kill(2) — Linux manual page


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

NAME         top

       kill - send signal to a process

SYNOPSIS         top

       #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

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 NOTES.)

RETURN VALUE         top

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

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 for.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

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 accidentally.

       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.

   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, were adopted in kernel 1.3.78.

BUGS         top

       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.

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),

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-10-10                          KILL(2)

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