How to construct a c++ fstream from a POSIX file descriptor?

From the answer given by Éric Malenfant:

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*.

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++.

libstdc++

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)) 

with description This constructor associates a file stream buffer with an open POSIX file descriptor.

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++

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
    }

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;
}

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 (This is the case for libstdc++, IIRC) or a FILE* as an input.

Another alternative would be to use a boost::iostreams::file_descriptor device, which you could wrap in a boost::iostreams::stream if you want to have an std::stream interface to it.

There's a good chance your compiler offers a FILE-based fstream constructor, even though it's non-standard. For example:

FILE* f = fdopen(my_fd, "a");
std::fstream fstr(f);
fstr << "Greetings\n";

But as far as I know, there's no portable way to do this.

Part of the original (unstated) motivation of this question is to have the ability to pass data either between programs or between two parts of a test program using a safely created temporary file, but tmpnam() throws a warning in gcc, so I wanted to use mkstemp() instead. Here is a test program that I wrote based on the answer given by Éric Malenfant but using mkstemp() instead of fdopen(); this works on my Ubuntu system with Boost libraries installed:

#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <string>
#include <iostream>
#include <boost/filesystem.hpp>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>

using boost::iostreams::stream;
using boost::iostreams::file_descriptor_sink;
using boost::filesystem::path;
using boost::filesystem::exists;
using boost::filesystem::status;
using boost::filesystem::remove;

int main(int argc, const char *argv[]) {
  char tmpTemplate[13];
  strncpy(tmpTemplate, "/tmp/XXXXXX", 13);
  stream<file_descriptor_sink> tmp(mkstemp(tmpTemplate));
  assert(tmp.is_open());
  tmp << "Hello mkstemp!" << std::endl;
  tmp.close();
  path tmpPath(tmpTemplate);
  if (exists(status(tmpPath))) {
    std::cout << "Output is in " << tmpPath.file_string() << std::endl;
    std::string cmd("cat ");
    cmd += tmpPath.file_string();
    system(cmd.c_str());
    std::cout << "Removing " << tmpPath.file_string() << std::endl;
    remove(tmpPath);
  }
}