set_thread_area(2)            System Calls Manual           set_thread_area(2)

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)

Linux man-pages 6.7               2023-10-31                set_thread_area(2)