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.
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 4.10 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/.