探寻python多线程ctrl+c退出问题解决方案

经常会遇到下述问题：很多io busy的应用采取多线程的方式来解决，但这时候会发现python命令行不响应ctrl-c 了，而对应的java代码则没有问题：

代码如下:

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则会结束程序

而对应的python代码：

代码如下:

# -*- 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则完全不起作用了。

不成熟的分析：

首先单单设置 daemon 为 true 肯定不行，就不解释了。当daemon为 false 时，导入python线程库后实际上，threading会在主线程执行完毕后，检查是否有不是 daemon 的线程，有的化就wait，等待线程结束了，在主线程等待期间，所有发送到主线程的信号也会被阻测，可以在上述代码加入signal模块验证一下：

代码如下:

def sigint_handler(signum,frame):
print "main-thread exit"
sys.exit()
signal.signal(signal.SIGINT,sigint_handler)

在100秒内按下ctrl－c没有反应，只有当子线程结束后才会出现打印 "main-thread exit",可见 ctrl-c被阻测了

threading 中在主线程结束时进行的操作：

代码如下:

_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()

对所有的非daemon线程进行join等待，其中join中可自行察看源码，又调用了wait，同上文分析 ，主线程等待到了一把锁上。

不成熟的解决：

只能把线程设成daemon才能让主线程不等待，能够接受ctrl－c信号，但是又不能让子线程立即结束，那么只能采用传统的轮询方法了，采用sleep间歇省点cpu吧：

代码如下:

# -*- 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()

#主线程wait住了,不能接受信号了
#thread_.join()

def _exitCheckfunc():
print "ok"
try:
while 1:
alive=False
if thread_.isAlive():
alive=True
if not alive:
break
time.sleep(1)
#为了使得统计时间能够运行，要捕捉 KeyboardInterrupt :ctrl-c
except KeyboardInterrupt, e:
traceback.print_exc()
print "consume time :",time.time()-start

threading._shutdown=_exitCheckfunc

缺点：轮询总会浪费点cpu资源，以及battery.

有更好的解决方案敬请提出。

ps1: 进程监控解决方案 ：

用另外一个进程来接受信号后杀掉执行任务进程，牛

代码如下:

# -*- 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()一定要放在线程创建前，原因未知。。。。，否则立刻就结束