Python C 程序子进程在“for line in iter"处挂起 [英] Python C program subprocess hangs at "for line in iter"

问题描述

好的，我正在尝试从 python 脚本运行 C 程序.目前我正在使用一个测试 C 程序:

Ok so I'm trying to run a C program from a python script. Currently I'm using a test C program:

#include <stdio.h>

int main() {
    while (1) {
        printf("2000
");
        sleep(1);
    }
    return 0;
}

模拟我将使用的程序，该程序不断地从传感器获取读数.然后我试图用python中的子进程从C程序中读取输出(在本例中为2000"):

To simulate the program that I will be using, which takes readings from a sensor constantly. Then I'm trying to read the output (in this case "2000") from the C program with subprocess in python:

#!usr/bin/python
import subprocess

process = subprocess.Popen("./main", stdout=subprocess.PIPE)
while True:
    for line in iter(process.stdout.readline, ''):
            print line,

但这不起作用.从使用打印语句开始，它运行 .Popen 行，然后在 for line in iter(process.stdout.readline, ''): 处等待，直到我按下 Ctrl-C.

but this is not working. From using print statements, it runs the .Popen line then waits at for line in iter(process.stdout.readline, ''):, until I press Ctrl-C.

这是为什么?这正是我见过的大多数示例的代码，但它不读取文件.

Why is this? This is exactly what most examples that I've seen have as their code, and yet it does not read the file.

有没有办法让它只在需要阅读的时候运行?

Is there a way of making it run only when there is something to be read?

推荐答案

这是块缓冲问题.

以下是我对 Python:从 subprocess.communicate() 读取流式输入 问题.

stdio 的程序在终端中交互运行，则通常是行缓冲的，而当它们的标准输出重定向到管道时，它们会被块缓冲.在后一种情况下，在缓冲区溢出或刷新之前，您不会看到新行.

stdio-based programs as a rule are line buffered if they are running interactively in a terminal and block buffered when their stdout is redirected to a pipe. In the latter case, you won't see new lines until the buffer overflows or flushed.

为避免在每次 printf() 调用后调用 fflush()，您可以在开始时调用 C 程序来强制行缓冲输出:

To avoid calling fflush() after each printf() call, you could force line buffered output by calling in a C program at the very beginning:

setvbuf(stdout, (char *) NULL, _IOLBF, 0); /* make line buffered stdout */

在这种情况下，一旦打印了换行符，缓冲区就会被刷新.

As soon as a newline is printed the buffer is flushed in this case.

有 stdbuf 实用程序可以让您在不修改源代码的情况下更改缓冲类型，例如:

There is stdbuf utility that allows you to change buffering type without modifying the source code e.g.:

from subprocess import Popen, PIPE

process = Popen(["stdbuf", "-oL", "./main"], stdout=PIPE, bufsize=1)
for line in iter(process.stdout.readline, b''):
    print line,
process.communicate() # close process' stream, wait for it to exit

还有其他可用的实用程序，请参阅关闭管道中的缓冲.

There are also other utilities available, see Turn off buffering in pipe.

要诱使子进程认为它正在交互运行，您可以使用 pexpect 模块 或其类似物，有关使用 pexpect 和 pty 模块的代码示例，请参阅 Python 子进程 readlines() 挂起.这是那里提供的 pty 示例的变体(它应该适用于 Linux):

To trick the subprocess into thinking that it is running interactively, you could use pexpect module or its analogs, for code examples that use pexpect and pty modules, see Python subprocess readlines() hangs. Here's a variation on the pty example provided there (it should work on Linux):

#!/usr/bin/env python
import os
import pty
import sys
from select import select
from subprocess import Popen, STDOUT

master_fd, slave_fd = pty.openpty()  # provide tty to enable line buffering
process = Popen("./main", stdin=slave_fd, stdout=slave_fd, stderr=STDOUT,
                bufsize=0, close_fds=True)
timeout = .1 # ugly but otherwise `select` blocks on process' exit
# code is similar to _copy() from pty.py
with os.fdopen(master_fd, 'r+b', 0) as master:
    input_fds = [master, sys.stdin]
    while True:
        fds = select(input_fds, [], [], timeout)[0]
        if master in fds: # subprocess' output is ready
            data = os.read(master_fd, 512) # <-- doesn't block, may return less
            if not data: # EOF
                input_fds.remove(master)
            else:
                os.write(sys.stdout.fileno(), data) # copy to our stdout
        if sys.stdin in fds: # got user input
            data = os.read(sys.stdin.fileno(), 512)
            if not data:
                input_fds.remove(sys.stdin)
            else:
                master.write(data) # copy it to subprocess' stdin
        if not fds: # timeout in select()
            if process.poll() is not None: # subprocess ended
                # and no output is buffered <-- timeout + dead subprocess
                assert not select([master], [], [], 0)[0] # race is possible
                os.close(slave_fd) # subproces don't need it anymore
                break
rc = process.wait()
print("subprocess exited with status %d" % rc)

或者通过pexpect

使用pty

pexpect 将 pty 处理包装到 更高级别的接口:

Or use pty via pexpect

pexpect wraps pty handling into higher level interface:

#!/usr/bin/env python
import pexpect

child = pexpect.spawn("/.main")
for line in child:
    print line,
child.close()

问:为什么不直接使用管道 (popen())? 解释了为什么伪 TTY 很有用.

Q: Why not just use a pipe (popen())? explains why pseudo-TTY is useful.

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