Getting a FILE* from a std::fstream

Question

Is there a (cross-platform) way to get a C FILE* handle from a C++ std::fstream ?

The reason I ask is because my C++ library accepts fstreams and in one particular function I'd like to use a C library that accepts a FILE*.

Answer 1

Well, you can get the file descriptor - I forget whether the method is fd() or getfd(). The implementations I've used provide such methods, but the language standard doesn't require them, I believe - the standard shouldn't care whether your platform uses fd's for files.

From that, you can use fdopen(fd, mode) to get a FILE*.

However, I think that the mechanisms the standard requires for synching STDIN/cin, STDOUT/cout and STDERR/cerr don't have to be visible to you. So if you're using both the fstream and FILE*, buffering may mess you up.

Also, if either the fstream OR the FILE closes, they'll probably close the underlying fd, so you need to make sure you flush BOTH before closing EITHER.

Answer 2

The short answer is no.

The reason, is because the std::fstream is not required to use a FILE* as part of its implementation. So even if you manage to extract file descriptor from the std::fstream object and manually build a FILE object, then you will have other problems because you will now have two buffered objects writing to the same file descriptor.

The real question is why do you want to convert the std::fstream object into a FILE*?

Though I don't recommend it you could try looking up funopen()
Unfortunately this is not a POSIX API (it a BSD extension) so its portability is in question. Which is also probably why I can't find anybody that has wrapped a std::stream with an object like this.

FILE *funopen(
              const void *cookie,
              int    (*readfn )(void *, char *, int),
              int    (*writefn)(void *, const char *, int),
              fpos_t (*seekfn) (void *, fpos_t, int),
              int    (*closefn)(void *)
             );

This allows you to build a FILE object and specify some functions that will be used to do the actual work. If you write appropriate functions you can get them to read from the std::fstream object that actually has the file open.

Answer 3

There isn't a standardized way. I assume this is because the C++ standardization group didn't want to assume that a file handle can be represented as a fd.

Most platforms do seem to provide some non-standard way to do this.

http://www.ginac.de/~kreckel/fileno/ provides a good writeup of the situation and provides code that hides all the platform specific grossness, at least for GCC. Given how gross this is just on GCC, I think I'd avoid doing this all together if possible.

Answer 4

Please, look at this library

MDS utils

It solves the problem, because allows to treat a C FILE* as a C++ stream. It uses Boost C++ libraries. You have to use Doxygen to view the documentation.