NAME

       get_thread_area, set_thread_area - manipulate thread-local storage information

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 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 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" 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

       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 unlikely 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 5.10 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/.