Is it possible to write a custom allocator at all? where do I store internal data structure?

Question

Static writable data is not supported. I could store it in the heap itself, only if there is some way to store a flag indicating if initialization is called.

Has this ever been done before?

This is the failed code: https://github.com/oxfeeefeee/smalloc/blob/3ccb02ee416fa64943b06df7ebb3aa4684cada77/src/lib.rs

Answer 1

I took a closer look at the example https://github.com/solana-labs/solana-program-library/blob/master/examples/rust/custom-heap/src/entrypoint.rs, and there is a way to tell if "alloc" is called for the first time:

let mut pos = *POS_PTR;
        if pos == 0 {
            // First time, set starting position
            pos = TOP_ADDRESS;
        }

update: The code may need more polishing but passes basic tests: https://github.com/oxfeeefeee/smalloc