.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl)
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.TH SET_TID_ADDRESS 2 2014-07-08 "Linux" "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 );
.fi
.SH DESCRIPTION
For each thread, the kernel maintains two attributes (addresses) called
.I set_child_tid
and
.IR clear_child_tid .
These two attributes contain the value NULL by default.
.TP
.I set_child_tid
If a thread is started using
.BR clone (2)
with the
.B CLONE_CHILD_SETTID
flag,
.I set_child_tid
is set to the value passed in the
.I ctid
argument of that system call.
.IP
When
.I set_child_tid
is set, the very first thing the new thread does
is to write its thread ID at this address.
.TP
.I clear_child_tid
If a thread is started using
.BR clone (2)
with the
.B CLONE_CHILD_CLEARTID
flag,
.I clear_child_tid
is set to the value passed in the
.I ctid
argument of that system call.
.LP
The system call
.BR set_tid_address ()
sets the
.I clear_child_tid
value for the calling thread to
.IR tidptr .
.LP
When a thread whose
.I 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
.I 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 performing a futex wait on the memory location.
Errors from the futex wake operation are ignored.
.SH RETURN VALUE
.BR set_tid_address ()
always returns the caller's thread ID.
.SH ERRORS
.BR set_tid_address ()
always succeeds.
.SH VERSIONS
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),
.BR gettid (2)
.SH COLOPHON
This page is part of release 4.10 of the Linux
.I 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/.