This can be used to check the validity of the process parameter.
Processes can send signals to themselves.
Sending a signal does not imply that the operation is suc- cessful. All signal operations must pass the access checks prescribed by each enforced access control policy on the system. Otherwise, a value of -1 is returned and errno is set to indicate the error. The signal parameter is not a valid signal number. No process can be found corresponding to that specified by the process parameter.