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.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH SET_TID_ADDRESS 2 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
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set_tid_address \- set pointer to thread ID
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.SH SYNOPSIS
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.nf
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.B #include <linux/unistd.h>
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.PP
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.BI "long set_tid_address(int *" tidptr );
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.fi
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.PP
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.IR Note :
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There is no glibc wrapper for this system call; see NOTES.
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.SH DESCRIPTION
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For each thread, the kernel maintains two attributes (addresses) called
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.I set_child_tid
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and
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.IR clear_child_tid .
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These two attributes contain the value NULL by default.
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.TP
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.I set_child_tid
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If a thread is started using
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.BR clone (2)
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with the
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.B CLONE_CHILD_SETTID
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flag,
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.I set_child_tid
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is set to the value passed in the
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.I ctid
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argument of that system call.
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.IP
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When
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.I set_child_tid
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is set, the very first thing the new thread does
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is to write its thread ID at this address.
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.TP
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.I clear_child_tid
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If a thread is started using
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.BR clone (2)
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with the
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.B CLONE_CHILD_CLEARTID
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flag,
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.I clear_child_tid
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is set to the value passed in the
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.I ctid
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argument of that system call.
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.PP
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The system call
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.BR set_tid_address ()
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sets the
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.I clear_child_tid
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value for the calling thread to
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.IR tidptr .
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.PP
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When a thread whose
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.I clear_child_tid
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is not NULL terminates, then,
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if the thread is sharing memory with other threads,
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then 0 is written at the address specified in
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.I clear_child_tid
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and the kernel performs the following operation:
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.PP
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futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
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.PP
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The effect of this operation is to wake a single thread that
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is performing a futex wait on the memory location.
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Errors from the futex wake operation are ignored.
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.SH RETURN VALUE
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.BR set_tid_address ()
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always returns the caller's thread ID.
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.SH ERRORS
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.BR set_tid_address ()
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always succeeds.
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.SH VERSIONS
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This call is present since Linux 2.5.48.
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Details as given here are valid since Linux 2.5.49.
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.SH CONFORMING TO
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This system call is Linux-specific.
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.SH NOTES
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Glibc does not provide a wrapper for this system call; call it using
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.BR syscall (2).
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.SH SEE ALSO
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.BR clone (2),
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.BR futex (2),
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.BR gettid (2)
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.SH COLOPHON
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This page is part of release 4.15 of the Linux
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.I man-pages
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project.
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A description of the project,
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information about reporting bugs,
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and the latest version of this page,
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can be found at
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\%https://www.kernel.org/doc/man\-pages/.
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