C++ apt-get 处理 popen 的标准输出

Question

这听起来像是一个愚蠢的问题，我是 C++ 新手。

在 debian 上，我尝试使用 popen 调用 apt-get install。我需要解析这个程序的输出。不幸的是，我无法阅读 apt-get 的完整输出。

在某些时候 apt-get 可能会要求用户输入（询问是否应该安装依赖项）。我的程序无法输出这一行。

什么是最后一行（参见下面的示例，我的程序中缺少的行是：“您要继续 [Y/n] 吗？”）没有输出？

当我手动运行 apt-get 命令时，控制台输出如下所示：

$ sudo apt-get install python-wxtools
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  python-wxgtk2.8 python-wxversion
Suggested packages:
  wx2.8-doc wx2.8-examples ruby wish tk8.5 tcsh csh octave3.0 mksh pdksh
  python-xml editra
The following NEW packages will be installed:
  python-wxgtk2.8 python-wxtools python-wxversion
0 upgraded, 3 newly installed, 0 to remove and 8 not upgraded.
Need to get 5,942kB of archives.
After this operation, 25.0MB of additional disk space will be used.
Do you want to continue [Y/n]? 

注意：最后一行末尾没有换行符。

当我使用自己的程序时，最后一行丢失（输出）

$ sudo ./test/popen 
g++ -Wall -o test/popen test/popen.cpp
test/popen.cpp: In function ‘int main(int, char**, char**)’:
test/popen.cpp:22: warning: comparison between signed and unsigned integer expressions
apt-get install python-wxtools
Reading package lists...
Building dependency tree...
Reading state information...
The following extra packages will be installed:
  python-wxgtk2.8 python-wxversion
Suggested packages:
  wx2.8-doc wx2.8-examples ruby wish tk8.5 tcsh csh octave3.0 mksh pdksh
  python-xml editra
The following NEW packages will be installed:
  python-wxgtk2.8 python-wxtools python-wxversion
0 upgraded, 3 newly installed, 0 to remove and 8 not upgraded.
Need to get 5,942kB of archives.
After this operation, 25.0MB of additional disk space will be used.

注意：输出末尾的换行符

我在 c++ 中的 popen 参考实现如下所示：

// $Id: popen.cpp 126 2011-04-25 18:48:02Z wus $

#include <iostream>
#include <stdio.h>

using namespace std;

/**
 * run debians apt-get and check output
 */
int main(int argc, char **argv, char **envp) { 

    FILE *fp;
    char buffer[9];
    // must use a package which asks for dependencies
    char command[255] = "apt-get install python-wxtools";
    cout << command << endl;

    // Execute command, open /dev/stdout for reading
    fp = popen(command, "r");

    // read output character by character
    while (fread(buffer, 1, 1, fp) != EOF) {
        cout << buffer;
    }

    // close
    pclose(fp);
}

我尝试在 Linux 系统上这样做

$ uname -a
Linux shell1 2.6.35-28-server #50-Ubuntu SMP Fri Mar 18 18:59:25 UTC 2011 x86_64 GNU/Linux

$ gcc --version
gcc (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

尝试将-y 选项添加到apt-get。这将使它假设用户对所有事情都说“是”，这应该使它“正常工作”。我认为它现在失败了，因为它想提示用户，但它意识到没有可用的键盘输入（可能通过调用isatty(3)）所以它放弃了。

Answer 2

问题实际上不是输入缓冲，而是 cout 的输出缓冲。手动冲洗解决了这个问题：

while (fread(buffer, 1, 1, fp) != EOF) {
    cout << buffer << flush;
}