redhat man page for kill

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 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.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EINVAL An invalid signal was specified.

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

       EPERM  The process does not have permission to send the signal to any of the receiving processes.  For a process to have permission to send
	      a  signal to process pid it must either have root privileges, or the real or effective user ID of the sending process must equal the
	      real or saved set-user-ID of the receiving process.  In the case of SIGCONT it suffices when the	sending  and  receiving  processes
	      belong to the same session.

NOTES
       It  is  impossible to send a signal to task number one, the init process, for which it has not installed a signal handler.  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.

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, BSD 4.3, POSIX 1003.1-2001

SEE ALSO
       _exit(2), exit(3), signal(2), signal(7)

Linux 2.5.0							    2001-12-18								   KILL(2)