Windows, like practically any other mainstream multithreading operating system, provides a mechanism to allow programmers to efficiently store state on a per-thread basis. This capability is typically known as Thread Local Storage, and it’s quite handy in a number of circumstances where global variables might need to be instanced on a per-thread basis. Although the usage of TLS on Windows is fairly well documented, the implementation details of it are not so much (though there are a smattering of pieces of third party documentation floating out there).Conceptually, TLS is in principal not all that complicated (famous last words), at least from a high level. The general design is that all TLS accesses go through either a pointer or array that is present on the TEB, which is a system-defined data structure that is already instanced per thread. The “per-thread” resolution of the TEB is fairly well documented, but for the benefit of those that are unaware, the general idea is that one of the segment registers (fs on x86, gs on x64) is repurposed by the OS to point to the base address of the TEB for the current thread. This allows, say, an access to fs:[0x0] (or gs:[0x0] on x64) to always access the TEB allocated for the current thread, regardless of other threads in the address space. The TEB does really exist in the flat address space of the process (and indeed there is a field in the TEB that contains the flat virtual address of it), but the segmentation mechanism is simply used to provide a convenient way to access the TEB quickly without having to search through a list of thread IDs and TEB pointers (or other relatively slow mechanisms). On non-x86 and non-x64 architectures, the underlying mechanism by which the TEB is accessed varies, but the general theme is that there is a register of some sort which is always set to the base address of the current thread’s TEB for easy access. The TEB itself is probably one of the best-documented undocumented Windows structures, primarily because there is type information included for the debugger’s benefit in all recent ntdll and ntoskrnl.exe builds. With this information and a little disassembly work, it is not that hard to understand the implementation behind TLS.
Read more: Nynaeve Overview, Thread Local Storage, part 2: Explicit TLS, Thread Local Storage, part 3: Compiler and linker support for implicit TLS, Thread Local Storage, part 4: Accessing __declspec(thread) data, Thread Local Storage, part 5: Loader support for __declspec(thread) variables (process initialization time)
Read more: Nynaeve Overview, Thread Local Storage, part 2: Explicit TLS, Thread Local Storage, part 3: Compiler and linker support for implicit TLS, Thread Local Storage, part 4: Accessing __declspec(thread) data, Thread Local Storage, part 5: Loader support for __declspec(thread) variables (process initialization time)
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