匿名管道
管道是一個單向通道,有點類似共享內存緩存.管道有兩端,包括輸入端和輸出端.對于一個進程的而言,它只能看到管道一端,即要么是輸入端要么是輸出端.
os.pipe()返回2個文件描述符(r, w),表示可讀的和可寫的.示例代碼如下:
#!/usr/bin/python
import time
import os
def child(wpipe):
??? print('hello from child', os.getpid())
??? while True:
??????? msg = 'how are you\n'.encode()
??????? os.write(wpipe, msg)
??????? time.sleep(1)
def parent():
??? 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
我們也可以改進代碼,不用os.read()從管道中讀取二進制字節,而是從文件對象中讀取字符串.這時需要用到os.fdopen()把底層的文件描述符(管道)包裝成文件對象,然后再用文件對象中的readline()方法讀取.這里請注意文件對象的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,把父進程和子進程的管道中不用的一端關閉掉.
#!/usr/bin/python
import time
import os
def child(wpipe):
??? print('hello from child', os.getpid())
??? while True:
??????? msg = 'how are you\n'.encode()
??????? os.write(wpipe, msg)
??????? time.sleep(1)
def parent():
??? 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
如果要與子進程進行雙向通信,只有一個pipe管道是不夠的,需要2個pipe管道才行.以下示例在父進程新建了2個管道,然后再fork子進程.os.dup2()實現輸出和輸入的重定向.spawn功能類似于subprocess.Popen(),既能發送消息給子進程,由能從子子進程獲取返回數據.
#!/usr/bin/python
#coding=utf-8
import os, sys
def spawn(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)#輸入流綁定到管道,將輸入重定向到管道一端parentStdin
??????? os.dup2(parentStdout, stdoutFd)#輸出流綁定到管道,發送到子進程childStdin
??? else:
??????? os.close(parentStdin)
??????? os.close(parentStdout)
??????? os.dup2(childStdin, stdinFd)#輸入流綁定到管道
??????? 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 #打印到輸出流parentStdout, 經管道發送到子進程childStdin
??? sys.stdout.flush()
??? reply = raw_input()
??? sys.stderr.write('Parent got: "%s"\n' % reply)#stderr沒有綁定到管道上
??? print 'Hello 2 from parent', mypid
??? sys.stdout.flush()
??? reply = sys.stdin.readline()#另外一種方式獲得子進程返回信息
??? sys.stderr.write('Parent got: "%s"\n' % reply[:-1])
pipetest.py代碼如下:
#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()#從管道獲取數據,來源于父經常stdout
??? time.sleep(3)
??? send = 'Child %d got: [%s]' % (mypid, recv)
??? print(send)#stdout綁定到管道上,發送到父進程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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