kill - send signal to a process
The kill() system call can be used to send any signal to any process group or process.
pid 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.
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.
pid is less than -1, then
sig is sent to every
process in the process group whose ID is
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 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
process, are those for which
init has explicitly installed
signal handlers. This is done to assure the system is not brought down
POSIX.1 requires that
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.
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
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.
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