# Official way to get the size of a field

**URL:** <https://internals.rust-lang.org/t/official-way-to-get-the-size-of-a-field/22123>\
**Category:** language design\
**Created:** [January 6, 2025, 9:05pm UTC](https://internals.rust-lang.org/t/official-way-to-get-the-size-of-a-field/22123 "2025-01-06T21:05:52Z")\
**Posts on this page:** 1\
**Showing post:** 14

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**Author:** ![cosmicexplorer](https://sea2.discourse-cdn.com/flex002/user_avatar/internals.rust-lang.org/cosmicexplorer/32/13052_2.png) [@cosmicexplorer](https://internals.rust-lang.org/u/cosmicexplorer)\
**Post date:** [March 15, 2025, 1:58pm UTC](https://internals.rust-lang.org/t/official-way-to-get-the-size-of-a-field/22123/14 "2025-03-15T13:58:40Z")

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I'm currently wrapping the posix `readdir{,_r}()` in the manner performed by the rust stdlib for `fs::read_dir()`, which delegates to `readdir_r()` by default except for a specified list of platforms: [rust/library/std/src/sys/fs/unix.rs at aa95b9648ad0383a3fd73b5271dd86f848b7c00c · rust-lang/rust · GitHub](https://github.com/rust-lang/rust/blob/aa95b9648ad0383a3fd73b5271dd86f848b7c00c/library/std/src/sys/fs/unix.rs#L784-L825). If I understand correctly, the stdlib is relying on the result of `readdir_r()` always fitting into the stack-allocated `dirent` struct initialized with `mem::zeroed()` in the linked method body. In particular, contrast to the `fs::read_dir()` implementation immediately above used for linux, which must allocate an owned `CString` upon each iteration: [rust/library/std/src/sys/fs/unix.rs at aa95b9648ad0383a3fd73b5271dd86f848b7c00c · rust-lang/rust · GitHub](https://github.com/rust-lang/rust/blob/aa95b9648ad0383a3fd73b5271dd86f848b7c00c/library/std/src/sys/fs/unix.rs#L777).

The `dirent` struct used for these platforms has a `d_name` field which is a fixed-size array of `c_char`, which is why the rust stdlib is able to avoid allocating a new `CString` on these platforms without introducing unsafety. See macos ([libc/src/unix/bsd/apple/mod.rs at c6a3cd61445c38160c07b507617bb6099602759a · rust-lang/libc · GitHub](https://github.com/rust-lang/libc/blob/c6a3cd61445c38160c07b507617bb6099602759a/src/unix/bsd/apple/mod.rs#L1360)):

```rust
pub struct dirent {
    pub d_ino: u64,
    pub d_seekoff: u64,
    pub d_reclen: u16,
    pub d_namlen: u16,
    pub d_type: u8,
    pub d_name: [c_char; 1024],
}

```

I am looking to get that array length in `const` context so I can make a trait like:

```rust
trait HasMaxSizedName {
  const LEN: usize;
  fn name(&self) -> &[c_char; Self::LEN];
}

#[cfg(target_vendor = "apple")]
impl HasMaxSizedName for libc::dirent {
  // want # of array elements instead of byte length here 
  const LEN: usize = field_size!(Self, d_name);
  fn name(&self) -> &[c_char; Self::LEN] { &self.d_name } 
}

```

I would like to have this information available in the type system so I can use something like [`CStr::from_bytes_until_nul()`](https://doc.rust-lang.org/core/ffi/struct.CStr.html#method.from_bytes_until_nul) on a bounded-size array, as opposed to using the `unsafe` method `CStr::from_ptr()` like the stdlib currently does ([rust/library/std/src/sys/fs/unix.rs at aa95b9648ad0383a3fd73b5271dd86f848b7c00c · rust-lang/rust · GitHub](https://github.com/rust-lang/rust/blob/aa95b9648ad0383a3fd73b5271dd86f848b7c00c/library/std/src/sys/fs/unix.rs#L1041-L1043)).

For the stdlib's purposes, it actually _could_ use `CStr::from_bytes_until_nul()`, since it's dealing with a concrete type. But I'm trying to make this `HasMaxSizedName` trait to _differentiate_ platforms with a fixed-size immediate buffer from platforms like linux which must always eagerly allocate a new `CString` before moving to the next iteration result from `fs::read_dir()`. This is opposed to the stdlib's approach here, which forces all platforms into a single struct and uses `unsafe` methods for all of them, even though some of them could be safer. As of now, I am using lifetimes to propagate this ownership information, but being able to get the size of an array field in a type-level context like an associated constant would be much more ergonomic.

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