A custom Solana allocator with the following features:

• Supports heaps of arbitrary size. Default Solana allocator assumes 32 KiB of available space regardless of heap size configured via the ComputeBudgetInstruction. This allocator will use all the allotted space.

• Supports opportunistic freeing and resizing. The default Solana allocator never frees memory and calling msg! in a loop will eventually exhaust all memory. This allocator frees at least the last allocated object addressing simple case of temporary buffers.

• Allows declaring mutable global variables which is something Solana Virtual Machine doesn’t normally support. (Any program with mutable static variable will fail to deploy). This allocator offers a way to define such state.

The crate provides custom_heap and custom_global macros which make use of the allocator relatively straightforward. Note however, that solana_program::entrypoint and anchor::program macros define global allocator of their own unless custom-heap Cargo feature is enabled.

For a simple usage (without support for mutable static variables), add the dependency and use custom_heap macro. First in Cargo.toml:

[dependencies.solana-allocator]
name = "solana-allocator"
version = "0.1"
optional = true

[features]
default = ["custom-heap"]
custom-heap = ["dep:solana-allocator"]

And than in lib.rs:

#[cfg(feature = "custom-heap")]
solana_allocator::custom_heap();

For usage with the mutable global variables, additional bytemuck dependency must be added and custom_global macro used instead. First in Cargo.toml:

[dependencies.bytemuck]
version = "*"
optional = true

[dependencies.solana-allocator]
name = "solana-allocator"
version = "0.1"
optional = true

[features]
default = ["custom-heap"]
custom-heap = ["dep:bytemuck", "dep:solana-allocator"]

And then in lib.rs, for example:

solana_allocator::custom_global!(struct GlobalData {
    counter: Cell<usize>,
});

#[cfg(target_os = "solana")]
pub fn unique() -> usize {
    let counter = &global().counter;
    let value = counter.get();
    counter.set(value + 1);
    value
}

#[cfg(not(target_os = "solana"))]
pub fn unique() -> usize {
    use std::sync::atomic::{AtomicUsize, Ordering};
    static COUNTER: AtomicUsize = AtomicUsize::new(0);
    COUNTER.fetch_add(1, Ordering::SeqCst)
}

See documentation or Mutable global state in Solana article for more detailed description.