guaranteed to be available, or to
compile in C++ mode.

1.28.0 uses pthreads by default; this is safe but less efficient. You can
force asm/atomic.h to be used via #define BOOST_USE_ASM_ATOMIC_H, but this
is not recommended.

The recent atomic_count_gcc and lightweight_mutex_gcc patches (that aren't
part of the 1.28.0 release but are available from the CVS) use the atomic
operations in libstdc++, GCC 3.x's C++ standard library.

And of course you should keep in mind that in a program that doesn't use
threads, #define BOOST_DISABLE_THREADS will eliminate the thread safety
overhead in shared_ptr. Unfortunately, there is no way (I know of) to
autodetect whether the "real" pthreads library is being used (on this
platform.)