1KILL(2)                    Linux Programmer's Manual                   KILL(2)
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NAME

6       kill - send signal to a process
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SYNOPSIS

9       #include <sys/types.h>
10       #include <signal.h>
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12       int kill(pid_t pid, int sig);
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14   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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16       kill(): _POSIX_C_SOURCE
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DESCRIPTION

19       The  kill()  system  call can be used to send any signal to any process
20       group or process.
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22       If pid is positive, then signal sig is sent to the process with the  ID
23       specified by pid.
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25       If pid equals 0, then sig is sent to every process in the process group
26       of the calling process.
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28       If pid equals -1, then sig is sent to every process for which the call‐
29       ing  process  has  permission  to  send  signals,  except for process 1
30       (init), but see below.
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32       If pid is less than -1, then sig  is  sent  to  every  process  in  the
33       process group whose ID is -pid.
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35       If  sig  is  0,  then  no  signal is sent, but existence and permission
36       checks are still performed; this can be used to check for the existence
37       of  a  process  ID  or process group ID that the caller is permitted to
38       signal.
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40       For a process to have permission to send a signal, it  must  either  be
41       privileged  (under  Linux:  have  the  CAP_KILL  capability in the user
42       namespace of the target process), or the real or effective user  ID  of
43       the  sending  process  must  equal the real or saved set-user-ID of the
44       target process.  In the case of SIGCONT, it suffices when  the  sending
45       and receiving processes belong to the same session.  (Historically, the
46       rules were different; see NOTES.)
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RETURN VALUE

49       On success (at least one signal was sent), zero is returned.  On error,
50       -1 is returned, and errno is set appropriately.
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ERRORS

53       EINVAL An invalid signal was specified.
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55       EPERM  The  calling process does not have permission to send the signal
56              to any of the target processes.
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58       ESRCH  The target process or process group does not exist.   Note  that
59              an existing process might be a zombie, a process that has termi‐
60              nated execution, but has not yet been wait(2)ed for.
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CONFORMING TO

63       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
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NOTES

66       The only signals that can be sent to process ID 1,  the  init  process,
67       are  those  for  which  init  has explicitly installed signal handlers.
68       This is done to assure the system is not brought down accidentally.
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70       POSIX.1 requires that kill(-1,sig) send sig to all processes  that  the
71       calling process may send signals to, except possibly for some implemen‐
72       tation-defined system processes.  Linux  allows  a  process  to  signal
73       itself,  but on Linux the call kill(-1,sig) does not signal the calling
74       process.
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76       POSIX.1 requires that if a process sends a signal to  itself,  and  the
77       sending  thread  does  not have the signal blocked, and no other thread
78       has it unblocked or is waiting for  it  in  sigwait(3),  at  least  one
79       unblocked  signal  must  be  delivered to the sending thread before the
80       kill() returns.
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82   Linux notes
83       Across different kernel versions, Linux has  enforced  different  rules
84       for the permissions required for an unprivileged process to send a sig‐
85       nal to another process.  In kernels 1.0 to 1.2.2,  a  signal  could  be
86       sent  if  the effective user ID of the sender matched effective user ID
87       of the target, or the real user ID of the sender matched the real  user
88       ID  of  the  target.  From kernel 1.2.3 until 1.3.77, a signal could be
89       sent if the effective user ID of the sender matched either the real  or
90       effective  user  ID of the target.  The current rules, which conform to
91       POSIX.1, were adopted in kernel 1.3.78.
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BUGS

94       In 2.6 kernels up to and including 2.6.7, there was a  bug  that  meant
95       that  when  sending  signals to a process group, kill() failed with the
96       error EPERM if the caller did not have permission to send the signal to
97       any  (rather  than  all) of the members of the process group.  Notwith‐
98       standing this error return, the signal was still delivered  to  all  of
99       the processes for which the caller had permission to signal.
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SEE ALSO

102       kill(1),  _exit(2), pidfd_send_signal(2), signal(2), tkill(2), exit(3),
103       killpg(3), sigqueue(3), capabilities(7), credentials(7), signal(7)
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COLOPHON

106       This page is part of release 5.07 of the Linux  man-pages  project.   A
107       description  of  the project, information about reporting bugs, and the
108       latest    version    of    this    page,    can     be     found     at
109       https://www.kernel.org/doc/man-pages/.
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113Linux                             2019-10-10                           KILL(2)
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