Date: Tue, 15 Jan 2013 15:54:03 -0500 From: Trent Nelson <trent@snakebite.org> To: <freebsd-hackers@freebsd.org> Subject: Getting the current thread ID without a syscall? Message-ID: <20130115205403.GA52904@snakebite.org>
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Howdy,
I have an unusual requirement: I need to get the current thread ID
in as few instructions as possible. On Windows, I managed to come
up with this glorious hack:
#ifdef WITH_INTRINSICS
# ifdef MS_WINDOWS
# include <intrin.h>
# if defined(MS_WIN64)
# pragma intrinsic(__readgsdword)
# define _Py_get_current_process_id() (__readgsdword(0x40))
# define _Py_get_current_thread_id() (__readgsdword(0x48))
# elif defined(MS_WIN32)
# pragma intrinsic(__readfsdword)
# define _Py_get_current_process_id() (__readfsdword(0x20))
# define _Py_get_current_thread_id() (__readfsdword(0x24))
That exploits the fact that Windows uses the FS/GS registers to
store thread/process metadata. Could I use a similar approach on
FreeBSD to get the thread ID without the need for syscalls?
(I technically don't need the thread ID, I just need to get some
form of unique identifier for the current thread such that I can
compare it to a known global value that's been set to the "main
thread", in order to determine if I'm currently that thread or not.
As long as it's unique for each thread, and static for the lifetime
of the thread, that's fine.)
The "am I the main thread?" comparison is made every ~50-100 opcodes,
which is why it needs to have the lowest overhead possible.
Regards,
Trent.
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