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

       kill - send signal to a process

       #include <signal.h>

       int kill(pid_t pid, int sig);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


       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.)

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

       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.

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

       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.

       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.

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

       This page is part of release 5.13 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                              2021-03-22                            KILL(2)