SET_THREAD_AREA
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
get_thread_area, set_thread_area - set a GDT entry for thread-local storage
SYNOPSIS
#include <linux/unistd.h>
#include <asm/ldt.h>
int get_thread_area(struct user_desc *u_info);
int set_thread_area(struct user_desc *u_info);
Note:
There are no glibc wrappers for these system calls; see NOTES.
DESCRIPTION
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.
Both of these system calls take an argument that is a pointer
to a structure of the following type:
struct user_desc {
unsigned int entry_number;
unsigned long base_addr;
unsigned int limit;
unsigned int seg_32bit:1;
unsigned int contents:2;
unsigned int read_exec_only:1;
unsigned int limit_in_pages:1;
unsigned int seg_not_present:1;
unsigned int useable:1;
};
get_thread_area()
reads the GDT entry indicated by
u_info->entry_number
and fills in the rest of the fields in
u_info.
set_thread_area()
sets a TLS entry in the GDT.
The TLS array entry set by
set_thread_area()
corresponds to the value of
u_info->entry_number
passed in by the user.
If this value is in bounds,
set_thread_area()
writes the TLS descriptor pointed to by
u_info
into the thread's TLS array.
When
set_thread_area()
is passed an
entry_number
of -1, it searches for a free TLS entry.
If
set_thread_area()
finds a free TLS entry, the value of
u_info->entry_number
is set upon return to show which entry was changed.
A
user_desc
is considered "empty" if
read_exec_only
and
seg_not_present
are set to 1 and all of the other fields are 0.
If an "empty" descriptor is passed to
set_thread_area,
the corresponding TLS entry will be cleared.
See BUGS for additional details.
Since Linux 3.19,
set_thread_area()
cannot be used to write non-present segments, 16-bit segments, or code
segments, although clearing a segment is still acceptable.
RETURN VALUE
These system calls
return 0 on success, and -1 on failure, with
errno
set appropriately.
ERRORS
- EFAULT
-
u_info is an invalid pointer.
- EINVAL
-
u_info->entry_number is out of bounds.
- ENOSYS
-
get_thread_area()
or
set_thread_area()
was invoked as a 64-bit system call.
- ESRCH
-
(set_thread_area())
A free TLS entry could not be located.
VERSIONS
set_thread_area()
first appeared in Linux 2.5.29.
get_thread_area()
first appeared in Linux 2.5.32.
CONFORMING TO
set_thread_area()
is Linux-specific and should not be used in programs that are intended
to be portable.
NOTES
Glibc does not provide wrappers for these system calls,
since they are generally intended for use only by threading libraries.
In the unlikely event that you want to call them directly, use
syscall(2).
arch_prctl(2)
can interfere with
set_thread_area().
See
arch_prctl(2)
for more details.
This is not normally a problem, as
arch_prctl(2)
is normally used only by 64-bit programs.
BUGS
On 64-bit kernels before Linux 3.19,
one of the padding bits in
user_desc,
if set, would prevent the descriptor from being considered empty (see
modify_ldt(2)).
As a result, the only reliable way to clear a TLS entry is to use
memset(3)
to zero the entire
user_desc
structure, including padding bits, and then to set the
read_exec_only
and
seg_not_present
bits.
On Linux 3.19, a
user_desc
consisting entirely of zeros except for
entry_number
will also be interpreted as a request to clear a TLS entry, but this
behaved differently on older kernels.
Prior to Linux 3.19, the DS and ES segment registers must not reference
TLS entries.
SEE ALSO
arch_prctl(2),
modify_ldt(2)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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