get_thread_area

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

NAME
       get_thread_area,  set_thread_area - manipulate thread-local storage in-
       formation

SYNOPSIS
       #include <linux/unistd.h>

       #if defined __i386__ || defined __x86_64__
       # include <asm/ldt.h>

       int get_thread_area(struct user_desc *u_info);
       int set_thread_area(struct user_desc *u_info);

       #elif defined __m68k__

       int get_thread_area(void);
       int set_thread_area(unsigned long tp);

       #elif defined __mips__

       int set_thread_area(unsigned long addr);

       #endif

       Note: There are no glibc wrappers for these system calls; see NOTES.

DESCRIPTION
       These calls provide architecture-specific support  for  a  thread-local
       storage  implementation.  At the moment, set_thread_area() is available
       on  m68k,  MIPS,  and  x86   (both   32-bit   and   64-bit   variants);
       get_thread_area() is available on m68k and x86.

       On m68k and MIPS, set_thread_area() allows storing an arbitrary pointer
       (provided in the tp argument on m68k and in the addr argument on  MIPS)
       in  the  kernel data structure associated with the calling thread; this
       pointer can later be retrieved using get_thread_area() (see also  NOTES
       for information regarding obtaining the thread pointer on MIPS).

       On x86, Linux dedicates three global descriptor table (GDT) entries for
       thread-local storage.  For more information about the GDT, see the  In-
       tel  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 int  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;
           #ifdef __x86_64__
               unsigned int  lm:1;
           #endif
           };

       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" descrip-
       tor 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  seg-
       ment is still acceptable.

RETURN VALUE
       On x86, these system calls return 0 on success, and -1 on failure, with
       errno set appropriately.

       On MIPS  and  m68k,  set_thread_area()  always  returns  0.   On  m68k,
       get_thread_area() returns the thread area pointer value (previously set
       via set_thread_area()).

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() and get_thread_area() are 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  un-
       likely event that you want to call them directly, use syscall(2).

       arch_prctl(2)   can  interfere  with  set_thread_area()  on  x86.   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  MIPS,  the current value of the thread area pointer can be obtained
       using the instruction:

           rdhwr dest, $29

       This instruction traps and is handled by kernel.

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),  ptrace(2)  (PTRACE_GET_THREAD_AREA  and
       PTRACE_SET_THREAD_AREA)

COLOPHON
       This  page  is  part of release 5.05 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                             2020-02-09                SET_THREAD_AREA(2)