1KILL(3P) POSIX Programmer's Manual KILL(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 kill — send a signal to a process or a group of processes
14
16 #include <signal.h>
17 int kill(pid_t pid, int sig);
19
21 The kill() function shall send a signal to a process or a group of pro‐
22 cesses specified by pid. The signal to be sent is specified by sig and
23 is either one from the list given in <signal.h> or 0. If sig is 0 (the
24 null signal), error checking is performed but no signal is actually
25 sent. The null signal can be used to check the validity of pid.
26 For a process to have permission to send a signal to a process desig‐
28 nated by pid, unless the sending process has appropriate privileges,
29 the real or effective user ID of the sending process shall match the
30 real or saved set-user-ID of the receiving process.
31 If pid is greater than 0, sig shall be sent to the process whose
33 process ID is equal to pid.
34 If pid is 0, sig shall be sent to all processes (excluding an unspeci‐
36 fied set of system processes) whose process group ID is equal to the
37 process group ID of the sender, and for which the process has permis‐
38 sion to send a signal.
39 If pid is −1, sig shall be sent to all processes (excluding an unspeci‐
41 fied set of system processes) for which the process has permission to
42 send that signal.
43 If pid is negative, but not −1, sig shall be sent to all processes
45 (excluding an unspecified set of system processes) whose process group
46 ID is equal to the absolute value of pid, and for which the process has
47 permission to send a signal.
48 If the value of pid causes sig to be generated for the sending process,
50 and if sig is not blocked for the calling thread and if no other thread
51 has sig unblocked or is waiting in a sigwait() function for sig, either
52 sig or at least one pending unblocked signal shall be delivered to the
53 sending thread before kill() returns.
54 The user ID tests described above shall not be applied when sending
56 SIGCONT to a process that is a member of the same session as the send‐
57 ing process.
58 An implementation that provides extended security controls may impose
60 further implementation-defined restrictions on the sending of signals,
61 including the null signal. In particular, the system may deny the exis‐
62 tence of some or all of the processes specified by pid.
63 The kill() function is successful if the process has permission to send
65 sig to any of the processes specified by pid. If kill() fails, no sig‐
66 nal shall be sent.
67
69 Upon successful completion, 0 shall be returned. Otherwise, −1 shall be
70 returned and errno set to indicate the error.
71
73 The kill() function shall fail if:
74 EINVAL The value of the sig argument is an invalid or unsupported sig‐
76 nal number.
77 EPERM The process does not have permission to send the signal to any
79 receiving process.
80 ESRCH No process or process group can be found corresponding to that
82 specified by pid.
83 The following sections are informative.
85
87 None.
88
90 None.
91
93 The semantics for permission checking for kill() differed between Sys‐
94 tem V and most other implementations, such as Version 7 or 4.3 BSD. The
95 semantics chosen for this volume of POSIX.1‐2008 agree with System V.
96 Specifically, a set-user-ID process cannot protect itself against sig‐
97 nals (or at least not against SIGKILL) unless it changes its real user
98 ID. This choice allows the user who starts an application to send it
99 signals even if it changes its effective user ID. The other semantics
100 give more power to an application that wants to protect itself from the
101 user who ran it.
102 Some implementations provide semantic extensions to the kill() function
104 when the absolute value of pid is greater than some maximum, or other‐
105 wise special, value. Negative values are a flag to kill(). Since most
106 implementations return [ESRCH] in this case, this behavior is not
107 included in this volume of POSIX.1‐2008, although a conforming imple‐
108 mentation could provide such an extension.
109 The unspecified processes to which a signal cannot be sent may include
111 the scheduler or init.
112 There was initially strong sentiment to specify that, if pid specifies
114 that a signal be sent to the calling process and that signal is not
115 blocked, that signal would be delivered before kill() returns. This
116 would permit a process to call kill() and be guaranteed that the call
117 never return. However, historical implementations that provide only the
118 signal() function make only the weaker guarantee in this volume of
119 POSIX.1‐2008, because they only deliver one signal each time a process
120 enters the kernel. Modifications to such implementations to support
121 the sigaction() function generally require entry to the kernel follow‐
122 ing return from a signal-catching function, in order to restore the
123 signal mask. Such modifications have the effect of satisfying the
124 stronger requirement, at least when sigaction() is used, but not neces‐
125 sarily when signal() is used. The standard developers considered making
126 the stronger requirement except when signal() is used, but felt this
127 would be unnecessarily complex. Implementors are encouraged to meet the
128 stronger requirement whenever possible. In practice, the weaker
129 requirement is the same, except in the rare case when two signals
130 arrive during a very short window. This reasoning also applies to a
131 similar requirement for sigprocmask().
132 In 4.2 BSD, the SIGCONT signal can be sent to any descendant process
134 regardless of user-ID security checks. This allows a job control shell
135 to continue a job even if processes in the job have altered their user
136 IDs (as in the su command). In keeping with the addition of the concept
137 of sessions, similar functionality is provided by allowing the SIGCONT
138 signal to be sent to any process in the same session regardless of user
139 ID security checks. This is less restrictive than BSD in the sense that
140 ancestor processes (in the same session) can now be the recipient. It
141 is more restrictive than BSD in the sense that descendant processes
142 that form new sessions are now subject to the user ID checks. A similar
143 relaxation of security is not necessary for the other job control sig‐
144 nals since those signals are typically sent by the terminal driver in
145 recognition of special characters being typed; the terminal driver
146 bypasses all security checks.
147 In secure implementations, a process may be restricted from sending a
149 signal to a process having a different security label. In order to
150 prevent the existence or nonexistence of a process from being used as a
151 covert channel, such processes should appear nonexistent to the sender;
152 that is, [ESRCH] should be returned, rather than [EPERM], if pid refers
153 only to such processes.
154 Existing implementations vary on the result of a kill() with pid indi‐
156 cating an inactive process (a terminated process that has not been
157 waited for by its parent). Some indicate success on such a call (sub‐
158 ject to permission checking), while others give an error of [ESRCH].
159 Since the definition of process lifetime in this volume of POSIX.1‐2008
160 covers inactive processes, the [ESRCH] error as described is inappro‐
161 priate in this case. In particular, this means that an application can‐
162 not have a parent process check for termination of a particular child
163 with kill(). (Usually this is done with the null signal; this can be
164 done reliably with waitpid().)
165 There is some belief that the name kill() is misleading, since the
167 function is not always intended to cause process termination. However,
168 the name is common to all historical implementations, and any change
169 would be in conflict with the goal of minimal changes to existing
170 application code.
171
173 None.
174
176 getpid(), raise(), setsid(), sigaction(), sigqueue(), wait()
177 The Base Definitions volume of POSIX.1‐2008, <signal.h>, <sys_types.h>
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181 Portions of this text are reprinted and reproduced in electronic form
182 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
183 -- Portable Operating System Interface (POSIX), The Open Group Base
184 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
185 cal and Electronics Engineers, Inc and The Open Group. (This is
186 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
187 event of any discrepancy between this version and the original IEEE and
188 The Open Group Standard, the original IEEE and The Open Group Standard
189 is the referee document. The original Standard can be obtained online
190 at http://www.unix.org/online.html .
191 Any typographical or formatting errors that appear in this page are
193 most likely to have been introduced during the conversion of the source
194 files to man page format. To report such errors, see https://www.ker‐
195 nel.org/doc/man-pages/reporting_bugs.html .
196IEEE/The Open Group 2013 KILL(3P)