SET_THREAD_AREA(2)        Linux Programmer's Manual       SET_THREAD_AREA(2)

NAME         top

       get_thread_area,  set_thread_area  - set a GDT entry for thread-local

SYNOPSIS         top

       #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         top

       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

       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_num‐
       ber 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 clear‐
       ing a segment is still acceptable.

RETURN VALUE         top

       These system calls return 0 on success, and -1 on failure, with errno
       set appropriately.

ERRORS         top

       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         top

       set_thread_area() first appeared in Linux 2.5.29.  get_thread_area()
       first appeared in Linux 2.5.32.

CONFORMING TO         top

       set_thread_area() is Linux-specific and should not be used in
       programs that are intended to be portable.

NOTES         top

       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         top

       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

       Prior to Linux 3.19, the DS and ES segment registers must not
       reference TLS entries.

SEE ALSO         top

       arch_prctl(2), modify_ldt(2)

COLOPHON         top

       This page is part of release 4.16 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

Linux                            2017-09-15               SET_THREAD_AREA(2)

Pages that refer to this page: strace(1)arch_prctl(2)clone(2)modify_ldt(2)ptrace(2)syscalls(2)