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


       #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  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 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" 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.


       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)


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