|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
SET_THREAD_AREA(2) Linux Programmer's Manual SET_THREAD_AREA(2)
get_thread_area, set_thread_area - set a GDT entry for thread-local
storage
#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.
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.
These system calls return 0 on success, and -1 on failure, with errno
set appropriately.
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.
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.
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.
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.
arch_prctl(2), modify_ldt(2)
This page is part of release 4.12 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/.
Linux 2016-10-08 SET_THREAD_AREA(2)
Pages that refer to this page: arch_prctl(2), clone(2), modify_ldt(2), ptrace(2), syscalls(2)