Re: [boost] [log] how to set attributes under multi-threaded application

Subject: Re: [boost] [log] how to set attributes under multi-threaded application
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2011-08-03 14:02:31

On 08/03/2011 08:29 AM, Jinqiang Zhang wrote:
> In boost::log documentation, there's some code as following:
>
> *    typedef attrs::mutable_constant<  int>  int_constant_t;
>      boost::shared_ptr<  int_constant_t>  attr(new int_constant_t(-5));
>      lg.add_attribute("MyInteger", attr);
>      BOOST_LOG(lg)<<  "This record has MyInteger == -5";
>
>      **// Change the attribute value
>      attr->set_value(100);
>      BOOST_LOG(lg)<<  "This record has MyInteger == 100";
> *
> Now I have a situation in my multi-threading app:
> the 'attr->set_value()' could be called in different threads, so the
> following 'BOOST_LOG(lg)...' could access wrong attributes that is set by
> another thread.  If I use a mutex to protect this, it would be stupid,
> because the log line might be filtered out, that means the 'locking mutex'
> is not neccessary.

If thread synchronizaion is what bothers you, mutable constant has 
internal locking ability, you can specify mutex and lock types in the 
mutable_constant template parameters. You can find an example in the docs:

<http://boost-log.sourceforge.net/libs/log/doc/html/log/detailed/attributes.html#log.detailed.attributes.mutable_constant>

So, when internal locking is used, you don't have to externally 
synchronize access to this attribute, neither when you update its value 
nor when you write log records involving it.

On the other hand, if you simply try to tag a specific log record with 
an attribute (e.g. you want to make sure that a particular record has 
MyInteger value -5 attached) then thread-specific attributes would be a 
better option. You can add a scoped thread-specific attribute for this 
record, this will not affect log records generated by other threads and 
will not require any synchronization either. You can read more about 
scoped attributes here:

<http://boost-log.sourceforge.net/libs/log/doc/html/log/detailed/utilities.html#log.detailed.utilities.scoped_attributes>