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  argu-
	      ment 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 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 3.44 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)