From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-09-23 11:30:31
Anthony Williams wrote:
>> I am trying to change my call_once implementation for boost.thread,
>> so it doesn't require windows.h to be included. In doing so, I tried
>> to use boost/detail/interlocked.hpp, and discovered that it doesn't
>> work on gcc-mingw-4.0.1, since the imported functions are declared
>> in boost::detail namespace, and the macros don't qualify the names.
>>
>> Attached is a patch for the file that changes the macros to refer to
>> the imported functions by fully qualified names.
>>
>> Please can someone check and commit this for me.
Apologies for not being able to respond earlier.
I have some (very) minor comments about your changes. Qualifying
_Interlocked* with :: appears to work as intended - although I haven't
tested it extensively - but looks very odd for someone reading the code;
these identifiers are reserved and there shouldn't be any namespace clashes,
so it might be best to leave them unqualified.
BOOST_INTERLOCKED_READ doesn't really belong in interlocked.hpp (macro vs
inline aside). The aim of this header is only to provide the Interlocked*
functions as specified and documented by Microsoft without including
<windows.h>; it is not meant to introduce new unspecified and undocumented
functionality.
Finally, I believe that for correct double-checked locking you only need a
load with acquire barrier on the fast path - which maps to an ordinary load
on x86(-64) and to ld.acq on IA-64 - and by using a fully locked cmpxchg
you're introducing a performance penalty (the philosophical debate of
whether InterlockedCompareExchange is guaranteed to enforce memory ordering
when the comparison fails aside.)