kill - send signal to a process
int kill(pid_t pid, int sig);
Feature Test Macro Requirements for glibc (see feature_test_macros
() system call can be used to send any signal to any process
group or process.
is positive, then signal sig
is sent to the process with
the ID specified by pid
equals 0, then sig
is sent to every process in the process
group of the calling process.
equals -1, then sig
is sent to every process for which the
calling process has permission to send signals, except for process 1 (
), but see below.
is less than -1, then sig
is sent to every process in the
process group whose ID is -pid
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
, 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.
- An invalid signal was specified.
- The calling process does not have permission to send the signal to any of
the target processes.
- 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
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)
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
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
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
if the caller did not have permission to send the signal to
(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.