syscalls with the same name present on these architectures, but they
simply allow setting some arbibtrary 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@xxxxxxxxx>
---
man2/set_thread_area.2 | 60 +++++++++++++++++++++++++++++++++++++++++++++-----
1 file changed, 55 insertions(+), 5 deletions(-)
diff --git a/man2/set_thread_area.2 b/man2/set_thread_area.2
index ad2f064..58eb16b 100644
--- a/man2/set_thread_area.2
+++ b/man2/set_thread_area.2
@@ -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
--
2.1.4
--
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