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NAME

       kill - send signal to a process

SYNOPSIS

       #include <sys/types.h>
       #include <signal.h>

       int kill(pid_t pid, int sig);

DESCRIPTION

       The  kill()  system  call can be used to send any signal to any process
       group or process.

       If pid is positive, then signal sig is sent to pid.

       If pid equals 0, then sig is sent to every process in the process group
       of the current process.

       If  pid  equals  -1,  then  sig  is sent to every process for which the
       calling process has permission to send signals, except  for  process  1
       (init), but see below.

       If  pid  is  less  than  -1,  then  sig is sent to every process in the
       process group -pid.

       If sig is 0, then no signal  is  sent,  but  error  checking  is  still
       performed.

       For  a  process  to  have permission to send a signal it must either be
       privileged (under Linux: have the CAP_KILL capability), or the real  or
       effective  user  ID of the sending process must equal the real or saved
       set-user-ID of the target process.  In the case of SIGCONT it  suffices
       when the sending and receiving processes belong to the same session.

RETURN VALUE

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

ERRORS

       EINVAL An invalid signal was specified.

       EPERM  The process does not have permission to send the signal  to  any
              of the target processes.

       ESRCH  The  pid or process group does not exist.  Note that an existing
              process might be a zombie, a  process  which  already  committed
              termination, but has not yet been wait()ed for.

NOTES

       The  only  signals  that can be sent task number one, the init process,
       are those for which init  has  explicitly  installed  signal  handlers.
       This is done to assure the system is not brought down accidentally.

       POSIX  1003.1-2001 requires that kill(-1,sig) send sig to all processes
       that the current process may send signals to, except possibly for  some
       implementation-defined  system  processes.   Linux  allows a process to
       signal itself, but on Linux the call kill(-1,sig) does not  signal  the
       current process.

       POSIX  1003.1-2003 requires that if a process sends a signal to itself,
       and that process does not have the signal blocked, and no other  thread
       has  it  unblocked  or  is  waiting  for  it in sigwait(), at least one
       unblocked signal must be delivered to the  sending  thread  before  the
       call of kill() returns.

BUGS

       In  2.6  kernels  up to and including 2.6.7, there was a bug that meant
       that when sending signals to a process group, kill()  failed  with  the
       error EPERM if the caller did have permission to send the signal to any
       (rather than all) of the embers of the process group.   Notwithstanding
       this  error  return,  the  signal  was  still  delivered  to all of the
       processes for which the caller had permission to signal.

LINUX HISTORY

       Across different kernel versions, Linux has  enforced  different  rules
       for  the  permissions  required  for  an unprivileged process to send a
       signal to another process.  In kernels 1.0 to 1.2.2, a signal could  be
       sent  if  the  effective  user  ID  of  the  sender matched that of the
       receiver, or the real user  ID  of  the  sender  matched  that  of  the
       receiver.   From  kernel  1.2.3 until 1.3.77, a signal could be sent if
       the effective user  ID  of  the  sender  matched  either  the  real  or
       effective user ID of the receiver.  The current rules, which conform to
       POSIX 1003.1-2001, were adopted in kernel 1.3.78.

CONFORMING TO

       SVr4, SVID, POSIX.1, X/OPEN, 4.3BSD, POSIX 1003.1-2001

SEE ALSO

       _exit(2),  killpg(2),  signal(2),   sigqueue(2),   tkill(2),   exit(3),
       capabilities(7), signal(7)