Possible Duplicate:
Getting a FILE* from a std::fstream
I am working on Linux and file descriptors are the main model in this OS.
I was wondering whether is there any library or any way to retrieve the native Linux file descriptor starting from a C++ std::fstream.
I thought about boost::iostream since there is a class called file_descriptor but I understood that its purpose is different from the one I want to achieve.
Do you know some way to do that?
You can go the other way: implement your own stream buffer that wraps a file descriptor and then use it with iostream instead of fstream. Using Boost.Iostreams can make the task easier.
Non-portable gcc solution is:
#include <ext/stdio_filebuf.h>
{
int fd = ...;
__gnu_cxx::stdio_filebuf<char> fd_file_buf{fd, std::ios_base::out | std::ios_base::binary};
std::ostream fd_stream{&fd_file_buf};
// Write into fd_stream.
// ...
// Flushes the stream and closes fd at scope exit.
}
There is no (standard) way to extract the file number from an std::fstream since the standard library does not mandate how file streams will be implemented.
Rather, you need to use the C file API if you want to do this (using FILE*).
There is no official way to get the private file handle of a file stream (or actualy a std::basic_filebuf), just because it should be portable and discourage use of platform-specific functions.
However, you can do ugly hack like inheriting std::basic_filebuf and from that try to pry out the file handle. It's not something I recommend though as it will probably break on different versions of the C++ library.
There is no support of exposing file descriptor neither in standard C++ nor in libstdc++.
