如何防止C共享库在python中的stdout上打印？ [英] How do I prevent a C shared library to print on stdout in python?

问题描述

我使用python库，该库导入了一个在stdout上打印的C共享库。我想要一个干净的输出以便与管道一起使用或在文件中重定向。打印是在python之外的共享库中完成的。

I work with a python lib that imports a C shared library that prints on stdout. I want a clean output in order to use it with pipes or to redirect in files. The prints are done outside of python, in the shared library.

一开始，我的方法是：

# file: test.py
import os
from ctypes import *
from tempfile import mktemp

libc = CDLL("libc.so.6")

print # That's here on purpose, otherwise hello word is always printed

tempfile = open(mktemp(),'w')
savestdout = os.dup(1)
os.close(1)
if os.dup(tempfile.fileno()) != 1:
    assert False, "couldn't redirect stdout - dup() error"

# let's pretend this is a call to my library
libc.printf("hello world\n")

os.close(1)
os.dup(savestdout)
os.close(savestdout)

第一种方法只能奏效：

-由于某种原因，它在移动stdout之前需要一个 print语句，否则总是打印问候词。因此，它将打印一个空行，而不是库通常输出的所有模糊测试。

-更令人讨厌的是，重定向到文件时失败：

This first approach is half working:
- For some reason, it needs a "print" statement just before moving stdout, otherwise hello word is always printed. As a result it will print an empty line instead of all the fuzz the library usually outputs.
- More annoying, it fails when redirecting to a file:

$python test.py > foo && cat foo

hello world

我的第二次python尝试是受到另一个类似的启发注释中给出的线程：

My second python attempt was inspired from another similar thread given in the comments:

import os
import sys
from ctypes import *
libc = CDLL("libc.so.6")

devnull = open('/dev/null', 'w')
oldstdout = os.dup(sys.stdout.fileno())
os.dup2(devnull.fileno(), 1)

# We still pretend this is a call to my library
libc.printf("hello\n")

os.dup2(oldstdout, 1)

这一个

由于我觉得这有点低，所以我决定完全使用ctypes。我从此C程序中获得了灵感，该程序不显示任何内容：

Since I felt this was a bit low level, I then decided to go completely with ctypes. I took inspiration from this C program, which does not print anything:

#include <stdio.h>

int main(int argc, const char *argv[]) {
    char buf[20];
    int saved_stdout = dup(1);
    freopen("/dev/null", "w", stdout);

    printf("hello\n"); // not printed

    sprintf(buf, "/dev/fd/%d", saved_stdout);
    freopen(buf, "w", stdout);

    return 0;
}

我建立了以下示例：

from ctypes import *
libc = CDLL("libc.so.6")

saved_stdout = libc.dup(1)
stdout = libc.fdopen(1, "w")
libc.freopen("/dev/null", "w", stdout);

libc.printf("hello\n")

libc.freopen("/dev/fd/" + str(saved_stdout), "w", stdout)

即使我在printf之后libc.fflush（stdout），它也会打印你好。我开始认为可能无法在python中做我想做的事情。也许我获取指向stdout的文件指针的方法不正确。

This prints "hello", even if I libc.fflush(stdout) just after the printf. I am starting to think it may be not possible to do what I want in python. Or maybe the way I get a file pointer to stdout is not right.

您怎么看？

推荐答案

基于 @Yinon Ehrlich的答案。此变体试图避免泄漏文件描述符：

Based on @Yinon Ehrlich's answer. This variant tries to avoid leaking file descriptors:

import os
import sys
from contextlib import contextmanager

@contextmanager
def stdout_redirected(to=os.devnull):
    '''
    import os

    with stdout_redirected(to=filename):
        print("from Python")
        os.system("echo non-Python applications are also supported")
    '''
    fd = sys.stdout.fileno()

    ##### assert that Python and C stdio write using the same file descriptor
    ####assert libc.fileno(ctypes.c_void_p.in_dll(libc, "stdout")) == fd == 1

    def _redirect_stdout(to):
        sys.stdout.close() # + implicit flush()
        os.dup2(to.fileno(), fd) # fd writes to 'to' file
        sys.stdout = os.fdopen(fd, 'w') # Python writes to fd

    with os.fdopen(os.dup(fd), 'w') as old_stdout:
        with open(to, 'w') as file:
            _redirect_stdout(to=file)
        try:
            yield # allow code to be run with the redirected stdout
        finally:
            _redirect_stdout(to=old_stdout) # restore stdout.
                                            # buffering and flags such as
                                            # CLOEXEC may be different

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