volatile-mem

A rust library for managing volatile memory.

lib.rs:

This crate provides a type, Volatile, for managing volatile memory or data.

The Volatile object does not contain a pointer or reference to the volatile memory, but is a container of the volatile data itself. This means that a pointer to a Volatile object is a pointer to the volatile memory. As such, it would make little or no sense to create a local variable or parameter of type Volatile. You would typically use some kind of pointer or reference to the Volatile object instead.

Besides Volatile, the crate provides two additional volatile types. They are VolatileReadOnly, and VolatileWriteOnly. These are technically just type definitions which alias read-only and write-only variants of Volatile, respectively. However, those variants are only available through these aliases. The default variant for Volatile allows both reads and writes.

Volatile is meant for reading from or writing to memory used for communication with some process external to the program. A common use case would be memory-mapped I/O.

Safety

Typically, Volatile would be created from a raw pointer, which carries with it the typical pointer safety concerns. In particular, the following must be guaranteed regarding the location of the volatile memory.

• The memory must be valid for reads and/or writes.

• The memory must be properly aligned.

• The memory must point to a properly initialized for the data type, unless the Volatile is write-only.

Note that even if the data has size zero, the pointer must be non-NULL and properly aligned.

Do not forget that even creating a reference to uninitialized data (even if that data is never used) is immediate undefined behavior. As such, do not at any point create a reference directly to uninitialized data (as opposed to a reference to VolatileWriteOnly or MaybeUninit, each of which can safely handle uninitialized data).

Just like in C, whether an operation is volatile has no bearing whatsoever on questions involving concurrent access from multiple threads. Volatile accesses behave exactly like non-atomic accesses in that regard. In particular, a race between a write operation and any other operation (reading or writing) to the same location is undefined behavior.

Disclaimer

The Rust documentation contains the following note regarding volatile reads and writes:

Rust does not currently have a rigorously and formally defined memory model, so the precise semantics of what "volatile" means here is subject to change over time. That being said, the semantics will almost always end up pretty similar to C11's definition of volatile.

The compiler shouldn't change the relative order or number of volatile memory operations. However, volatile memory operations on zero-sized types [...] are noops and may be ignored.