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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);
DESCRIPTION
       For  each  process,  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 process 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 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 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 process to tidptr.
       When  a  process whose clear_child_tid is not NULL terminates, then, if
       the process is sharing memory with other processes or 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 process 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 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 3.53 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2012-07-19                SET_TID_ADDRESS(2)