場景：
經常會遇到下述問題：很多io busy的應用采取多線程的方式來解決，但這時候會發現python命令行不響應ctrl-c 了，而對應的java代碼則沒有問題：
public class Test {?
??? public static void main(String[] args) throws Exception {?
?
??????? new Thread(new Runnable() {?
?
??????????? public void run() {?
??????????????? long start = System.currentTimeMillis();?
??????????????? while (true) {?
??????????????????? try {?
??????????????????????? Thread.sleep(1000);?
??????????????????? } catch (Exception e) {?
??????????????????? }?
??????????????????? System.out.println(System.currentTimeMillis());?
??????????????????? if (System.currentTimeMillis() - start > 1000 * 100) break;?
??????????????? }?
??????????? }?
??????? }).start();?
?
??? }?
}?
java Test
ctrl-c則會結束程序
而對應的python代碼：
# -*- coding: utf-8 -*-?
import time?
import threading?
start=time.time()?
def foreverLoop():?
??? start=time.time()?
??? while 1:?
??????? time.sleep(1)?
??????? print time.time()?
??????? if time.time()-start>100:?
??????????? break?
??????????????
thread_=threading.Thread(target=foreverLoop)?
#thread_.setDaemon(True)?
thread_.start()?
python p.py
后ctrl-c則完全不起作用了。
不成熟的分析：
首先單單設置 daemon 為 true 肯定不行，就不解釋了。當daemon為 false 時，導入python線程庫后實際上，threading會在主線程執行完畢后，檢查是否有不是 daemon 的線程，有的化就wait，等待線程結束了，在主線程等待期間，所有發送到主線程的信號也會被阻測，可以在上述代碼加入signal模塊驗證一下：
def sigint_handler(signum,frame):???
??? print "main-thread exit"?
??? sys.exit()???
signal.signal(signal.SIGINT,sigint_handler)?
在100秒內按下ctrl－c沒有反應，只有當子線程結束后才會出現打印 "main-thread exit",可見 ctrl-c被阻測了
threading 中在主線程結束時進行的操作：
_shutdown = _MainThread()._exitfunc?
def _exitfunc(self):?
??????? self._Thread__stop()?
??????? t = _pickSomeNonDaemonThread()?
??????? if t:?
??????????? if __debug__:?
??????????????? self._note("%s: waiting for other threads", self)?
??????? while t:?
??????????? t.join()?
??????????? t = _pickSomeNonDaemonThread()?
??????? if __debug__:?
??????????? self._note("%s: exiting", self)?
??????? self._Thread__delete()?
?
?對所有的非daemon線程進行join等待，其中join中可自行察看源碼，又調用了wait，同上文分析 ，主線程等待到了一把鎖上。
不成熟的解決：
只能把線程設成daemon才能讓主線程不等待，能夠接受ctrl－c信號，但是又不能讓子線程立即結束，那么只能采用傳統的輪詢方法了，采用sleep間歇省點cpu吧：
?
# -*- coding: utf-8 -*-?
import time,signal,traceback?
import sys?
import threading?
start=time.time()?
def foreverLoop():?
??? start=time.time()?
??? while 1:?
??????? time.sleep(1)?
??????? print time.time()?
??????? if time.time()-start>5:?
??????????? break?
?????????????
thread_=threading.Thread(target=foreverLoop)?
thread_.setDaemon(True)?
thread_.start()?
?
#主線程wait住了,不能接受信號了?
#thread_.join()?
?
def _exitCheckfunc():?
??? print "ok"?
??? try:?
??????? while 1:?
??????????? alive=False?
??????????? if thread_.isAlive():?
??????????????? alive=True?
??????????? if not alive:?
??????????????? break?
??????????? time.sleep(1)???
??? #為了使得統計時間能夠運行，要捕捉? KeyboardInterrupt :ctrl-c???????
??? except KeyboardInterrupt, e:?
??????? traceback.print_exc()?
??? print "consume time :",time.time()-start?
?????????
threading._shutdown=_exitCheckfunc?
?? 缺點：輪詢總會浪費點cpu資源，以及battery.
有更好的解決方案敬請提出。
ps1: 進程監控解決方案 ：
用另外一個進程來接受信號后殺掉執行任務進程，牛
# -*- coding: utf-8 -*-?
import time,signal,traceback,os?
import sys?
import threading?
start=time.time()?
def foreverLoop():?
??? start=time.time()?
??? while 1:?
??????? time.sleep(1)?
??????? print time.time()?
??????? if time.time()-start>5:?
??????????? break?
?
class Watcher:?
??? """this class solves two problems with multithreaded
??? programs in Python, (1) a signal might be delivered
??? to any thread (which is just a malfeature) and (2) if
??? the thread that gets the signal is waiting, the signal
??? is ignored (which is a bug).
?
??? The watcher is a concurrent process (not thread) that
??? waits for a signal and the process that contains the
??? threads.? See Appendix A of The Little Book of Semaphores.
??? http://greenteapress.com/semaphores/
?
??? I have only tested this on Linux.? I would expect it to
??? work on the Macintosh and not work on Windows.
??? """?
?
??? def __init__(self):?
??????? """ Creates a child thread, which returns.? The parent
??????????? thread waits for a KeyboardInterrupt and then kills
??????????? the child thread.
??????? """?
??????? self.child = os.fork()?
??????? if self.child == 0:?
??????????? return?
??????? else:?
??????????? self.watch()?
?
??? def watch(self):?
??????? try:?
??????????? os.wait()?
??????? except KeyboardInterrupt:?
??????????? # I put the capital B in KeyBoardInterrupt so I can?
??????????? # tell when the Watcher gets the SIGINT?
??????????? print 'KeyBoardInterrupt'?
??????????? self.kill()?
??????? sys.exit()?
?
??? def kill(self):?
??????? try:?
??????????? os.kill(self.child, signal.SIGKILL)?
??????? except OSError: pass?
?
Watcher()?????????????
thread_=threading.Thread(target=foreverLoop)?
thread_.start()?
?注意 watch()一定要放在線程創建前，原因未知。。。。，否則立刻就結束
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