.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\" 
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" 
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.TH SET_TID_ADDRESS 2 2004-09-10 "Linux 2.6" "Linux Programmer's Manual"
.SH NAME
set_tid_address \- set pointer to thread ID
.SH SYNOPSIS
.nf
.B #include <linux/unistd.h>
.sp
.BI "long set_tid_address(int *" tidptr );
.SH DESCRIPTION
The kernel keeps for each process two values called
.I set_child_tid
and
.I clear_child_tid
that are NULL by default.
.SS set_child_tid
If a process is started using
.BR clone (2)
with the
.B CLONE_CHILD_SETTID
flag,
.I set_child_tid
is set to
.IR child_tidptr ,
the fifth parameter of that system call.
.LP
When
.I set_child_tid
is set, the very first thing the new process does
is writing its PID at this address.
.SS clear_child_tid
If a process is started using
.BR clone (2)
with the
.B CLONE_CHILD_CLEARTID
flag, 
.I clear_child_tid
is set to
.IR child_tidptr ,
the fifth parameter of that system call.
.LP
The system call
.BR set_tid_address ()
sets the
.I clear_child_tid
value for the calling process to
.IR tidptr .
.LP
When
.I 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
.I 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.
.SH "RETURN VALUE"
.BR set_tid_address ()
returns the PID of the current process.
.SH HISTORY
This call is present since Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.
.SH "CONFORMING TO"
This system call is Linux specific.
.SH "SEE ALSO"
.BR clone (2),
.BR futex (2)