SYNOPSIS
       #include <linux/unistd.h>

       long set_tid_address(int *tidptr);

DESCRIPTION
       The  kernel  keeps for each process two values called set_child_tid and
       clear_child_tid that are NULL by default.

   set_child_tid
       if a process is started  using  clone(2)  with  the  clone_child_settid
       flag,  set_child_tid is set to child_tidptr, the fifth argument of that
       system call.

       When set_child_tid is set, the very first thing the new process does is
       writing its PID at this address.

   clear_child_tid
       if  a  process  is started using clone(2) with the clone_child_cleartid
       flag, clear_child_tid is set to child_tidptr,  the  fifth  argument  of
       that system call.

       The  system  call  set_tid_address() sets the clear_child_tid value for
       the calling process to tidptr.

       When clear_child_tid is set, and the process exits, and the process was
       sharing  memory  with  other processes or threads, then 0 is written at
       this address, and a futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL,  0);
       call  is done.  (That is, wake a single process waiting on this futex.)
       Errors are ignored.

RETURN VALUE
       set_tid_address() always returns the PID of the calling process.

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.

SEE ALSO
       clone(2), futex(2)

COLOPHON
       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can