Re: [boost] [threads] namespace & externs

Subject: Re: [boost] [threads] namespace & externs
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2011-01-27 04:20:16

Scot Shinderman <scot_at_[hidden]> writes:

> Anthony Williams <anthony.ajw <at> gmail.com> writes:
>
>> 
>> scot shinderman <scot <at> imageworks.com> writes:
>> 
>> >     In boost/thread/src/pthread/thread.cpp we see extern "C" {}
>> > around code such as tls_destructor.  Is this needed so it
>> > interfaces w/ native pthread local storage?
>> 
>> Yes.
>> 
>> >      When a vendor program is built against a specific version
>> > of boost it usually becomes necessary to isolate other users
>> > via a namespace.  However, anything with extern "C"
>> > style defeats this.  Aside from explicit linking to native c
>> > api's are there reasons for this; also what's considered the best
>> > style to isolate different versions -- currently we modify every header
>> > with a version-ed namespace wrapper.
>> 
>> extern "C" is only used where it is necessary (at least, that is the
>> intent). If you need the extern "C" stuff to be versioned then you'll
>> have to rename it. Renaming the namespace seems a sensible option for
>> the rest of the code.
>
> A little confused by this last part -- if the routine tls_destructor can be
> renamed something i.e. tls_destructor_v2 then why does it need to be extern "C"
> at all (why wouldn't the name-mangled version work as well)?   If the pthread
> library cannot find something called tls_destructor what will occur?     So far
> in small tests we haven't seen problems;  with our vendor's version of boost
> however thrown into the mix something stills seems awry especially during thread
> tear down...

In this case, extern "C" is not used for the name mangling, but for the
calling convention. pthread_key_create requires a pointer to an extern
"C" function for the destructor function. Passing a pointer to a normal
C++ function is not guaranteed to be portable.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976