man2/kill.2

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   3 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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  25 .\" Modified by Michael Haardt <michael@moria.de>
  26 .\" Modified by Thomas Koenig <ig25@rz.uni-karlsruhe.de>
  27 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified 1993-07-25 by Rik Faith <faith@cs.unc.edu>
  29 .\" Modified 1995-11-01 by Michael Haardt
  30 .\"  <michael@cantor.informatik.rwth-aachen.de>
  31 .\" Modified 1996-04-14 by Andries Brouwer <aeb@cwi.nl>
  32 .\"  [added some polishing contributed by Mike Battersby <mib@deakin.edu.au>]
  33 .\" Modified 1996-07-21 by Andries Brouwer <aeb@cwi.nl>
  34 .\" Modified 1997-01-17 by Andries Brouwer <aeb@cwi.nl>
  35 .\" Modified 2001-12-18 by Andries Brouwer <aeb@cwi.nl>
  36 .\" Modified 2002-07-24 by Michael Kerrisk <mtk.manpages@gmail.com>
  37 .\"     Added note on historical rules enforced when an unprivileged process
  38 .\"     sends a signal.
  39 .\" Modified 2004-06-16 by Michael Kerrisk <mtk.manpages@gmail.com>
  40 .\"     Added note on CAP_KILL
  41 .\" Modified 2004-06-24 by aeb
  42 .\" Modified, 2004-11-30, after idea from emmanuel.colbus@ensimag.imag.fr
  43 .\"
  44 .TH KILL 2 2008-04-18 "Linux" "Linux Programmer's Manual"
  45 .SH NAME
  46 kill \- send signal to a process
  47 .SH SYNOPSIS
  48 .nf
  49 .B #include <sys/types.h>
  50 .br
  51 .B #include <signal.h>
  52 .sp
  53 .BI "int kill(pid_t " pid ", int " sig );
  54 .fi
  55 .sp
  56 .in -4n
  57 Feature Test Macro Requirements for glibc (see
  58 .BR feature_test_macros (7)):
  59 .in
  60 .sp
  61 .BR kill ():
  62 _POSIX_C_SOURCE || _XOPEN_SOURCE
  63 .SH DESCRIPTION
  64 The
  65 .BR kill ()
  66 system call
  67 can be used to send any signal to any process group or process.
  68 .PP
  69 If \fIpid\fP is positive, then signal \fIsig\fP is sent to the
  70 process with the ID specified by \fIpid\fP.
  71 .PP
  72 If \fIpid\fP equals 0, then \fIsig\fP is sent to every process in the
  73 process group of the calling process.
  74 .PP
  75 If \fIpid\fP equals \-1, then \fIsig\fP is sent to every process
  76 for which the calling process has permission to send signals,
  77 except for process 1 (\fIinit\fP), but see below.
  78 .PP
  79 If \fIpid\fP is less than \-1, then \fIsig\fP is sent to every process
  80 in the process group whose ID is \fI\-pid\fP.
  81 .PP
  82 If \fIsig\fP is 0, then no signal is sent, but error checking is still
  83 performed;
  84 this can be used to check for the existence of a process ID or
  85 process group ID.
  86
  87 For a process to have permission to send a signal
  88 it must either be privileged (under Linux: have the
  89 .B CAP_KILL
  90 capability), or the real or effective
  91 user ID of the sending process must equal the real or
  92 saved set-user-ID of the target process.
  93 In the case of
  94 .B SIGCONT
  95 it suffices when the sending and receiving
  96 processes belong to the same session.
  97 .SH "RETURN VALUE"
  98 On success (at least one signal was sent), zero is returned.
  99 On error, \-1 is returned, and
 100 .I errno
 101 is set appropriately.
 102 .SH ERRORS
 103 .TP
 104 .B EINVAL
 105 An invalid signal was specified.
 106 .TP
 107 .B EPERM
 108 The process does not have permission to send the signal
 109 to any of the target processes.
 110 .TP
 111 .B ESRCH
 112 The pid or process group does not exist.
 113 Note that an existing process might be a zombie,
 114 a process which already committed termination, but
 115 has not yet been
 116 .BR wait (2)ed
 117 for.
 118 .SH "CONFORMING TO"
 119 SVr4, 4.3BSD, POSIX.1-2001.
 120 .SH NOTES
 121 The only signals that can be sent to process ID 1, the
 122 .I init
 123 process, are those for which
 124 .I init
 125 has explicitly installed signal handlers.
 126 This is done to assure the
 127 system is not brought down accidentally.
 128 .LP
 129 POSIX.1-2001 requires that \fIkill(\-1,sig)\fP send \fIsig\fP
 130 to all processes that the calling process may send signals to,
 131 except possibly for some implementation-defined system processes.
 132 Linux allows a process to signal itself, but on Linux the call
 133 \fIkill(\-1,sig)\fP does not signal the calling process.
 134 .LP
 135 POSIX.1-2001 requires that if a process sends a signal to itself,
 136 and the sending thread does not have the signal blocked,
 137 and no other thread
 138 has it unblocked or is waiting for it in
 139 .BR sigwait (3),
 140 at least one
 141 unblocked signal must be delivered to the sending thread before the
 142 .BR kill ().
 143 .SS "Linux Notes"
 144 Across different kernel versions, Linux has enforced different rules
 145 for the permissions required for an unprivileged process
 146 to send a signal to another process.
 147 .\" In the 0.* kernels things chopped and changed quite
 148 .\" a bit - MTK, 24 Jul 02
 149 In kernels 1.0 to 1.2.2, a signal could be sent if the
 150 effective user ID of the sender matched that of the receiver,
 151 or the real user ID of the sender matched that of the receiver.
 152 From kernel 1.2.3 until 1.3.77, a signal could be sent if the
 153 effective user ID of the sender matched either the real or effective
 154 user ID of the receiver.
 155 The current rules, which conform to POSIX.1-2001, were adopted
 156 in kernel 1.3.78.
 157 .SH BUGS
 158 In 2.6 kernels up to and including 2.6.7,
 159 there was a bug that meant that when sending signals to a process group,
 160 .BR kill ()
 161 failed with the error
 162 .B EPERM
 163 if the caller did have permission to send the signal to \fIany\fP (rather
 164 than \fIall\fP) of the members of the process group.
 165 Notwithstanding this error return, the signal was still delivered
 166 to all of the processes for which the caller had permission to signal.
 167 .SH "SEE ALSO"
 168 .BR _exit (2),
 169 .BR killpg (2),
 170 .BR signal (2),
 171 .BR sigqueue (2),
 172 .BR tkill (2),
 173 .BR exit (3),
 174 .BR capabilities (7),
 175 .BR credentials (7),
 176 .BR signal (7)