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

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

       #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.

       Linux dedicates three global descriptor table (GDT) entries for thread-
       local storage.  For more information about the GDT, see the Intel Soft-
       ware 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 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_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.

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

       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.

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

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

       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.

       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.

       arch_prctl(2), modify_ldt(2)

       This  page  is  part of release 4.10 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

Linux				  2016-10-08		    SET_THREAD_AREA(2)