使用Python从正在运行的C程序中捕获标准输出

Question

I've got this C program:

#include <stdio.h>
#include <Windows.h>

int main() {
    for (int i=0;; i++) {
        printf("%i\n",i);
        Sleep(100);
    }
    return 0;
}

And I have a Python script that tries to capture its output and do something with it:

from subprocess import Popen, PIPE

p = Popen("a.exe", stdout=PIPE, shell=True)
print p.stdout.readline()

... and it hangs on the last line without printing anything on a screen.

I've tried to solve this problem using the Python shell and found this:

>>> from test import *
>>> p.stdout.flush()
>>> p.stdout.readline()
'0\r\n'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
^CKeyboardInterrupt

It can actually read the output but only when I send KeyboardInterrupt. p.stdout.read(1) behaves the same way.

So, what's a working, correct way to do what I want?

Edit:
Ok, looks like it is impossible on Windows, see comments to first answer.

Answer 1

The output is being buffered so you need to use iter(p.stdout.readline,"")

p = Popen("a.exe", stdout=PIPE)

for line in iter(p.stdout.readline,""):
     print line

Try flushing stdout from c if sys.stdout.flush() if is not working, as far as I know lines are block buffered when writing to a pipe:

int main() {
    for (int i=0;; i++) {
        printf("%i\n",i);
        Sleep(100);
        fflush(stdout);

    }
    return 0;
}

Answer 2

Rough example of something I did a long time ago.

process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
while process.poll() is None:
    txt = process.stdout.read().decode("utf-8")
    result += txt

You can make your own StringIO which handles file io operations like read and write.

https://docs.python.org/3/library/io.html

I have not tested the code below!

import io
buffer = io.StringIO()
p = Popen(["a.exe"], stdout=buffer)

Answer 3

以无缓冲模式运行python：

python -u myprogram.py