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SET_TID_ADDRESS(2)         Linux Programmer's Manual        SET_TID_ADDRESS(2)
NAME
       set_tid_address - set pointer to thread ID
SYNOPSIS
       #include <linux/unistd.h>
       long set_tid_address(int *tidptr);
       Note: There is no glibc wrapper for this system call; see NOTES.
DESCRIPTION
       For each thread, the kernel maintains two attributes (addresses) called
       set_child_tid and clear_child_tid.  These two  attributes  contain  the
       value NULL by default.
       set_child_tid
              If  a thread is started using clone(2) with the CLONE_CHILD_SET-
              TID flag, set_child_tid is set to the value passed in  the  ctid
              argument of that system call.
              When  set_child_tid  is set, the very first thing the new thread
              does is to write its thread ID at this address.
       clear_child_tid
              If   a   thread   is   started   using   clone(2)    with    the
              CLONE_CHILD_CLEARTID  flag,  clear_child_tid is set to the value
              passed in the ctid argument of that system call.
       The system call set_tid_address() sets the  clear_child_tid  value  for
       the calling thread to tidptr.
       When  a  thread  whose clear_child_tid is not NULL terminates, then, if
       the thread is sharing memory with other threads, then 0 is  written  at
       the  address  specified  in clear_child_tid and the kernel performs the
       following operation:
           futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
       The effect of this operation is to wake a single thread  that  is  per-
       forming  a  futex  wait  on the memory location.  Errors from the futex
       wake operation are ignored.
RETURN VALUE
       set_tid_address() always returns the caller's thread ID.
ERRORS
       set_tid_address() always succeeds.
VERSIONS
       This call is present since Linux 2.5.48.  Details  as  given  here  are
       valid since Linux 2.5.49.
CONFORMING TO
       This system call is Linux-specific.
NOTES
       Glibc  does  not  provide a wrapper for this system call; call it using
       syscall(2).
SEE ALSO
       clone(2), futex(2), gettid(2)
COLOPHON
       This page is part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.
Linux                             2017-09-15                SET_TID_ADDRESS(2)