.\" Copyright (C) 2008 Michael Kerrisk <tmk.manpages@gmail.com>

.\" and Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

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.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

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.\" 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.\" manual page may be incorrect or out-of-date.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,

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.\"

.\" 2004-05-31, added tgkill, ahu, aeb

.\" 2008-01-15 mtk -- rewrote DESCRIPTION

.\"

.TH TKILL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

tkill, tgkill \- send a signal to a thread

.SH SYNOPSIS

.nf

.BI "int tkill(int " tid ", int " sig );

.PP

.BI "int tgkill(int " tgid ", int " tid ", int " sig );

.fi

.PP

.IR Note :

There are no glibc wrappers for these system calls; see NOTES.

.SH DESCRIPTION

.BR tgkill ()

sends the signal

.I sig

to the thread with the thread ID

.I tid

in the thread group

.IR tgid .

(By contrast,

.BR kill (2)

can be used to send a signal only to a process (i.e., thread group)

as a whole, and the signal will be delivered to an arbitrary

thread within that process.)

.PP

.BR tkill ()

is an obsolete predecessor to

.BR tgkill ().

It allows only the target thread ID to be specified,

which may result in the wrong thread being signaled if a thread

terminates and its thread ID is recycled.

Avoid using this system call.

.\" FIXME Maybe say something about the following:

.\" http://sourceware.org/bugzilla/show_bug.cgi?id=12889

.\"

.\" Quoting Rich Felker <bugdal@aerifal.cx>:

.\"

.\" There is a race condition in pthread_kill: it is possible that,

.\" between the time pthread_kill reads the pid/tid from the target

.\" thread descriptor and the time it makes the tgkill syscall,

.\" the target thread terminates and the same tid gets assigned

.\" to a new thread in the same process.

.\"

.\" (The tgkill syscall was designed to eliminate a similar race

.\" condition in tkill, but it only succeeded in eliminating races

.\" where the tid gets reused in a different process, and does not

.\" help if the same tid gets assigned to a new thread in the

.\" same process.)

.\"

.\" The only solution I can see is to introduce a mutex that ensures

.\" that a thread cannot exit while pthread_kill is being called on it.

.\"

.\" Note that in most real-world situations, like almost all race

.\" conditions, this one will be extremely rare. To make it

.\" measurable, one could exhaust all but 1-2 available pid values,

.\" possibly by lowering the max pid parameter in /proc, forcing

.\" the same tid to be reused rapidly.

.PP

These are the raw system call interfaces, meant for internal

thread library use.

.SH RETURN VALUE

On success, zero is returned.

On error, \-1 is returned, and \fIerrno\fP

is set appropriately.

.SH ERRORS

.TP

.B EINVAL

An invalid thread ID, thread group ID, or signal was specified.

.TP

.B EPERM

Permission denied.

For the required permissions, see

.BR kill (2).

.TP

.B ESRCH

No process with the specified thread ID (and thread group ID) exists.

.TP

.B EAGAIN

The

.B RLIMIT_SIGPENDING

resource limit was reached and

.I sig

is a real-time signal.

.TP

.B EAGAIN

Insufficient kernel memory was available and

.I sig

is a real-time signal.

.SH VERSIONS

.BR tkill ()

is supported since Linux 2.4.19 / 2.5.4.

.BR tgkill ()

was added in Linux 2.5.75.

.SH CONFORMING TO

.BR tkill ()

and

.BR tgkill ()

are Linux-specific and should not be used

in programs that are intended to be portable.

.SH NOTES

See the description of

.B CLONE_THREAD

in

.BR clone (2)

for an explanation of thread groups.

.PP

Glibc does not provide wrappers for these system calls; call them using

.BR syscall (2).

.SH SEE ALSO

.BR clone (2),

.BR gettid (2),

.BR kill (2),

.BR rt_sigqueueinfo (2)

.SH COLOPHON

This page is part of release 4.15 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/.