avr-progmem

Progmem utilities for the AVR architectures.

This crate provides unsafe utilities for working with data stored in the program memory of an AVR micro-controller. Additionally, it defines a 'best-effort' safe wrapper struct ProgMem to simplify working with it, as well as a PmString wrapper for string handling.

This crate is implemented only in Rust and some short assembly, it does NOT depend on the avr-libc or any other C-library. However, due to the use of inline assembly, this crate may only be compiled using a nightly Rust compiler (as of mid 2022, inline assembly for AVR is still 'experimental').

MSRV

This crate works with a Rust nightly-2023-08-08 compiler. All versions 0.4.x will adhere to work with nightly-2023-08-08. Other Rust compiler version (particularly newer ones) might also work, but due to the use of experimental compiler features it is possible that some future Rust compiler version will fail to work.

Future versions such as 0.5.x might required a newer Rust compiler version.

AVR Memory

This crate is specifically for AVR-base micro-controllers such as the Arduino Uno (and some other Arduino boards, but not all), which have a modified Harvard architecture which implies the strict separation of program code and data while having special instructions to read and write to program memory.

While, of course, all ordinary data is stored in the data domain where it is perfectly usable, the harsh constraints of most AVR processors make it very appealing to use the program memory (also referred to as progmem) for storing constant values. However, due to the Harvard design, those values are not usable with normal instructions (i.e. those emitted from normal Rust code). Instead, special instructions are required to load data from the program code domain, such as the lpm (load [from] program memory) instruction. And because there is no way to emit it from Rust code, this crate uses inline assembly to emit that instruction.

However, since a pointer into program code cannot be differentiated from a normal data pointer, it is entirely up to the programmer to ensure that these different 'pointer-types' are not accidentally mixed. In other words, this is unsafe in the context of Rust.

Loading Data from Program Memory

The first part of this crate simply provides a few functions (e.g. read_byte) to load constant data (i.e. a Rust static that is immutable) from the program memory into the data domain, so that sub-sequentially it is normal usable data, i.e. as owned data on the stack.

Because, as aforementioned, a simple *const u8 in Rust does not specify whether is lives in the program code domain or the data domain, all functions which simply load a given pointer from the program memory are inherently unsafe.

Notice that using references (e.g. &u8) to the program code domain should generally be avoided because references in Rust should be dereferencable, which the program code domain is not.

Additionally references can be easily dereferenced by safe code, which would be undefined behavior if that reference points into the program memory. Therefore, a Rust reference to a static that is stored in program memory must be considered hazardous (if not UB), and it is recommended to only use raw pointers to those statics, e.g. obtained via the addr_of! macro, which directly creates raw pointers without needing a reference.

Example

use avr_progmem::raw::read_byte;
use core::ptr::addr_of;

// This `static` must never be directly dereferenced/accessed!
// So a `let data: u8 = P_BYTE;` ⚠️ is **undefined behavior**!!!
/// Static byte stored in progmem!
#[link_section = ".progmem.data"]
static P_BYTE: u8 = b'A';

// Load the byte from progmem
// Here, it is sound, because due to the link_section it is indeed in the
// program code memory.
let data: u8 = unsafe { read_byte(addr_of!(P_BYTE)) };
assert_eq!(b'A', data);

The best-effort Wrapper

Since working with progmem data is inherently unsafe and rather difficult to do correctly, this crate introduces the best-effort 'safe' wrapper ProgMem, that is supposed to only wrap data in progmem, thus offering only functions to load its content using the progmem loading function introduced above. Using these functions is sound, if that the wrapper data is really stored in the program memory. Therefore, to enforce this invariant, the constructor of ProgMem is unsafe.

Additionally, since proper Rust references (unlike pointers) come with a lot special requirements, it should be considered hazardous to have a reference to data stored in program memory. Instead, only raw pointers to this kind of data should be kept, created e.g. via the addr_of! macro. Consequently, the ProgMem just wrap a pointer to data in progmem, which in turn must be stored in a static marked with #[link_section = ".progmem.data"]. However, since, safe Rust can always create a "normal" Rust reference to any (accessible) static, it must be considered hazardous if not just unsound, to expose such a static to safe Rust code.

To also make this easier (and less hazardous), this crate provides the progmem! macro, which will create a hidden static in program memory initialized with the data you give it, wrap it's pointer in the ProgMem struct, and put that wrapper into yet another (normal RAM) static, so you can access it. This will ensure that the static that is stored in program memory can not be referenced by safe Rust code (because it is not accessible), while the accessible ProgMem wrapper allows access to the underling data by loading it correctly from program memory.

