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 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.

SEE ALSO
       clone(2), futex(2), gettid(2)

COLOPHON
       This  page  is  part of release 4.10 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				  2014-07-08		    SET_TID_ADDRESS(2)