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 脚本执行它。 （我对编写脚本非常陌生）并且我在通过管道读取输出时遇到问题。从我所看到的来看，如果没有 EOF，readline() 似乎无法工作，但我希望能够在程序中间读取并使脚本响应输出的内容。它不读取输出，而是挂起 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分搜索解决问题，但现在我只想能够读取一行而不是文件末尾

Answer 1

正如 @Ron Reiter 指出的，您不能使用 readline() 因为 cout 不会隐式打印换行符 - 您在这里需要 std::endl 或 "\n" 。

对于交互式使用，当您无法更改子程序时，pexpect module 提供了几种方便的方法（一般来说它解决了免费：直接从终端输入/输出（在标准输入/标准输出之外）和块缓冲问题）：

#!/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)

它可以工作，但它是二进制搜索，因此 （传统上）可能存在错误。

下面是使用 subprocess 模块进行比较的类似代码：

#!/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

我非常确定在 C++ 程序中添加换行符会导致读取行返回。

Answer 3

您可能必须显式关闭stdin，以便子进程将停止挂起，我认为这就是您的代码所发生的情况 - 这可以通过在终端上运行 top 并检查是否 randomnumber 的状态保持睡眠状态，如果它在执行所需的预期时间之后使用 0% CPU。

简而言之，如果您在 rng=subprocess(...) 调用之后添加 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]分别是stdout和stderr。您可以在此处<找到有关沟通的信息< /a>.