"""

 IPC

 http://www.cnblogs.com/BoyXiao/archive/2011/01/01/1923828.html

 """

 import os,time

 import threading

 def child(pipeout):

     zzz = 0

     while True:

         time.sleep(zzz)                                             # make parent wait

         msg=('spam %03d \n' % zzz).encode()                         # pipes are binary bytes

         os.write(pipeout,msg)                                       # send to parent

         zzz = (zzz+1)%5

 def parent(pipein):

     while True:

         line = os.read(pipein,32)                                   # blocks until data sent

         print('parent %d got [%s]  at [%s]'%(os.getpid(),line,time.ctime()))

 pipein,pipeout = os.pipe()

 threading.Thread(target = child,args=(pipeout,)).start()

 parent(pipein)

 """

 location: programing python 4td page 229

 spawn a child process/program,connect my stdin/stdput to child process's stdout/stdin

 my reads and writes map to output and input streams of the spawn program;much like typing

  together streams with subprocess module

 """

 import  os,sys

 def spawn(prog,*args):

     stdinFd = sys.stdin.fileno()            #pass progname,cmdline args

     stdoutFd =sys.stdin.fileno()            # get desciptors for streams,normally stdin =0,stdout =1

     parentStdin,childStdout = os.pipe()     # make two IPC pipe changels

     childStdin,parentStdout = os.pipe()     # pipe returns(inputfd,outputfd)

     pid = os.fork()                         #make a copy of this process

     if pid:

         os.close(childStdout)               # in parent process after fork

         os.close(childStdin)                # close child ends in parent

         os.dump2(parentStdin,stdinFd)       # my sys.stdin copy = pipe1[0]

         os.dump2(parentStdout,stdoutFd)     # my sys.stdout copy = pipe2[1]

     else:

         os.close(parentStdin)               # in child process afte fork:

         os.close(parentStdout)              # close parent ends in child

         os.dump2(childStdin,stdinFd)        # my sys.stdin copy = pipe2[0]

         os.dump2(childStdout,stdoutFd)      # my sysout copu = pipe1[1]

         args = (prog,)+args

         os.execvo(prog,args)                # new program in this process

         assert False,'execvp failed'        # os.exec call never returns here

 if __name__=='__main__':

     mypid = os.getpid()

     spawn('python','pipes-testchild.py','spawm') # fork child program

     print('hello  1 from parent',mypid)      # to child's stdin

     sys.stdout.flush()                       # subvert stdio buffering

     reply = input()                          # from child's stdout

     sys.stderr.write('parent got: "%s"\n' % reply) # stderr not tied to pipe

     print('hello 2 from parent',mypid)

     sys.stdout.flush()

     reply = sys.stdin.readline()

     sys.stderr.write('parent got:"%s"\n' % reply[:-1])
     

 """

 name pipes;os.mkfifo is not available on windown

 thare is no reason to fork here ,since fifo file ipes

 are external to proceses--shared fds in parent/child processes

 are irrelevent;

 """

 import os,time,sys

 fifoname ='/tmp/pipefifo'

 def child():

     pipeout = os.open(filename,os.O_WRONLY)                 # open fifo pipe as fd

     zzz = 0

     while True:

         time.sleep(zzz)

         msg =('spam %03d\n' %zzz).encode()

         os.write(pipeout,msg)

         zzz =(zzz+1)%5

 def parent():

     pipein = open(fifoname,'r')                                             # open file as text file object

     while True:

         line = pipein.readline()[:-1]                                       #block until data sent

         print('parent %d got "%s" at %s' % (os.getpid(),line,time.ctime()))

 if __name__ =='__main__':

     if not os.path.exits(filename):

         os.mkfifo(fifename)                                                 #create a named pipe file

     if len(sys.argv) == 1:

         parent()                                                            # run as parent if no args

     else:

         child()                                                             # else run as child process