如何从POSIX文件描述符构造一个c ++ fstream？ [英] How to construct a c++ fstream from a POSIX file descriptor?

问题描述

我基本上是在寻找一个C ++版本的fdopen（）。我做了一些研究，这是一些似乎应该很容易，但事实证明是非常复杂的事情之一。我在这个信念中缺少一些东西（即它真的很容易）？

I'm basically looking for a C++ version of fdopen(). I did a bit of research on this and it is one of those things that seems like it should be easy, but turns out to be very complicated. Am I missing something in this belief (i.e. it really is easy)? If not, is there a good library out there somewhere to handle this?

编辑：将我的示例解决方案移动到一个单独的答案。

Moved my example solution to a separate answer.

推荐答案

从ÉricMalenfant给出的答案：

From the answer given by Éric Malenfant:

AFAIK没有办法这样做在
标准C ++。根据您的
平台，您的
标准库的实现可能提供（作为
非标准扩展）fstream
构造函数，以文件描述符
作为输入。 （这是
libstdc ++，IIRC的情况）或文件*。

AFAIK, there is no way to do this in standard C++. Depending on your platform, your implementation of the standard library may offer (as a nonstandard extension) a fstream constructor taking a file descriptor as input. (This is the case for libstdc++, IIRC) or a FILE*.

基于上述观察和我的研究有两个变体的工作代码;一个用于libstdc ++，另一个用于Microsoft Visual C ++。

Based on above observations and my research below there's working code in two variants; one for libstdc++ and another one for Microsoft Visual C++.

有非标准的 __ gnu_cxx :: stdio_filebuf 类模板，它继承了 std :: basic_streambuf 并具有以下构造函数

There's non-standard __gnu_cxx::stdio_filebuf class template which inherits std::basic_streambuf and has the following constructor

stdio_filebuf (int __fd, std::ios_base::openmode __mode, size_t __size=static_cast< size_t >(BUFSIZ)) 

与描述此构造函数将文件流缓冲区与打开的POSIX文件描述符相关联。

我们创建它传递POSIX句柄（行1），然后我们将它传递给istream的构造函数作为basic_streambuf（行2）：

We create it passing POSIX handle (line 1) and then we pass it to istream's constructor as basic_streambuf (line 2):

#include <ext/stdio_filebuf.h>
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    ofstream ofs("test.txt");
    ofs << "Writing to a basic_ofstream object..." << endl;
    ofs.close();

    int posix_handle = fileno(::fopen("test.txt", "r"));

    __gnu_cxx::stdio_filebuf<char> filebuf(posix_handle, std::ios::in); // 1
    istream is(&filebuf); // 2

    string line;
    getline(is, line);
    cout << "line: " << line << std::endl;
    return 0;
}

---

Microsoft Visual C ++ < h1>

以前是非标准的版本 ifstream的构造函数接受POSIX文件描述符，但它从当前文档和从代码。还有另一个非标准版本的ifstream的构造函数取FILE *

---

Microsoft Visual C++

There used to be non-standard version of ifstream's constructor taking POSIX file descriptor but it's missing both from current docs and from code. There is another non-standard version of ifstream's constructor taking FILE*

explicit basic_ifstream(_Filet *_File)
    : _Mybase(&_Filebuffer),
        _Filebuffer(_File)
    {   // construct with specified C stream
    }

它没有记录（我甚至不能找到任何旧的文档，它会存在）。我们将其称为（第1行），参数是调用 _fdopen的结果从POSIX文件句柄中获取C流FILE *。

and it's not documented (I couldn't even find any old documentation where it would be present). We call it (line 1) with the parameter being the result of calling _fdopen to get C stream FILE* from POSIX file handle.

#include <cstdio>
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    ofstream ofs("test.txt");
    ofs << "Writing to a basic_ofstream object..." << endl;
    ofs.close();

    int posix_handle = ::_fileno(::fopen("test.txt", "r"));

    ifstream ifs(::_fdopen(posix_handle, "r")); // 1

    string line;
    getline(ifs, line);
    ifs.close();
    cout << "line: " << line << endl;
    return 0;
}

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