Python multiprocessing.Process.is_alive() 返回 True 虽然进程已经完成，为什么？

multiprocessing.Process.is_alive() returns True although process has finished, why?

提问by zhanxw

I use multiprocess.Processto create a child process and then call os.wait4until child exists. When the actual child process finishes, multiprocess.Process.is_alive()still returns True. That's contradicting. Why?

我multiprocess.Process用来创建一个子进程，然后调用os.wait4直到子进程存在。当实际的子进程完成时，multiprocess.Process.is_alive()仍然返回True。这很矛盾。为什么？

Code:

代码：

from multiprocessing import Process
import os, sys

proc = Process(target=os.system, args= ("sleep 2", ))
proc.start()

print "is_alive()", proc.is_alive()
ret = os.wait4(proc.pid, 0)
procPid, procStatus, procRes = ret
print "wait4 = ", ret

## Puzzled!
print "----Puzzled below----"
print "is_alive()", proc.is_alive()
if os.WIFEXITED(procStatus):
    print "exit with status", os.WEXITSTATUS(procStatus)
print "is_alive()", proc.is_alive()
sys.exit(1)

Output:

输出：

is_alive() True
wait4 =  (11137, 0, resource.struct_rusage(ru_utime=0.0028959999999999997, ru_stime=0.003189, ru_maxrss=1363968, ru_ixrss=0, ru_idrss=0, ru_isrss=0, ru_minflt=818, ru_majflt=0, ru_nswap=0, ru_inblock=0, ru_oublock=0, ru_msgsnd=0, ru_msgrcv=0, ru_nsignals=0, ru_nvcsw=1, ru_nivcsw=9))
----Puzzled below----
is_alive() True
exit with status 0
is_alive() True

My question is about the last three output lines. Why is_alive()return Truewhen the actual process is finished. How can that happen?

我的问题是关于最后三个输出行。为什么在实际过程完成时is_alive()返回True。怎么会这样？

采纳答案by falsetru

You should use Process.joininstead of os.wait4.

您应该使用Process.join而不是os.wait4.

• Process.is_alivecalls waitpidinternally through multiprocessing.forking.Popen.poll.
• If you call os.wait4, waitpidraises os.errorwhich cause the poll()to return None. (http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/forking.py#l141)
• is_alive()use that return value to determine whether the process is alive. (http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/process.py#l159)
  • => return True.

• Process.is_alivewaitpid通过内部调用multiprocessing.forking.Popen.poll。
• 如果您调用os.wait4，则waitpid引发os.error导致poll()返回的None。（http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/forking.py#l141）
• is_alive()使用该返回值来确定进程是否处于活动状态。（http://hg.python.org/cpython/file/c167ab1c49c9/Lib/multiprocessing/process.py#l159）
  • => 返回True。

http://asciinema.org/a/5901

http://asciinema.org/a/5901

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Replace following line:

替换以下行：

ret = os.wait4(proc.pid, 0)

with:

和：

proc.join()