sagoez · October 22, 2023 12:00 · Oct 22, 2023
diff --git a/DirectoryIterator.rs b/DirectoryIterator.rs
@@ -0,0 +1,114 @@
+// Exercise: https://google.github.io/comprehensive-rust/exercises/day-3/safe-ffi-wrapper.html
+
+mod ffi {
+    use std::os::raw::{c_char, c_int};
+    #[cfg(not(target_os = "macos"))]
+    use std::os::raw::{c_long, c_uchar, c_ulong, c_ushort};
+
+    // Opaque type. See https://doc.rust-lang.org/nomicon/ffi.html.
+    #[repr(C)]
+    pub struct DIR {
+        _data: [u8; 0],
+        _marker: core::marker::PhantomData<(*mut u8, core::marker::PhantomPinned)>,
+    }
+
+    // Layout according to the Linux man page for readdir(3), where ino_t and
+    // off_t are resolved according to the definitions in
+    // /usr/include/x86_64-linux-gnu/{sys/types.h, bits/typesizes.h}.
+    #[cfg(not(target_os = "macos"))]
+    #[repr(C)]
+    pub struct dirent {
+        pub d_ino: c_ulong,
+        pub d_off: c_long,
+        pub d_reclen: c_ushort,
+        pub d_type: c_uchar,
+        pub d_name: [c_char; 256],
+    }
+
+    // Layout according to the macOS man page for dir(5).
+    #[cfg(target_os = "macos")]
+    #[repr(C)]
+    pub struct dirent {
+        pub d_fileno: u64,
+        pub d_seekoff: u64,
+        pub d_reclen: u16,
+        pub d_namlen: u16,
+        pub d_type: u8,
+        pub d_name: [c_char; 1024],
+    }
+
+    extern "C" {
+        pub fn opendir(s: *const c_char) -> *mut DIR;
+
+        #[cfg(not(all(target_os = "macos", target_arch = "x86_64")))]
+        pub fn readdir(s: *mut DIR) -> *const dirent;
+
+        // See https://github.com/rust-lang/libc/issues/414 and the section on
+        // _DARWIN_FEATURE_64_BIT_INODE in the macOS man page for stat(2).
+        //
+        // "Platforms that existed before these updates were available" refers
+        // to macOS (as opposed to iOS / wearOS / etc.) on Intel and PowerPC.
+        #[cfg(all(target_os = "macos", target_arch = "x86_64"))]
+        #[link_name = "readdir$INODE64"]
+        pub fn readdir(s: *mut DIR) -> *const dirent;
+
+        pub fn closedir(s: *mut DIR) -> c_int;
+    }
+}
+
+use std::ffi::{CStr, CString, OsStr, OsString};
+use std::os::unix::ffi::OsStrExt;
+
+#[derive(Debug)]
+struct DirectoryIterator {
+    path: CString,
+    dir: *mut ffi::DIR,
+}
+
+impl DirectoryIterator {
+    fn new(path: &str) -> Result<DirectoryIterator, String> {
+        // Call opendir and return a Ok value if that worked,
+        // otherwise return Err with a message.
+
+        // Hint: you will need to convert the path to a CString.
+        let path = CString::new(path).map_err(|err| format!("path is not valid: {}", err))?;
+        let dir = unsafe { ffi::opendir(path.as_ptr()) };
+
+        if dir.is_null() {
+            return Err("opendir failed".to_string());
+        }
+
+        Ok(DirectoryIterator { path, dir })
+    }
+}
+
+impl Iterator for DirectoryIterator {
+    type Item = OsString;
+    fn next(&mut self) -> Option<OsString> {
+        // Keep calling readdir until we get a NULL pointer back.
+        // If we get a NULL pointer, return None.
+        // Otherwise, return Some with the name of the file.
+        let dirent: *const ffi::dirent = unsafe { ffi::readdir(self.dir) };
+        if dirent.is_null() {
+            return None;
+        }
+        // SAFETY: We just checked that dirent is not null so it is safe to dereference.
+        let dir_name: &CStr = unsafe { CStr::from_ptr((*dirent).d_name.as_ptr()) };
+        Some(OsStr::from_bytes(dir_name.to_bytes()).into())
+    }
+}
+
+impl Drop for DirectoryIterator {
+    fn drop(&mut self) {
+        // Call closedir as needed.
+        if !self.dir.is_null() && unsafe { ffi::closedir(self.dir) } != 0 {
+            panic!("closedir failed");
+        }
+    }
+}
+
+fn main() -> Result<(), String> {
+    let iter = DirectoryIterator::new(".")?;
+    println!("files: {:#?}", iter.collect::<Vec<_>>());
+    Ok(())
+}
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