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       kill - send signal to a process


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

       int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       kill(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE


       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 the process with the  ID
       specified by pid.

       If pid equals 0, then sig is sent to every process in the process group
       of the calling 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 whose ID is -pid.

       If  sig  is  0,  then  no  signal  is sent, but error checking is still
       performed; this can be used to check for the existence of a process  ID
       or process group ID.

       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.


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


       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(2)ed for.


       SVr4, 4.3BSD, POSIX.1-2001.


       The  only  signals  that can be sent to process ID 1, 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 calling 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
       calling 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(3), at least
       one unblocked signal must be delivered to the sending thread before the
       kill() returns.

   Linux Notes
       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.1-2001, were adopted in kernel 1.3.78.


       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
       processes for which the caller had permission to signal.


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


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