system calls allow sending a signal, specified by the
argument, to some threads in a process.
function, signalled threads are always limited to the current process.
argument specifies the process with threads to be signalled.
argument specifies which threads get the signal.
is equal to -1, all threads in the specified process are signalled.
Otherwise, only the thread with the thread identifier equal to the
argument is signalled.
argument defines the delivered signal.
It must be a valid signal number or zero.
In the latter case no signal is actually sent, and the call is used to
verify the liveness of the thread.
The signal is delivered with