#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid , int sig );
DESCRIPTION
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 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.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EINVAL
An invalid signal was specified.
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 wait()ed for.
EPERM
The process does not have permission to send the signal
to any of the receiving processes.
For a process to have permission to send a signal to process pid it must either have root privileges, or the real or effective
user ID of the sending process must equal the real or
saved set-user-ID of the receiving process.
In the case of SIGCONT it suffices when the sending and receiving
processes belong to the same session.
NOTES
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 process.
LINUX HISTORY
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.