set_thread_area(2) — Linux manual page


set_thread_area(2)         System Calls Manual        set_thread_area(2)

NAME         top

       get_thread_area, set_thread_area - manipulate thread-local
       storage information

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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);


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

DESCRIPTION         top

       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;

       get_thread_area() reads the GDT entry indicated by
       u_info->entry_number and fills in the rest of the fields in

       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

       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         top

       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         top

       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         top


HISTORY         top

              Linux 2.5.29.

              Linux 2.5.32.

NOTES         top

       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         top

       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         top

       arch_prctl(2), modify_ldt(2), ptrace(2) (PTRACE_GET_THREAD_AREA

Linux man-pages (unreleased)   2024-05-02             set_thread_area(2)

Pages that refer to this page: strace(1)arch_prctl(2)clone(2)modify_ldt(2)ptrace(2)syscalls(2)