NAME

       get_thread_area, set_thread_area - manipulate thread-local storage information

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/syscall.h>     /* Definition of SYS_* constants */
       #include <unistd.h>

       #if defined __i386__ || defined __x86_64__
       # include <asm/ldt.h>        /* Definition of struct user_desc */

       int syscall(SYS_get_thread_area, struct user_desc *u_info);
       int syscall(SYS_set_thread_area, struct user_desc *u_info);

       #elif defined __m68k__

       int syscall(SYS_get_thread_area);
       int syscall(SYS_set_thread_area, unsigned long tp);

       #elif defined __mips__

       int syscall(SYS_set_thread_area, unsigned long addr);

       #endif

       Note:  glibc  provides  no  wrappers  for  these  system  calls,  necessitating the use of
       syscall(2).

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 to
       indicate the error.

       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.

STANDARDS

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

NOTES

       These system calls are generally intended for use only by threading libraries.

       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)