This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The kill() function shall send a signal to a process or a group of
processes specified by pid. The signal to be sent is specified by
sig and is either one from the list given in <signal.h> or 0. If sig
is 0 (the null signal), error checking is performed but no signal is
actually sent. The null signal can be used to check the validity of
For a process to have permission to send a signal to a process
designated by pid, unless the sending process has appropriate
privileges, the real or effective user ID of the sending process
shall match the real or saved set-user-ID of the receiving process.
If pid is greater than 0, sig shall be sent to the process whose
process ID is equal to pid.
If pid is 0, sig shall be sent to all processes (excluding an
unspecified set of system processes) whose process group ID is equal
to the process group ID of the sender, and for which the process has
permission to send a signal.
If pid is −1, sig shall be sent to all processes (excluding an
unspecified set of system processes) for which the process has
permission to send that signal.
If pid is negative, but not −1, sig shall be sent to all processes
(excluding an unspecified set of system processes) whose process
group ID is equal to the absolute value of pid, and for which the
process has permission to send a signal.
If the value of pid causes sig to be generated for the sending
process, and if sig is not blocked for the calling thread and if no
other thread has sig unblocked or is waiting in a sigwait() function
for sig, either sig or at least one pending unblocked signal shall be
delivered to the sending thread before kill() returns.
The user ID tests described above shall not be applied when sending
SIGCONT to a process that is a member of the same session as the
An implementation that provides extended security controls may impose
further implementation-defined restrictions on the sending of
signals, including the null signal. In particular, the system may
deny the existence of some or all of the processes specified by pid.
The kill() function is successful if the process has permission to
send sig to any of the processes specified by pid. If kill() fails,
no signal shall be sent.
The kill() function shall fail if:
EINVAL The value of the sig argument is an invalid or unsupported
EPERM The process does not have permission to send the signal to any
ESRCH No process or process group can be found corresponding to that
specified by pid.
The following sections are informative.
The semantics for permission checking for kill() differed between
System V and most other implementations, such as Version 7 or 4.3
BSD. The semantics chosen for this volume of POSIX.1‐2008 agree with
System V. Specifically, a set-user-ID process cannot protect itself
against signals (or at least not against SIGKILL) unless it changes
its real user ID. This choice allows the user who starts an
application to send it signals even if it changes its effective user
ID. The other semantics give more power to an application that wants
to protect itself from the user who ran it.
Some implementations provide semantic extensions to the kill()
function when the absolute value of pid is greater than some maximum,
or otherwise special, value. Negative values are a flag to kill().
Since most implementations return [ESRCH] in this case, this behavior
is not included in this volume of POSIX.1‐2008, although a conforming
implementation could provide such an extension.
The unspecified processes to which a signal cannot be sent may
include the scheduler or init.
There was initially strong sentiment to specify that, if pid
specifies that a signal be sent to the calling process and that
signal is not blocked, that signal would be delivered before kill()
returns. This would permit a process to call kill() and be guaranteed
that the call never return. However, historical implementations that
provide only the signal() function make only the weaker guarantee in
this volume of POSIX.1‐2008, because they only deliver one signal
each time a process enters the kernel. Modifications to such
implementations to support the sigaction() function generally require
entry to the kernel following return from a signal-catching function,
in order to restore the signal mask. Such modifications have the
effect of satisfying the stronger requirement, at least when
sigaction() is used, but not necessarily when signal() is used. The
standard developers considered making the stronger requirement except
when signal() is used, but felt this would be unnecessarily complex.
Implementors are encouraged to meet the stronger requirement whenever
possible. In practice, the weaker requirement is the same, except in
the rare case when two signals arrive during a very short window.
This reasoning also applies to a similar requirement for
In 4.2 BSD, the SIGCONT signal can be sent to any descendant process
regardless of user-ID security checks. This allows a job control
shell to continue a job even if processes in the job have altered
their user IDs (as in the su command). In keeping with the addition
of the concept of sessions, similar functionality is provided by
allowing the SIGCONT signal to be sent to any process in the same
session regardless of user ID security checks. This is less
restrictive than BSD in the sense that ancestor processes (in the
same session) can now be the recipient. It is more restrictive than
BSD in the sense that descendant processes that form new sessions are
now subject to the user ID checks. A similar relaxation of security
is not necessary for the other job control signals since those
signals are typically sent by the terminal driver in recognition of
special characters being typed; the terminal driver bypasses all
In secure implementations, a process may be restricted from sending a
signal to a process having a different security label. In order to
prevent the existence or nonexistence of a process from being used as
a covert channel, such processes should appear nonexistent to the
sender; that is, [ESRCH] should be returned, rather than [EPERM], if
pid refers only to such processes.
Existing implementations vary on the result of a kill() with pid
indicating an inactive process (a terminated process that has not
been waited for by its parent). Some indicate success on such a call
(subject to permission checking), while others give an error of
[ESRCH]. Since the definition of process lifetime in this volume of
POSIX.1‐2008 covers inactive processes, the [ESRCH] error as
described is inappropriate in this case. In particular, this means
that an application cannot have a parent process check for
termination of a particular child with kill(). (Usually this is done
with the null signal; this can be done reliably with waitpid().)
There is some belief that the name kill() is misleading, since the
function is not always intended to cause process termination.
However, the name is common to all historical implementations, and
any change would be in conflict with the goal of minimal changes to
existing application code.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 KILL(3P)