Global variables are bad

<This post is about C++ (C# folks are saved from this pain)>

One of our devs taking care of NETCF on Nokia S60 reported that after the latest code reached their branch things are working very slow. It was observed that Garbage Collector is getting kicked in way more than it should. Investigation showed something interesting.

I added a global var gcConfig which had a fairly complex constructor and that among other things sets the various member variables like the GC trigger threshold to default value (1mb). All of these works fine on Windows variants.

However, TCPL states “It is generally best to minimize the use of global variables and in particular to limit the use of global variables requiring complicated initialization. ”. This is especially true for dlls. We tend to ignore good advice sometimes :)

For Symbian OS (S60) on device complex ctors of global objects are not run (without any warning). The members are all set to 0 (default member initialization). So in the gcConfig GC-trigger was being set to 0 instead of 1mb. The allocation byte count is compared against this value to figure out when it’s time to start a GC. Since it was 0 the collection condition is being met for every allocation and hence for every allocation request GC was being fired!!!

Actually considering that even after that the app was actually running shows that we have pretty good perf ;)

A good alternative of using global vars is as follows

 MyClass& useMC()
{
    static MyClass mc; // static ensures objects are not created on each call
    return mc;
}

MyClass& mc = useMC();

Do note that this has some multi-threading issue. See https://blogs.msdn.com/oldnewthing/archive/2004/03/08/85901.aspx.

Comments

• Anonymous
  December 17, 2009
  C# guy will still suffer it with static type or static field, or even local variables in a thread call back function(when the thread running for a long time). global issue will never be ended and hard to realize by now. Don't you think? ;)

• Anonymous
  December 17, 2009
  Aaron that is true but on the .NET platform the rules of such initialization is much more rigid