KILL(2)                    Linux Programmer's Manual                   KILL(2)
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 call-
       ing 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 pro-
       cess group -pid.

       If sig is 0, then no signal is sent, but error checking is  still  per-
       formed.

       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.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.1-2001  requires  that if a process sends a signal to itself, and
       the sending thread 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
       kill().

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 members of the process group.  Notwithstanding
       this  error  return,  the signal was still delivered to all of the pro-
       cesses 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 sig-
       nal 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  effec-
       tive  user  ID  of  the  receiver.  The current rules, which conform to
       POSIX.1-2001, were adopted in kernel 1.3.78.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001

SEE ALSO
       _exit(2), killpg(2), signal(2), sigqueue(2), tkill(2),  exit(3),  capa-
       bilities(7), signal(7)

Linux 2.6.7                       2004-06-24                           KILL(2)