How can I read/write a thread local variable from another thread? That is, in Thread A I would like to access the variable in Thread B's thread local storage area. I know the ID of the other thread.
The variable is declared as __thread in GCC. Target platform is Linux, but independence might be nice (GCC specific is okay however).
Lacking a thread-start hook there is no way I can simply track this value at the start of each thread. All threads need to be tracked this way (not just specially started ones).
A higher level wrapper like boost thread_local_storage or using pthread keys is not an option. I require the performance of using a true __thread local variable.
FIRST ANSWER IS WRONG: One cannot use global variables for what I want to do. Each thread must have its own copy of the variable. Furthermore, those variables must be __thread variables for performance reasons (an equally efficient solution would also be okay, but I know of none). I also don't control the thread entry points, thus there is no possibility for those threads to register any kind of structure.
Thread Local is not private: Another misunderstanding about thread-local variables. These are in no way some kind of private variable for the thread. They are globally addressable memory, with the restriction that their lifetime is tied to the thread. Any function, from any thread, if given a pointer to these variables can modify them. The question above is essentially about how to get that pointer address.
I was unfortunately never able to find a way to do this.
Without some kind of thread init hook there just doesn't appear to be a way to get at that pointer (short of ASM hacks that would be platform dependent).