# XO1401: allocation in a real-time function

A function declared `uses realtime` (core 6.5) may not allocate: list, map, and set literals, string interpolation, `+` and `+=` on Str, List, or Bytes, slicing, closures, storing into a Map, and every prelude method that builds a new collection or Str or grows one (`push`, `put`, `insert`, `sort`, `map`, ...). Allocate during initialization instead (in code that is not real-time), and write preallocated Lists by index: `xs[i] = v` is in place when the List is unique. The message of `fault(...)` is exempt, since a fault ends the real-time section. Native builds also count allocations at run time and fault with XO-F007 when a section allocated (a shared List copied on write). See decision 0133.

## Example

```xo
fn tick(var st: State, x: Float) uses realtime {
  st.history.push(x)
}
```

## Fix

```xo
fn tick(var st: State, x: Float) uses realtime {
  st.history[st.pos] = x
  st.pos = (st.pos + 1) % st.history.len()
}
```

Run `xo explain XO1401` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).