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KILL(2)                    Linux Programmer's Manual                   KILL(2)

       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 pro-

       If pid is positive, then signal sig is sent to the process with the ID specified by

       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

       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().

   Linux Notes
       Across  different  kernel versions, Linux has enforced different rules for the per-
       missions 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

       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  mem-
       bers 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)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2008-08-29                           KILL(2)

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