Python - 如何从长时间运行的子进程中读取？

Question

Reading from a long-running subprocess seems to be possible, but I can only get it to work if the subprocess is python3 -i :

>>> sub = subprocess.Popen(['python3', '-i'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>>> sub.stdin.write(b'2+3\n')
4
>>> sub.stdin.flush()
>>> sub.stdout.readline()
b'5\n'

Attempting the same thing with a small echo program does not work:

>>> sub = subprocess.Popen(['./echo'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>>> sub.stdin.write(b'2+3\n')
4
>>> sub.stdin.flush()
>>> sub.stdout.readline()
^ Hangs here until interrupted with Ctrl-C

echo reads from STDIO and echoes when it gets a newline. It works fine when run directly. Here is the code for it:

#include <iostream>
#include <unistd.h>

int main() {
  char message[64];
  uint8_t cursor = 0;
  
  while(true) {
    read(STDIN_FILENO, message + cursor, 1);
    
    if(message[cursor++] == '\n') {
      message[cursor] = 0;
      printf(message);
      cursor = 0;
    }
  }
}

I've tried a few other techniques like .communicate() with similar results. Oddly, subprocess.Popen(['python3'], ...) fails where the same call works when running python3 -i , even though python3 and python3 -i seem to do the same thing when run at the terminal.

Answer 1

The issue turned out to be buffering within the subprocess' stdout. I solved it by setting line-buffering using setvbuf (stdout, NULL, _IOLBF, 1024); . If no buffering at all is desired, setvbuf (stdout, NULL, _IONBF, 0); can be used.