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

Question

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.当实际子进程完成时,multiprocess.Process.is_alive()仍然返回True.这是矛盾的.为什么？

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)

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

我的问题是关于最后三个输出线.为什么在实际过程结束时is_alive()返回True.怎么会发生这种情况？

Answer 1

你应该使用Process.join而不是os.wait4.

• Process.is_alivewaitpid通过内部呼叫multiprocessing.forking.Popen.poll.
• 如果你打电话os.wait4,waitpid加注os.error导致poll()返回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

替换以下行:

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

有:

proc.join()