.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH set_tid_address 2 2022-10-30 "Linux man-pages 6.03"
.SH NAME
set_tid_address \- set pointer to thread ID
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "pid_t syscall(SYS_set_tid_address, int *" tidptr );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR set_tid_address (),
necessitating the use of
.BR syscall (2).
.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.
.PP
The system call
.BR set_tid_address ()
sets the
.I clear_child_tid
value for the calling thread to
.IR tidptr .
.PP
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:
.PP
.in +4n
.EX
futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
.EE
.in
.PP
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 STANDARDS
This system call is Linux-specific.
.SH SEE ALSO
.BR clone (2),
.BR futex (2),
.BR gettid (2)