kill(2) — Linux manual page


kill(2)                    System Calls Manual                   kill(2)

NAME         top

       kill - send signal to a process

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <signal.h>

       int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see


DESCRIPTION         top

       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 existence and permission
       checks are still performed; this can be used to check for the
       existence of a process ID or process group ID that the caller is
       permitted to signal.

       For a process to have permission to send a signal, it must either
       be privileged (under Linux: have the CAP_KILL capability in the
       user namespace of the target process), 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

RETURN VALUE         top

       On success (at least one signal was sent), zero is returned.  On
       error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       EINVAL An invalid signal was specified.

       EPERM  The calling process does not have permission to send the
              signal to any of the target processes.

       ESRCH  The target process or process group does not exist.  Note
              that an existing process might be a zombie, a process that
              has terminated execution, but has not yet been wait(2)ed

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr4, 4.3BSD.

   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 Linux 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 Linux
       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 Linux 1.3.78.

NOTES         top

       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

       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.

BUGS         top

       In Linux 2.6 up to and including Linux 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.

SEE ALSO         top

       kill(1), _exit(2), pidfd_send_signal(2), signal(2), tkill(2),
       exit(3), killpg(3), sigqueue(3), capabilities(7), credentials(7),

Linux man-pages (unreleased)     (date)                          kill(2)

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