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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

       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

       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

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


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