comprehensive-rust/src/unsafe-rust/exercise.md

minutes

minutes
30

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 for the libc functions you would use from C to read the names of files in a directory.

You will want to consult the manual pages:

• opendir(3)
• readdir(3)
• closedir(3)

You will also want to browse the std::ffi module. There you find a number of string types which you need for the exercise:

Types	Encoding	Use
str and String	UTF-8	Text processing in Rust
CStr and CString	NUL-terminated	Communicating with C functions
OsStr and OsString	OS-specific	Communicating with the OS

You will convert between all these types:

• &str to CString: you need to allocate space for a trailing \0 character,
• CString to *const i8: you need a pointer to call C functions,
• *const i8 to &CStr: you need something which can find the trailing \0 character,
• &CStr to &[u8]: a slice of bytes is the universal interface for "some unknown data",
• &[u8] to &OsStr: &OsStr is a step towards OsString, use OsStrExt to create it,
• &OsStr to OsString: you need to clone the data in &OsStr to be able to return it and call readdir again.

The Nomicon also has a very useful chapter about FFI.

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

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

{{#include exercise.rs:ffi}}

{{#include exercise.rs:DirectoryIterator}}
        todo!()
    }
}

{{#include exercise.rs:Iterator}}
        todo!()
    }
}

{{#include exercise.rs:Drop}}
        todo!()
    }
}

{{#include exercise.rs:main}}

FFI binding code is typically generated by tools like bindgen, rather than being written manually as we are doing here. However, bindgen can't run in an online playground.