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Martin Geisler c9f66fd425 Format all Markdown files with dprint (#1157 )

This is the result of running `dprint fmt` after removing `src/` from
the list of excluded directories.

This also reformats the Rust code: we might want to tweak this a bit in
the future since some of the changes removes the hand-formatting. Of
course, this formatting can be seen as a mis-feature, so maybe this is
good overall.

Thanks to mdbook-i18n-helpers 0.2, the POT file is nearly unchanged
after this, meaning that all existing translations remain valid! A few
messages were changed because of stray whitespace characters:

     msgid ""
     "Slices always borrow from another object. In this example, `a` has to remain "
    -"'alive' (in scope) for at least as long as our slice. "
    +"'alive' (in scope) for at least as long as our slice."
     msgstr ""

The formatting is enforced in CI and we will have to see how annoying
this is in practice for the many contributors. If it becomes annoying,
we should look into fixing dprint/check#11 so that `dprint` can annotate
the lines that need fixing directly, then I think we can consider more
strict formatting checks.

I added more customization to `rustfmt.toml`. This is to better emulate
the dense style used in the course:

- `max_width = 85` allows lines to take up the full width available in
our code blocks (when taking margins and the line numbers into account).
- `wrap_comments = true` ensures that we don't show very long comments
in the code examples. I edited some comments to shorten them and avoid
unnecessary line breaks — please trim other unnecessarily long comments
when you see them! Remember we're writing code for slides 😄
- `use_small_heuristics = "Max"` allows for things like struct literals
and if-statements to take up the full line width configured above.

The formatting settings apply to all our Rust code right now — I think
we could improve this with https://github.com/dprint/dprint/issues/711
which lets us add per-directory `dprint` configuration files. However,
the `inherit: true` setting is not yet implemented (as far as I can
tell), so a nested configuration file will have to copy most or all of
the top-level file.

2023-12-31 00:15:07 +01:00

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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:

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}}
        unimplemented!()
    }
}

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

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

{{#include exercise.rs:main}}

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Safe FFI Wrapper

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