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



NAME
       kill - send signal to a process

SYNOPSIS
       #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

DESCRIPTION
       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 sig-
       nals, 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.

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

ERRORS
       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.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

NOTES
       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 sig-
       nal 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 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.

BUGS
       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  deliv-
       ered to all of the processes for which the caller had permission to signal.

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-29                           KILL(2)