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Linux 2.6 - man page for set_tid_address (linux section 2)

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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_SETTID   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 shar-
       ing 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 performing 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 3.55 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)
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