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__ || defined __csky__

       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, C-SKY, and x86 (both 32-bit and  64-bit  variants);
       get_thread_area() is available on m68k and x86.

       On  m68k,  MIPS  and  C-SKY,  set_thread_area()  allows  storing an arbitrary pointer (provided in the tp
       argument on m68k and in the addr argument on MIPS and C-SKY) 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 C-SKY, 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.

STANDARDS

       Linux.

HISTORY

       set_thread_area()
              Linux 2.5.29.

       get_thread_area()
              Linux 2.5.32.

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)