Python程序間通訊之匿名管道

管道是乙個單向通道,有點類似共享記憶體快取.管道有兩端,包括輸入端和輸出端.對於乙個程序的而言,它只能看到管道一端,即要麼是輸入端要麼是輸出端.

os.pipe()返回2個檔案描述符(r, w),表示可讀的和可寫的.示例**如下:

#!/usr/bin/python
import time
import os
defchild
(wpipe):
print('hello from child', os.getpid())
while
true:
msg = 'how are you\n'.encode()
os.write(wpipe, msg)
time.sleep(1)
defparent
():    rpipe, wpipe = os.pipe()
pid = os.fork()
if pid == 0:
child(wpipe)
assert
false, 'fork child process error!'
else:
os.close(wpipe)
print('hello from parent', os.getpid(), pid)
fobj = os.fdopen(rpipe, 'r')
while
true:
recv = os.read(rpipe, 32)
print recv
parent()

輸出如下:

('hello from parent', 5053, 5054)
('hello from child', 5054)
how are you
how are you
how are you
how are you

我們也可以改進**,不用os.read()從管道中讀取二進位制位元組,而是從檔案物件中讀取字串.這時需要用到os.fdopen()把底層的檔案描述符(管道)包裝成檔案物件,然後再用檔案物件中的readline()方法讀取.這裡請注意檔案物件的readline()方法總是讀取有換行符』\n』的一行,而且連換行符也讀取出來.還有一點要改進的地方是,把父程序和子程序的管道中不用的一端關閉掉.

#!/usr/bin/python
import time
import os
defchild
(wpipe):
print('hello from child', os.getpid())
while
true:
msg = 'how are you\n'.encode()
os.write(wpipe, msg)
time.sleep(1)
defparent
():    rpipe, wpipe = os.pipe()
pid = os.fork()
if pid == 0:
os.close(rpipe)
child(wpipe)
assert
false, 'fork child process error!'
else:
os.close(wpipe)
print('hello from parent', os.getpid(), pid)
fobj = os.fdopen(rpipe, 'r')
while
true:
recv = fobj.readline()[:-1]
print recv
parent()

輸出如下:

('hello from parent', 5108, 5109)
('hello from child', 5109)
how are you
how are you
how are you

如果要與子程序進行雙向通訊,只有乙個pipe管道是不夠的,需要2個pipe管道才行.以下示例在父程序新建了2個管道,然後再fork子程序.os.dup2()實現輸出和輸入的重定向.spawn功能類似於subprocess.popen(),既能傳送訊息給子程序,由能從子子程序獲取返回資料.

#!/usr/bin/python
#coding=utf-8
import os, sys
defspawn
(prog, *args):
stdinfd = sys.stdin.fileno()
stdoutfd = sys.stdout.fileno()
parentstdin, childstdout = os.pipe()
childstdin, parentstdout= os.pipe()
pid = os.fork()
if pid:
os.close(childstdin)
os.close(childstdout)
os.dup2(parentstdin, stdinfd)#輸入流繫結到管道,將輸入重定向到管道一端parentstdin
os.dup2(parentstdout, stdoutfd)#輸出流繫結到管道,傳送到子程序childstdin
else:
os.close(parentstdin)
os.close(parentstdout)
os.dup2(childstdin, stdinfd)#輸入流繫結到管道
os.dup2(childstdout, stdoutfd)
args = (prog, ) + args
os.execvp(prog, args)
assert
false, 'execvp failed!'
if __name__ == '__main__':
mypid = os.getpid()
spawn('python', 'pipetest.py', 'spam')
print
'hello 1 from parent', mypid #列印到輸出流parentstdout, 經管道傳送到子程序childstdin
sys.stdout.flush()
reply = raw_input()
sys.stderr.write('parent got: "%s"\n' % reply)#stderr沒有繫結到管道上
print
'hello 2 from parent', mypid
sys.stdout.flush()
reply = sys.stdin.readline()#另外一種方式獲得子程序返回資訊
sys.stderr.write('parent got: "%s"\n' % reply[:-1])

pipetest.py**如下:

#coding=utf-8
import os, time, sys
mypid = os.getpid()
parentpid = os.getppid()
sys.stderr.write('child %d of %d got arg: "%s"\n'
%(mypid, parentpid, sys.argv[1]))
for i in range(2):
time.sleep(3)
recv = raw_input()#從管道獲取資料,**于父經常stdout
time.sleep(3)
send = 'child %d got: [%s]' % (mypid, recv)
print(send)#stdout繫結到管道上,傳送到父程序stdin
sys.stdout.flush()

輸出:

child 7265 of 7264 got arg: "spam"
parent got: "child 7265 got: [hello 1 from parent 7264]"
parent got: "child 7265 got: [hello 2 from parent 7264]"

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