Editing Operating system (section)

=====Signal=====
To generate ''software interrupts'' in [[Unix-like]] operating systems, the <code>kill(pid,signum)</code> [[system call]] will send a [[Signal (IPC)|signal]] to another process.<ref name="duos-p200">{{cite book
  | last = Bach
  | first = Maurice J.
  | title = The Design of the UNIX Operating System
  | publisher = Prentice-Hall
  | year = 1986
  | page = 200
  | isbn = 0-13-201799-7
}}</ref> <code>pid</code> is the [[process identifier]] of the receiving process. <code>signum</code> is the signal number (in [[mnemonic]] format){{efn|Examples include [[Signal (IPC)#SIGINT|SIGINT]], [[SIGSEGV]], and [[SIGBUS]].}} to be sent. (The abrasive name of <code>kill</code> was chosen because early implementations only terminated the process.)<ref name="lpi-ch20-p400">{{cite book
  | last = Kerrisk
  | first = Michael
  | title = The Linux Programming Interface
  | publisher = No Starch Press
  | year = 2010
  | page = 400
  | isbn = 978-1-59327-220-3
}}</ref>

In Unix-like operating systems, ''signals'' inform processes of the occurrence of asynchronous events.<ref name="duos-p200"/> To communicate asynchronously, interrupts are required.<ref name="sco-ch5-p308_b">{{cite book
  | last = Tanenbaum
  | first = Andrew S.
  | title = Structured Computer Organization, Third Edition
  | publisher = Prentice Hall
  | year = 1990
  | page = [https://archive.org/details/structuredcomput00tane/page/308 308]
  | isbn = 978-0-13-854662-5
  | url = https://archive.org/details/structuredcomput00tane/page/308
  }}</ref> One reason a process needs to asynchronously communicate to another process solves a variation of the classic [[Readers–writers problem|reader/writer problem]].<ref name="osc-p182">{{cite book
  | last = Silberschatz
  | first = Abraham
  | title = Operating System Concepts, Fourth Edition
  | publisher = Addison-Wesley
  | year = 1994
  | page = 182
  | isbn = 978-0-201-50480-4
}}</ref> The writer receives a pipe from the [[Shell (computing)|shell]] for its output to be sent to the reader's input stream.<ref name="usp-ch6-p153">{{cite book
  | last1 = Haviland
  | first1 = Keith
  | last2 = Salama
  | first2 = Ben
  | title = UNIX System Programming
  | publisher = Addison-Wesley Publishing Company
  | year = 1987
  | page = 153
  | isbn = 0-201-12919-1
}}</ref> The [[Command-line interface|command-line]] syntax is <code>alpha | bravo</code>. <code>alpha</code> will write to the pipe when its computation is ready and then sleep in the wait queue.<ref name="usp-ch6-p148">{{cite book
  | last1 = Haviland
  | first1 = Keith
  | last2 = Salama
  | first2 = Ben
  | title = UNIX System Programming
  | publisher = Addison-Wesley Publishing Company
  | year = 1987
  | page = 148
  | isbn = 0-201-12919-1
}}</ref> <code>bravo</code> will then be moved to the [[Multilevel feedback queue|ready queue]] and soon will read from its input stream.<ref name="usp-ch6-p149">{{cite book
  | last1 = Haviland
  | first1 = Keith
  | last2 = Salama
  | first2 = Ben
  | title = UNIX System Programming
  | publisher = Addison-Wesley Publishing Company
  | year = 1987
  | page = 149
  | isbn = 0-201-12919-1
}}</ref> The kernel will generate ''software interrupts'' to coordinate the piping.<ref name="usp-ch6-p149"/>

''Signals'' may be classified into 7 categories.<ref name="duos-p200"/> The categories are:
# when a process finishes normally.
# when a process has an error exception.
# when a process runs out of a system resource.
# when a process executes an illegal instruction.
# when a process sets an alarm event.
# when a process is aborted from the keyboard.
# when a process has a tracing alert for debugging.