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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.
       (Historically, the rules were different; see NOTES.)


       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.


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


       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 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 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 effective user ID
       of the target, or the real user ID of the sender matched the real  user
       ID  of  the  target.  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 target.  The current rules, which conform to
       POSIX.1, 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 not 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),   tkill(2),  exit(3),  sigqueue(3),
       capabilities(7), credentials(7), signal(7)


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