Example

use avr_progmem::progmem;

// It will be wrapped in the ProgMem struct and expand to:
// ```
// static P_BYTE: ProgMem<u8> = {
//     #[link_section = ".progmem.data"]
//     static INNER_HIDDEN: u8 = 42;
//     unsafe { ProgMem::new(addr_of!(INNER_HIDDEN)) }
// };
// ```
// Thus it is impossible for safe Rust to directly access the progmem data!
progmem! {
    /// Static byte stored in progmem!
    static progmem P_BYTE: u8 = 42;
}

// Load the byte from progmem
// This is sound, because the `ProgMem` always uses the special operation to
// load the data from program memory.
let data: u8 = P_BYTE.load();
assert_eq!(42, data);

Strings

Using strings such as &str with ProgMem is rather difficult, and surprisingly hard if Unicode support is needed (see issue #3). Thus, to make working with string convenient the PmString struct is provided on top of ProgMem.

PmString stores any given &str as statically sized UTF-8 byte array (with full Unicode support). To make its content usable, it provides a Display & uDisplay implementation, a lazy chars iterator, and load function similar to ProgMem's load, that yields a LoadedString, which in turn defers to &str.

For more details see the string module.

Example

use avr_progmem::progmem;

progmem! {
    // A simple Unicode string in progmem.
    static progmem string TEXT = "Hello 大賢者";
}

// You can load it and use that as `&str`
let buffer = TEXT.load();
assert_eq!("Hello 大賢者", &*buffer);

// Or you use directly the `Display` impl
assert_eq!("Hello 大賢者", format!("{}", TEXT));

Additionally, two special macros are provided similar to the F macro of the Arduino IDE, that allows to "mark" a string as to be stored in progmem while being returned at this place as a loaded &str.

// Or you skip the static and use in-line progmem strings:
use avr_progmem::progmem_str as F;
use avr_progmem::progmem_display as D;

// Either as `&str`
assert_eq!("Foo 大賢者", F!("Foo 大賢者"));

// Or as some `impl Display + uDisplay`
assert_eq!("Bar 大賢者", format!("{}", D!("Bar 大賢者")));

If you enabled the ufmt crate feature (its a default feature), you can also use uDisplay in addition to Display.

use avr_progmem::progmem;
use avr_progmem::progmem_str as F;
use avr_progmem::progmem_display as D;

fn foo<W: ufmt::uWrite>(writer: &mut W) {
    progmem! {
        // A simple Unicode string in progmem.
        static progmem string TEXT = "Hello 大賢者";
    }

    // You can use the `uDisplay` impl
    ufmt::uwriteln!(writer, "{}", TEXT);

    // Or use the in-line `&str`
    writer.write_str(F!("Foo 大賢者\n"));

    // Or the in-line `impl uDisplay`
    ufmt::uwriteln!(writer, "{}", D!("Bar 大賢者"));
}
//

Other Architectures

As mentioned before, this crate is specifically designed to be use with AVR-base micro-controllers. But since most of us don't write their programs on an AVR system but e.g. on x86 systems, and might want to test them there (well as far as it is possible), this crate also has a fallback implementation for all other architectures that are not AVR, falling back to a simple Rust static in the default data segment. And all the data loading functions will just dereference the pointed-to data, assuming that they just live in the default location.

This fallback is perfectly safe on x86 and friend, and should also be fine on all further architectures, otherwise normal Rust statics would be broken. However, it is an important point to know when for instance writing a library that is not limited to AVR.

Implementation Limitations

Aside from what has been already been covered, the current implementation has two further limitations.

First, since this crate uses an inline assembly loop on a 8-bit architecture, the loop counter only allows values up to 255. This means that not more that 255 bytes can be loaded at once with any of the methods of this crate. However, this only applies to a single continuous load operation, so for instance ProgMem<[u8;1024]>::load() will panic, but accessing such a big type in smaller chunks e.g. ProgMem<[u8;1024]>::load_sub_array::<[u8;128]>(512) is perfectly fine because the to be loaded type [u8;128] is only 128 bytes in size. Notice that the same limitation holds for PmString<N>::load() (i.e. you can only use it if N <= 255 holds. On the other hand, there is no such limitation on PmString<N>::chars() and PmString's Display/uDisplay implementation, because those, just load each char individually (i.e. no more that 4 bytes at a time).

Second, since this crate only uses the lpm instruction, which is limited by a 16-bit pointer, this crate may only be used with data stored in the lower 64 kiB of program memory. Since this property has not be tested it is unclear whether it will cause a panic or right-up undefined behavior, so be very wary when working with AVR chips that have more then 64 kiB of program memory.

License

Licensed under Apache License, Version 2.0 (LICENSE or https://www.apache.org/licenses/LICENSE-2.0).

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.