kill(2) - Linux manual page (original) (raw)

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 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
   HISTORY.)

RETURN VALUE top

   On success (at least one signal was sent), zero is returned.  On
   error, -1 is returned, and _[errno](../man3/errno.3.html)_ 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)](../man2/wait.2.html)ed
          for.

STANDARDS top

   POSIX.1-2008.

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
   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)](../man3/sigwait.3.html), 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)](../man1/kill.1.html), [_exit(2)](../man2/%5Fexit.2.html), [pidfd_send_signal(2)](../man2/pidfd%5Fsend%5Fsignal.2.html), [signal(2)](../man2/signal.2.html), [tkill(2)](../man2/tkill.2.html),
   [exit(3)](../man3/exit.3.html), [killpg(3)](../man3/killpg.3.html), [sigqueue(3)](../man3/sigqueue.3.html), [capabilities(7)](../man7/capabilities.7.html), [credentials(7)](../man7/credentials.7.html),
   [signal(7)](../man7/signal.7.html)

COLOPHON top

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   man-pages@man7.org

Linux man-pages 6.10 2024-07-23 kill(2)

Pages that refer to this page:capsh(1), fuser(1), kill(1@@coreutils), kill(1), kill(1@@procps-ng), killall(1), pgrep(1), skill(1), strace(1), clone(2), _exit(2), fcntl(2), getpid(2), getrlimit(2), pause(2), pidfd_open(2), pidfd_send_signal(2), ptrace(2), rt_sigqueueinfo(2), setfsgid(2), setfsuid(2), sigaction(2), signal(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), syscalls(2), tkill(2), wait(2), gsignal(3), id_t(3type), killpg(3), psignal(3), pthread_kill(3), raise(3), sd_event_add_child(3), sigpause(3), sigqueue(3), sigset(3), sigvec(3), systemd.exec(5), systemd.kill(5), capabilities(7), cpuset(7), credentials(7), pid_namespaces(7), pthreads(7), signal(7), signal-safety(7), systemd-coredump(8)