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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.
       (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

       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 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.1-2001, 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 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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