XO1401: allocation in a real-time function

error · reported by the checker

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

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

Fix

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.

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XO1401: allocation in a real-time function

Example

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