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initializationundefinedwebassemblyspecificationslocal-variables

What is the value of a WebAssembly local that has not been set?

If I use local.get to access the value of a WebAssembly local which I have not already set via local.set or local.tee, am I guaranteed to get a specific default value, or is this undefined behavior? For instance:

(module
  (func (result i32) (local i32)
    local.get 0))

Running this in Chrome gives zero, but I would like to know if that is guaranteed by the spec.

Solution

  • Yes, the Wasm spec demands that all local variables are zero initialized. Thus you'd get the same behavior on all spec compliant runtimes.

    You can find this in the spec here: https://webassembly.github.io/spec/core/exec/instructions.html#exec-invoke