用alarm信号处理阻塞socket的超时

以前在使用阻塞（blocked）socket的connect，send，recv函数的时候，通常都使用settimeout来设置超时。
如下，

#!/usr/bin/env python

import socket
import time

fd = socket.socket()

try:
    print "connect", time.ctime()
    fd.settimeout(5)
    fd.connect(("twitter.com", 22))
except:
    print "timeout", time.ctime()
finally:
    fd.settimeout(0)

今晚看一本《TCP/IP高效编程》，看到了一种使用alarm的方法，

#!/usr/bin/env python
import signal
import socket
import time

def alarm_handler(signum, frame):
    print "Singal number is", signum
    raise Exception("Timeout")

fd = socket.socket()

try:
    print "connect", time.ctime()
    signal.signal(signal.SIGALRM, alarm_handler)
    # Timeout set to 5 seconds
    signal.alarm(5)
    fd.connect(("twitter.com", 22))
except:
    print "timeout", time.ctime()
finally:
    # Disable alarm
    signal.alarm(0)

两种方法的测试运行结果：

root@xiaoxia-pc:~/test# ./timeout.py
connect Tue Nov 22 21:38:29 2011
timeout Tue Nov 22 21:38:34 2011
root@xiaoxia-pc:~/test# ./alarm.py
connect Tue Nov 22 21:38:38 2011
Singal number is 14
timeout Tue Nov 22 21:38:43 2011