Is getting the thread ID expensive in terms of performance?

Question

I need to access the thread ID from threads that I don\'t control (it\'s in an asynchronous callback function, and it get called from a set of different threads).

I

Answer 1

Windows stores all the thread specific information in the so called TEB. In x86 the fs register points to the start of this structure, in x64 it is the gs register.

In x86 windows the thread id is stored at FS:[0x24], which presumably should be rather cheap to access. Storing the information in thread local storage involves one extra indirection (we get the address of the TLS from the TEB), so it's basically the same as your handrolled private cache - just less work for you.