How Can I Obtain a C FILE* Handle from a C std::fstream Object?

Obtaining a FILE* Handle from an std::fstream Object

Problem:

Can C std::fstream objects be easily converted into C FILE* handles?

Background:

This conversion is desirable because certain C libraries accept FILE* handles as parameters, but some C libraries use std::fstream for file access.

Answer:

Unfortunately, there is no direct and cross-platform method to obtain a FILE* handle from an std::fstream object.

Explanation:

The implementation of std::fstream does not mandate the use of a FILE* handle. Therefore, it is possible that extracting the file descriptor from the std::fstream object and manually constructing a FILE object may result in conflicts and unpredictable behavior.

Alternative Approaches:

Instead of attempting the conversion, consider why a FILE handle is required. If possible, explore alternative methods within the C library that do not require FILE handles.

funopen() Function:

As a less recommended approach, you could investigate the use of the funopen() function. This BSD extension allows you to create a FILE object by providing functions that perform the actual file operations. By implementing appropriate functions, you could read from the std::fstream object within your FILE object. However, this approach is not portable and is not part of the POSIX standard.

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