comprehensive-rust/src/exercises/day-3/safe-ffi-wrapper.md

# Safe FFI Wrapper

Rust has great support for calling functions through a _foreign function
interface_ (FFI). We will use this to build a safe wrapper the `glibc` functions
you would use from C to read the filenames of a directory.

You will want to consult the manual pages:

* [`opendir(3)`](https://man7.org/linux/man-pages/man3/opendir.3.html)
* [`readdir(3)`](https://man7.org/linux/man-pages/man3/readdir.3.html)
* [`closedir(3)`](https://man7.org/linux/man-pages/man3/closedir.3.html)

You will also want to browse the [`std::ffi`] module, particular for [`CStr`]
and [`CString`] types which are used to hold NUL-terminated strings coming from
C. The [Nomicon] also has a very useful chapter about FFI.

[`std::ffi`]: https://doc.rust-lang.org/std/ffi/
[`CStr`]: https://doc.rust-lang.org/std/ffi/struct.CStr.html
[`CString`]: https://doc.rust-lang.org/std/ffi/struct.CString.html
[Nomicon]: https://doc.rust-lang.org/nomicon/ffi.html

Copy the code below to <https://play.rust-lang.org/> and fill in the missing
functions and methods:

```rust,should_panic
// TODO: remove this when you're done with your implementation.
#![allow(unused_imports, unused_variables, dead_code)]

{{#include safe-ffi-wrapper.rs:ffi}}

{{#include safe-ffi-wrapper.rs:DirectoryIterator}}
        unimplemented!()
    }
}

{{#include safe-ffi-wrapper.rs:Iterator}}
        unimplemented!()
    }
}

{{#include safe-ffi-wrapper.rs:Drop}}
        unimplemented!()
    }
}

{{#include safe-ffi-wrapper.rs:main}}
```