__thread specifier doesn't work in C++ class

Question

I am trying to use the __thread specifier to create a thread local variable. This works OK in the following code:

#include <stdio.h>
#include <pthread.h>

static __thread int val;

int main()
{
  val = 10;
}

But if I try to use the __thread specifier in a class as follows:

#include <stdio.h>
#include <pthread.h>

class A
{
public:
  A();
  static __thread int val;
};

A::A()
{
  val = 10;
}

int main()
{
  A a;
}

I get the compiler error: undefined reference to 'A::val'

possible duplicate of [C++: undefined reference to static class member](http://stackoverflow.com/questions/272900/c-undefined-reference-to-static-class-member) — jogojapan, Oct 30 '12 at 13:04

score 3 · Answer 1 · answered Sep 07 '12 at 15:15

3

You've only declared the static variable; you must also define it outside the class (in just one source file, if you have multiple source files):

int __thread A::val;

answered Sep 07 '12 at 15:15

Mike Seymour

@KerrekSB: Which I'm also not familiar with :). But now I've managed to fire up the compiler, and confirm that you do need it in the definition. – Mike Seymour Sep 07 '12 at 15:20
1

@KerrekSB: Unfortunately, it is not, since __thread does not know about ctors and dtors. – PlasmaHH Sep 07 '12 at 15:26
@PlasmaHH: Are you sure, even in `g++` with `-std=c++11`? Interesting. But I think it plays the same grammatical role. – Kerrek SB Sep 07 '12 at 15:27
@KerrekSB: Yes I am (at least for gcc 4.7, don't know if there are any plans to change it). Note that in C++11 thread_local is a storage class specifier on its own, while the gcc extension is just a "special static". So "__thread static" is similar to "thread_local". – PlasmaHH Sep 10 '12 at 09:40

score 0 · Answer 2 · answered Sep 07 '12 at 15:17

0

Static variables have to defined outside the class declaration scope. Like this:

int A::val;

answered Sep 07 '12 at 15:17

user1531111

*non-integral* static variables have to be *initialized* outside the class declaration, but any type can be declared within the class. – ssube Sep 07 '12 at 19:28

score 0 · Answer 3 · edited Aug 18 '17 at 11:38

0

You should define it like:

/\*static\*/ __thread int A::val;

The __thread keyword must be ahead of int.

edited Aug 18 '17 at 11:38

dbajtr

answered Aug 18 '17 at 08:37

Yongquan

3 Answers3