CODEWITHSUNDEEP
c++thread-safety
Stick it to THE MAN
Stick it to THE MAN

Reputation: 5701

Thread-safe variables in Linux programming

I am writing a shared library that will allow linked applications to query a resource.

The resource class is implemented with only static methods (see below). It also uses a global object (well scoped in an anonymous namespace). The reason for the global variable is that I do not want to expose users of the library to the internals of the system. I suppose I could have used a pimpl idiom, but that still does not address the issue of thread safety.

The class looks something like this:

//Header
class A
{
  public:
     static int foo();
     static double foobar();
};


// Source
namespace
{
   SomeResourceObject globvar;  // <- how can this variable be made thread safe ?
}

int A::foo(){}
double A::foobar(){}

Some of the applications using this library will be multithreaded and thus may call methods on A from different threads.

My question therefore is how to implement globvar so as to be threadsafe?

I am developing using gcc 4.4.1 on Ubuntu 9.10

Upvotes: 4

Views: 912

Answers (3)

kfmfe04
kfmfe04

Reputation: 15327

If you do not need to share globvar between threads and you aren't spawning a gazillion threads, you should also consider using thread-local storage.

The great thing about TLS is, there is no need for mutexes, so there is no blocking.

Upvotes: 0

Chris Dennett
Chris Dennett

Reputation: 22721

Wrap your objects to be operated upon in re-entrant locks wherever you access it :) There's some code in C++ here which allows you to implement a locking mechanism. Needs Boost though:

http://the-lazy-programmer.com/blog/?p=39

Seems quite cool :)

LOCK (myObject) {
    do something with myObject
}

Make sure you look at the comments to see any fixes people have made to the code.

Upvotes: 1

dirkgently
dirkgently

Reputation: 111120

How about wrapping up the globvar object in a class and providing accessors/mutators which inherently use mutexes? That ought to give you some thread safety.

Upvotes: 2

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