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)):
kill():
_POSIX_C_SOURCE
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
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
https://www.kernel.org/doc/man-pages/.
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),
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),
system_data_types(7),
systemd-coredump(8)
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