Sentinel

sentinel is a sentinel-terminated slice library.

How it works

Rust Slices

In Rust, the slice type &[T] is basically defined like that: (*const T, usize). The usize indicates the number of Ts referenced at the *const T. Knowing in advance the size of an array, like that, has numerous advantages, which won't be discussed here.

There is however two main problems with the &[T] type:

1. It is not (at least, yet) FFI-safe. One cannot create an extern "C" fn(s: &[u32]) function and expect it to work when calling it from C-code.

2. The size of &[T] has the size of two usizes.

Sentinels?

A sentinel is a special value that is used to determine the end of an array. For example, in C, the char * type can be a pointer to a "null-terminated" string. This is an example of sentinel-terminated slice.

CString:
char *ptr
 |
'H' 'e' 'l' 'l' 'o' '\0'
                      ^ sentinel, anything after this point may be invalid.
str:
*const u8, 5
 |
'H' 'e' 'l' 'l' 'o'
                    ^ no sentinel, we know the slice contains 5 elements.

This crate remains generic over how sentinels are defined. It uses the Sentinel trait, which is roughly defined like that:

trait Sentinel<T> {
    fn is_sentinel(val: &T) -> bool;
}

It is used to determine whether a specific instance of T should be treated as a "sentinel" value.

SSlice

Finally, in conjonction with the Sentinel trait, this crate defines the SSlice<T, S> type. It is generic over T, the type of stored elements, and over S: Sentinel<T>, defining which instances of T should be considered sentinel values.

struct SSlice<T, S: Sentinel<T>> {
    _marker: PhantomData<(T, S)>,
}

Note that this type actually contains no data. Only references to this type can be created (i.e. &SSlice<T, S> or &mut SSlice<T, S>), and those references have the size a single usize.

Features

• alloc - adds support for the alloc crate. This adds the SBox<T, S> type.

• cstr - this feature enables the CStr type, as well as a generic Null which implements the Sentinel trait for common types (integers, pointers, Option).

• nightly - makes use of the unstable extern_type feature to make sure no instance of SSlice<T, S> can be created on the stack by making it !Sized. This feature also enables support for the new allocator_api unstable feature.

null and alloc are enabled by default.

Old sentinel crate

The name sentinel was kindly given to me by the previous maintainer of this project.

Every pre-0.2 versions (on crates.io) contain the source code of that crate.