Re: What is the purpose of %fs?

Josef Moellers wrote:
> While trying to analyse a problem with snmpd, I stumbled across
> instructions in the tls/libc that reference %fs. Where could I find
> information about the contents of that segment?
>
> I've single-stepped through a small printf("Hello, world")-program but
> have found no instruction which populates this segment, yet it seems to
> exist and contains data.

It provides an addressing space for thread-local storage ("TLS").
See __NR_set_thread_area, __NR_get_thread_area in /usr/include/asm/unistd.h
and the code for clone() in the glibc sources in
glibc-2.3.5/sysdeps/unix/sysv/linux/i386/clone.S .
Note that clone() takes 7 arguments as glibc routine,
but only 5 as a system call. One of the arguments is
the address to use for the base of %fs (or %gs, depending on machine.)
Note that glibc considers this to be none of your business.

--
.

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