子进程 readline 挂起等待 EOF

Question

我有一个简单的 c++ 程序，我试图通过 python 脚本来执行它。 （我对编写脚本非常陌生）并且我无法通过管道读取输出。从我所看到的情况来看，如果没有 EOF，readline() 似乎将无法工作，但我希望能够在程序中间读取并让脚本响应输出的内容。而不是读取输出，它只是挂起 python脚本：

#!/usr/bin/env python
import subprocess
def call_random_number():
    print "Running the random guesser"
    rng = subprocess.Popen("./randomNumber", stdin=subprocess.PIPE, stdout=subprocess.PIPE, shell=True)
    i = 50
    rng.stdin.write("%d\n" % i)
    output = rng.stdout.readline()
    output = rng.stdout.readline()

call_random_number()

和 c++ 文件，它生成一个介于 1 到 100 之间的随机数，然后检查用户的猜测，直到他们猜对为止

#include<iostream>
#include<cstdlib>

int main(){
  std::cout<< "This program generates a random number from 1 to 100 and asks the user to enter guesses until they succuessfully guess the number.  It then tells the user how many guesses it took them\n";
  std::srand(std::time(NULL));
  int num = std::rand() % 100;
  int guessCount = 0;
  int guess = -1;
  std::cout << "Please enter a number:  ";
  std::cin >> guess;
  while(guess != num){
    if (guess > num){
        std::cout << "That guess is too high.  Please guess again:  ";
    } else {
        std::cout << "That guess is too low.  Please guess again:  ";
    }
    std::cin >> guess;
    guessCount++;
  }
  std::cout << "Congratulations!  You solved it in " << guessCount << " guesses!\n";
}

最终目标是让脚本通过二进制搜索解决问题，但现在我只想能够读取一行而不是文件末尾

Answer 1

作为@Ron Reiter pointed out，您不能使用readline()，因为cout 不会隐式打印换行符——您需要std::endl 或"\n" 这里。

对于交互式使用，当你无法更改子程序时，pexpect module 提供了几种方便的方法（一般为it solves for free: input/output directly from/to terminal (outside of stdin/stdout) and block-buffering issues）：

#!/usr/bin/env python
import sys

if sys.version_info[:1] < (3,):
    from pexpect import spawn, EOF # $ pip install pexpect
else:
    from pexpect import spawnu as spawn, EOF # Python 3

child = spawn("./randomNumber") # run command
child.delaybeforesend = 0 
child.logfile_read = sys.stdout # print child output to stdout for debugging
child.expect("enter a number: ") # read the first prompt
lo, hi = 0, 100
while lo <= hi:
    mid = (lo + hi) // 2
    child.sendline(str(mid)) # send number
    index = child.expect([": ", EOF]) # read prompt
    if index == 0: # got prompt
        prompt = child.before
        if "too high" in prompt:
            hi = mid - 1 # guess > num
        elif "too low" in prompt:
            lo = mid + 1 # guess < num
    elif index == 1: # EOF
        assert "Congratulations" in child.before
        child.close()
        break
else:
    print('not found')
    child.terminate()
sys.exit(-child.signalstatus if child.signalstatus else child.exitstatus)

它有效，但它是二进制搜索，因此(traditionally) there could be bugs。

这是一个类似的代码，使用subprocess 模块进行比较：

#!/usr/bin/env python
from __future__ import print_function
import sys
from subprocess import Popen, PIPE

p = Popen("./randomNumber", stdin=PIPE, stdout=PIPE,
          bufsize=1, # line-buffering
          universal_newlines=True) # enable text mode
p.stdout.readline() # discard welcome message: "This program gener...

readchar = lambda: p.stdout.read(1)
def read_until(char):
    buf = []
    for c in iter(readchar, char):
        if not c: # EOF
            break
        buf.append(c)
    else: # no EOF
        buf.append(char)
    return ''.join(buf).strip()

prompt = read_until(':') # read 1st prompt
lo, hi = 0, 100
while lo <= hi:
    mid = (lo + hi) // 2
    print(prompt, mid)
    print(mid, file=p.stdin) # send number
    prompt = read_until(':') # read prompt
    if "Congratulations" in prompt:
        print(prompt)
        print(mid)
        break # found
    elif "too high" in prompt:
        hi = mid - 1 # guess > num
    elif "too low" in prompt:
        lo = mid + 1 # guess < num
else:
    print('not found')
    p.kill()
for pipe in [p.stdin, p.stdout]:
    try:
        pipe.close()
    except OSError:
        pass
sys.exit(p.wait())

Answer 2

您可能必须显式关闭stdin，因此子进程将停止挂起，我认为这是您的代码正在发生的事情——这可以通过在终端上运行 top 并检查是否有 randomnumber 来验证s 状态保持睡眠，如果它在预期的执行时间之后使用 0% CPU。

简而言之，如果您在rng=subprocess(...) 调用之后立即添加rng.stdin.close()，它可能会毫无问题地恢复。另一种选择是执行output=rng.communicate(stdin="%d\n" % i) 并查看output[0]和output[1]，他们分别是stdout 和stderr。你可以在communicatehere找到信息。

Answer 3

我很确定在您的 C++ 程序中添加换行符会导致 readlines 返回。