kill — send signal to a process
#include <sys/types.h> #include <signal.h>
int
kill( |
pid_t | pid, |
int | sig) ; |
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 pid
.
If pid
equals 0,
then sig
is sent to
every process in the process group of the current
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 −pid
.
If sig
is 0, then
no signal is sent, but error checking is still performed.
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.
An invalid signal was specified.
The process does not have permission to send the signal to any of the target processes.
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 wait(2)ed for.
The only signals that can be sent 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 current 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 current 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
().
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 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), sigqueue(2), tkill(2), exit(3), capabilities(7), credentials(7), signal(7)
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