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set_thread_area.2: Describe set_thread_area()/get_thread_area() on m68k/MIPS 

There are system calls of the same name present on the m86k and
MIPS  architectures, but they simply allow setting some arbitrary
value which can be interpreted as a thread pointer by a threading
library.

* man2/set_thread_area.2 (.SH NAME): Rephrase in order to not
  mention GDT.
(.SH SYNOPSIS): Add declarations for MIPS and m68k.
(.SH DESCRIPTION, .SH RETURN VALUE): Add description for MIPS
  and m68k.
(.SH NOTES): Mention a way to get thread pointer on MIPS.

Signed-off-by: Eugene Syromyatnikov <evgsyr@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Eugene Syromyatnikov 2018-04-14 19:47:39 +02:00 committed by Michael Kerrisk
parent ff2c46188e
commit f7c3bc5ce6
1 changed files with 55 additions and 5 deletions
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60
man2/set_thread_area.2
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@ -8,20 +8,54 @@
.\"
.TH SET_THREAD_AREA 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
get_thread_area, set_thread_area \- set a GDT entry for thread-local storage
get_thread_area, set_thread_area \- manipulate thread-local storage information
.SH SYNOPSIS
.nf
.B #include <linux/unistd.h>
.B #include <asm/ldt.h>
.B #if defined __i386__ || defined __x86_64__
.B # include <asm/ldt.h>
.PP
.BI "int get_thread_area(struct user_desc *" u_info );
.BI "int set_thread_area(struct user_desc *" u_info );
.PP
.B #elif defined __m68k__
.PP
.B "int get_thread_area(void);
.BI "int set_thread_area(unsigned long " tp );
.PP
.B #elif defined __mips__
.PP
.BI "int set_thread_area(unsigned long " addr );
.PP
.B #endif
.fi
.PP
.IR Note :
There are no glibc wrappers for these system calls; see NOTES.
.SH DESCRIPTION
Linux dedicates three global descriptor table (GDT) entries for
These calls provide architecture-specific support for a thread-local storage
implementation.
At the moment,
.BR set_thread_area ()
is available on m68k, MIPS, and x86 (both 32-bit and 64-bit variants);
.BR get_thread_area ()
is available on m68k and x86.
.PP
On m68k and MIPS,
.BR set_thread_area ()
allows storing an arbitrary pointer (provided in the
.B tp
argument on m68k and in the
.B addr
argument on MIPS) in kernel's structure associated with the caller thread
and then retrieving it via the
.BR get_thread_area ()
syscall (see also
.I NOTES
for the information regarding obtaining thread pointer on MIPS).
.PP
On x86, Linux dedicates three global descriptor table (GDT) entries for
thread-local storage.
For more information about the GDT, see the
Intel Software Developer's Manual or the AMD Architecture Programming Manual.
@ -41,6 +75,9 @@ struct user_desc {
unsigned int limit_in_pages:1;
unsigned int seg_not_present:1;
unsigned int useable:1;
#ifdef __x86_64__
unsigned int lm:1;
#endif
};
.EE
.in
@ -93,10 +130,19 @@ Since Linux 3.19,
cannot be used to write non-present segments, 16-bit segments, or code
segments, although clearing a segment is still acceptable.
.SH RETURN VALUE
These system calls
On x86, these syscalls
return 0 on success, and \-1 on failure, with
.I errno
set appropriately.
.PP
On MIPS and m68k,
.BR set_thread_area ()
syscall always returns 0.
On m68k,
.BR get_thread_area ()
syscall returns (previously set via
.BR set_thread_area ())
thread area pointer value.
.SH ERRORS
.TP
.B EFAULT
@ -131,13 +177,17 @@ In the unlikely event that you want to call them directly, use
.PP
.BR arch_prctl (2)
can interfere with
.BR set_thread_area ().
.BR set_thread_area ()
on x86.
See
.BR arch_prctl (2)
for more details.
This is not normally a problem, as
.BR arch_prctl (2)
is normally used only by 64-bit programs.
.PP
On MIPS, in order to obtain thread area pointer, "rdhwr dest, $29" instruction
can be used; it traps and is handled by kernel.
.SH BUGS
On 64-bit kernels before Linux 3.19,
.\" commit e30ab185c490e9a9381385529e0fd32f0a399495