Rust no_std: Building a Minimal Memory Allocator

The no_std Rust environment is crucial for embedded systems and bare-metal programming where the standard library, which relies on heap allocation and other OS services, is unavailable. This challenge focuses on implementing a fundamental component for no_std development: a simple, custom memory allocator.

Problem Description

Your task is to implement a basic memory allocator that can function within a no_std Rust environment. This allocator will be responsible for managing a fixed-size region of memory, satisfying allocation and deallocation requests from your no_std application. You will need to integrate this custom allocator with Rust's alloc crate to enable dynamic memory allocation (like Box or Vec) in your no_std program.

Key requirements:

• Global Allocator: Implement the GlobalAlloc trait from core::alloc. This trait defines the interface for memory allocators.
• Fixed Memory Pool: Manage a pre-defined, contiguous region of memory (e.g., a static byte array). You will not be requesting memory from the operating system or any external source at runtime.
• Basic Allocation Strategy: Implement a simple allocation strategy. A common approach for minimal allocators is a bump allocator or a first-fit allocator. For this challenge, a bump allocator is sufficient. A bump allocator simply allocates memory from the next available address in the pool. Deallocation in a simple bump allocator is often a no-op, meaning memory is only reclaimed when the entire pool is reset.
• #[global_allocator] Attribute: Mark your allocator implementation with the #[global_allocator] attribute.
• no_std Target: The solution must compile and run without the standard library.

Expected Behavior:

• Your allocator should successfully handle alloc and dealloc calls.
• When alloc is called, it should return a pointer to a valid, aligned memory region within the allocated memory pool.
• When dealloc is called, it should do nothing if you are implementing a simple bump allocator (as is common in no_std for simplicity and performance).
• If the memory pool is exhausted, alloc should return a null pointer (or handle the error according to the GlobalAlloc trait contract, which usually involves returning a null pointer).

Edge Cases:

• Alignment: Ensure that allocated memory is properly aligned according to the requested alignment.
• Zero-sized allocations: Handle requests for zero-sized allocations.
• Pool Exhaustion: The allocator must correctly indicate when it cannot fulfill an allocation request due to insufficient memory.

Examples

Example 1: Basic Allocation

// Assume a global allocator is defined elsewhere that uses a memory pool of 1024 bytes.

use core::alloc::{GlobalAlloc, Layout};

// ... (allocator implementation using a 1024 byte buffer)

#[no_mangle]
pub extern "C" fn entry_point() {
    let layout = Layout::from_size_align(100, 8).unwrap();
    let ptr = unsafe { alloc::alloc::alloc(layout) };

    if !ptr.is_null() {
        // Successfully allocated 100 bytes with 8-byte alignment.
        // The pointer `ptr` now points to a valid memory region.
        // Further operations can be performed on this memory.
        // For this challenge, simply verifying it's not null is enough.
        println!("Allocation successful!");
    } else {
        println!("Allocation failed!");
    }

    // Deallocation in a simple bump allocator is often a no-op,
    // but the trait requires it.
    unsafe { alloc::alloc::dealloc(ptr, layout) };
}

Output:

Allocation successful!

Explanation:

A request is made to allocate 100 bytes with an alignment of 8 bytes. The custom allocator finds a suitable region within its memory pool, returns a pointer to it, and the program prints a success message.

Example 2: Pool Exhaustion

// Assume a global allocator is defined elsewhere that uses a memory pool of 100 bytes.

use core::alloc::{GlobalAlloc, Layout};

// ... (allocator implementation using a 100 byte buffer)

#[no_mangle]
pub extern "C" fn entry_point() {
    // First allocation that fits
    let layout1 = Layout::from_size_align(50, 8).unwrap();
    let ptr1 = unsafe { alloc::alloc::alloc(layout1) };

    if !ptr1.is_null() {
        println!("First allocation successful.");
    } else {
        println!("First allocation failed.");
    }

    // Second allocation that would exhaust the pool
    let layout2 = Layout::from_size_align(60, 8).unwrap();
    let ptr2 = unsafe { alloc::alloc::alloc(layout2) };

    if !ptr2.is_null() {
        println!("Second allocation successful.");
    } else {
        println!("Second allocation failed (pool exhausted).");
    }

    unsafe {
        alloc::alloc::dealloc(ptr1, layout1);
        // ptr2 might be null, deallocating a null pointer is safe.
        alloc::alloc::dealloc(ptr2, layout2);
    }
}

Output:

First allocation successful.
Second allocation failed (pool exhausted).

Explanation:

The first allocation of 50 bytes succeeds. The second allocation request of 60 bytes exceeds the remaining capacity (50 bytes) of the 100-byte pool. The allocator returns a null pointer, and the program prints a failure message.

Constraints

• Memory Pool Size: The memory pool managed by your allocator must be at least 1 KiB (1024 bytes). You can define this as a static mut or use a similar mechanism.
• Alignment: The allocator must support alignments up to 64 bytes.
• No Standard Library: The solution must be strictly no_std. Do not use std::alloc or any other part of the standard library that requires std. You can use core::alloc.
• Target: The code should be compilable for a no_std target (e.g., x86_64-unknown-none).

Notes

• You'll need to add alloc to your dependencies in Cargo.toml and configure your build for a no_std target.
• Consider using core::sync::atomic for thread safety if you envision concurrent access, though for this basic challenge, a single-threaded assumption is acceptable. However, Rust's GlobalAlloc trait does require thread safety, so you'll likely need some form of synchronization. A simple spinlock or disabling interrupts (if on bare metal) could be options, but for a simulation, careful use of unsafe and compiler memory barriers might suffice if you are certain no other threads exist.
• A bump allocator is a good starting point: maintain a pointer to the next free byte and increment it on each allocation. Deallocation is typically a no-op.
• Remember to handle alignment correctly. The align_up method from core::mem or manual calculation can be used.
• The GlobalAlloc trait requires alloc and dealloc to be unsafe functions. You will need to implement them with unsafe blocks.