KILL(2) Linux Programmer's Manual KILL(2)
kill - send signal to a process
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 call‐
ing 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
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 appropriately.
EINVAL An invalid signal was specified.
EPERM The process does not have permission to send the signal to any
of the target processes.
ESRCH The process or process group does not exist. Note that an
existing process might be a zombie, a process that has termi‐
nated 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 implemen‐
tation-defined system processes. Linux allows a process to signal
itself, but on Linux the call kill(-1,sig) does not signal the calling
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
Across different kernel versions, Linux has enforced different rules
for the permissions required for an unprivileged process to send a sig‐
nal 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. Notwith‐
standing 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), signal(2), tkill(2), exit(3), killpg(3),
sigqueue(3), capabilities(7), credentials(7), signal(7)
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Linux 2017-09-15 KILL(2)