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@gmx.net>
  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@gmx.net>
  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 2004-06-24 "Linux 2.6.7" "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 .SH DESCRIPTION
  56 The
  57 .BR kill ()
  58 system call
  59 can be used to send any signal to any process group or process.
  60 .PP
  61 If \fIpid\fP is positive, then signal \fIsig\fP is sent to \fIpid\fP.
  62 .PP
  63 If \fIpid\fP equals 0, then \fIsig\fP is sent to every process in the
  64 process group of the current process.
  65 .PP
  66 If \fIpid\fP equals \-1, then \fIsig\fP is sent to every process
  67 for which the calling process has permission to send signals,
  68 except for process 1 (init), but see below.
  69 .PP
  70 If \fIpid\fP is less than \-1, then \fIsig\fP is sent to every process
  71 in the process group \fI\-pid\fP.
  72 .PP
  73 If \fIsig\fP is 0, then no signal is sent, but error checking is still
  74 performed.
  75
  76 For a process to have permission to send a signal
  77 it must either be privileged (under Linux: have the
  78 .B CAP_KILL
  79 capability), or the real or effective
  80 user ID of the sending process must equal the real or
  81 saved set-user-ID of the target process.
  82 In the case of SIGCONT it suffices when the sending and receiving
  83 processes belong to the same session.
  84 .SH "RETURN VALUE"
  85 On success (at least one signal was sent), zero is returned.
  86 On error, \-1 is returned, and
  87 .I errno
  88 is set appropriately.
  89 .SH ERRORS
  90 .TP
  91 .B EINVAL
  92 An invalid signal was specified.
  93 .TP
  94 .B EPERM
  95 The process does not have permission to send the signal
  96 to any of the target processes.
  97 .TP
  98 .B ESRCH
  99 The pid or process group does not exist.
 100 Note that an existing process might be a zombie,
 101 a process which already committed termination, but
 102 has not yet been
 103 .BR wait (2)ed
 104 for.
 105 .SH NOTES
 106 The only signals that can be sent process ID 1, the
 107 .I init
 108 process, are those for which
 109 .I init
 110 has explicitly installed signal handlers.
 111 This is done to assure the
 112 system is not brought down accidentally.
 113 .LP
 114 POSIX.1-2001 requires that \fIkill(\-1,sig)\fP send \fIsig\fP
 115 to all processes that the current process may send signals to,
 116 except possibly for some implementation-defined system processes.
 117 Linux allows a process to signal itself, but on Linux the call
 118 \fIkill(\-1,sig)\fP does not signal the current process.
 119 .LP
 120 POSIX.1-2001 requires that if a process sends a signal to itself,
 121 and the sending thread does not have the signal blocked,
 122 and no other thread
 123 has it unblocked or is waiting for it in
 124 .BR sigwait (3),
 125 at least one
 126 unblocked signal must be delivered to the sending thread before the
 127 .BR kill ().
 128 .SS "Linux Notes"
 129 Across different kernel versions, Linux has enforced different rules
 130 for the permissions required for an unprivileged process
 131 to send a signal to another process.
 132 .\" In the 0.* kernels things chopped and changed quite
 133 .\" a bit - MTK, 24 Jul 02
 134 In kernels 1.0 to 1.2.2, a signal could be sent if the
 135 effective user ID of the sender matched that of the receiver,
 136 or the real user ID of the sender matched that of the receiver.
 137 From kernel 1.2.3 until 1.3.77, a signal could be sent if the
 138 effective user ID of the sender matched either the real or effective
 139 user ID of the receiver.
 140 The current rules, which conform to POSIX.1-2001, were adopted
 141 in kernel 1.3.78.
 142 .SH BUGS
 143 In 2.6 kernels up to and including 2.6.7,
 144 there was a bug that meant that when sending signals to a process group,
 145 .BR kill ()
 146 failed with the error
 147 .B EPERM
 148 if the caller did have permission to send the signal to \fIany\fP (rather
 149 than \fIall\fP) of the members of the process group.
 150 Notwithstanding this error return, the signal was still delivered
 151 to all of the processes for which the caller had permission to signal.
 152 .SH "CONFORMING TO"
 153 SVr4, 4.3BSD, POSIX.1-2001
 154 .SH "SEE ALSO"
 155 .BR _exit (2),
 156 .BR killpg (2),
 157 .BR signal (2),
 158 .BR sigqueue (2),
 159 .BR tkill (2),
 160 .BR exit (3),
 161 .BR capabilities (7),
 162 .BR signal (7)