Python：os.read和f.read之间的区别是什么

Question

import os
from threading import Thread
import time
def handle(fd):
    while True:
        data=raw_input()
        if data:
            print('send',data)
            os.write(fd,data)
r,w=os.pipe()
pid=os.fork()
if pid==0:
    os.close()
    os.dup2(r,0)
    # f=os.fdopen(r,'r')
    print(1)
    while True:
        # data=f.read()
        data=os.read(r,1024)
        if data:
            print('get',data)
else:
    os.close(r)
    t=Thread(target=handle,args=(w,))
    t.setDaemon(True)
    t.start()
    time.sleep(5)
    print('parent exit')
    os._exit(0)

when use os.read,the outputs is:

> 1
> dir
> ('send','dir')
> ('get','dir')
> parent exit

when use f.read:

> 1
> dir
> ('send','dir')
> parent exit
> ('get','dir')

Obviously,the subprocess was blocking on f.read() until the write end closed which caused by the parent process' withdrawal.What causes their different behavior?I want to make clear of the operating system's actions after calling f.read and os.read. I would be grateful if you can recommend some blogs to me.

Answer 1

os.read is a low-level system function. No decoding, no buffering, it just returns raw bytes up to a given limit. All other reads are built on the top of this function and may call it repeatedly. They add some functionality like decoding bytes into strings, splitting text on newlines etc.

In your program the parent sends data and after 5 seconds closes the pipe (precisely, the system does it, because the parent terminates).

Keep in mind that a pipe is a device, not a file. This means there is a state when a read can wait for new data. In this state you see the difference between those two read functions:

1. When os.read finds no data in the pipe, it waits until the data arrives or the pipe is closed (EOF). When there is some data in the pipe it returns it immediately.

Read at most n bytes from file descriptor fd. Return a bytestring containing the bytes read. If the end of the file referred to by fd has been reached, an empty bytes object is returned.

2. When f.read is called without a size limit, it waits till EOF:

Read and return at most size characters from the stream as a single str. If size is negative or None, reads until EOF.