.\" Copyright (C) 2008 Michael Kerrisk <tmk.manpages@gmail.com>
.\" and Copyright 2003 Abhijit Menon-Sen <ams@wiw.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2004-05-31, added tgkill, ahu, aeb
.\" 2008-01-15 mtk -- rewrote DESCRIPTION
.\"
.TH tkill 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
tkill, tgkill \- send a signal to a thread
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.BR "#include <signal.h>" "           /* Definition of " SIG* " constants */"
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "[[deprecated]] int syscall(SYS_tkill, pid_t " tid ", int " sig );
.PP
.B #include <signal.h>
.PP
.BI "int tgkill(pid_t " tgid ", pid_t " tid ", int " sig );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR tkill (),
necessitating the use of
.BR syscall (2).
.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 to indicate the error.
.SH ERRORS
.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.
.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.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.BR tkill ()
Linux 2.4.19 / 2.5.4.
.TP
.BR tgkill ()
Linux 2.5.75,
glibc 2.30.
.SH NOTES
See the description of
.B CLONE_THREAD
in
.BR clone (2)
for an explanation of thread groups.
.SH SEE ALSO
.BR clone (2),
.BR gettid (2),
.BR kill (2),
.BR rt_sigqueueinfo (2)