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SET_TID_ADDRESS(2)         Linux Programmer's ManualSET_TID_ADDRESS(2)NAME       set_tid_address - set pointer to thread IDSYNOPSIS       #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 usingclone(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   usingclone(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 usingsyscall(2).SEE ALSOclone(2),futex(2),gettid(2)COLOPHON       This page is part of release 5.05 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                             2017-09-15SET_TID_ADDRESS(2)