LD_PRELOAD and thread local variable - c++

Question

I have a shared library (libtest.cpp) and a simple program (test.cpp). I want them to share a thread local variable gVar. The shared library is linked through LD_PRELOAD.
Here is my code for the shared library libtest.cpp:
#include<stdio.h>
__thread int gVar;
void print_gVar(){
printf("%d\n", gVar);
}
Below is the code for test.cpp.
#include<stdio.h>
__thread int gVar;
void __attribute__((weak)) print_gVar();
int main(){
gVar = 10;
print_gVar();
return 0;
}
And I use the following script to compile and run them.
g++ -g -shared -fPIC -olibtest.so libtest.cpp
g++ -g -fPIC -o test test.cpp
LD_PRELOAD=./libtest.so ./test
The expected result is 10 because the assignment in test.cpp will affect the gVar in libtest.cpp. However, I only got 0. It seems that the the gVar in libtest.cpp and the gVar in test.cpp are not linked.
I did some additional tests:
If I add __attribute__((weak)) to the declaration of gVar in any of the files, the output is still 0.
If I remove __thread from both files, then the result is 10 (successful).
If I add extern and __attribute__((weak)) to the declaration of gVar in libtest.cpp, there will be segmentation fault.
I guess there must be something wrong with LD_PRELOAD and __thread. But I cannot figure out.
Could anyone tell me how I can make it work? Thank you very much!

Answer 1

This is not possible, since thread-local-storage requires per-thread initialisation.
LD_PRELOAD will load the library even before the standard library is loaded, which messes up TLS initialisation.
Update:
Please read sections 2 and 3 of ELF Handling For Thread-Local Storage