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)):
kill(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_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
If pid equals -1, then sig is sent to every process for which the calling process has per-
mission 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
If sig is 0, then no signal is sent, but error checking is still performed; this can be
used to check for the existence of a process ID or process group ID.
For a process to have permission to send a signal it must either be privileged (under
Linux: have the CAP_KILL capability), 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.
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 pro-
ESRCH The pid or process group does not exist. Note that an existing process might be a
zombie, a process which already committed termination, but has not yet been
SVr4, 4.3BSD, POSIX.1-2001.
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.1-2001 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 sig-
nal the calling process.
POSIX.1-2001 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.
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 that of
the receiver, or the real user ID of the sender matched that of the receiver. 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 receiver. The current rules, which conform to
POSIX.1-2001, 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.
_exit(2), killpg(2), signal(2), tkill(2), exit(3), sigqueue(3), capabilities(7), creden-
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Linux 2013-02-05 KILL(2)