The kill system call can be used to send any signal to any process group
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 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
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.
It is impossible to send a signal to task number one, the init process, for
which it has not installed a signal handler. This is done to assure the system
is not brought down accidentally.
POSIX 1003.1-2001 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
POSIX 1003.1-2003 requires that if a process sends a signal to itself, and that
process does not have the signal blocked, and no other thread has it unblocked
or is waiting for it in sigwait(), at least one unblocked signal must
be delivered to the sending thread before the call of 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 1003.1-2001, were adopted in kernel 1.3.78.