The start function in WebAssembly

Initialization that runs before any export can be called.

(module
  ;; The start function runs once at instantiation, before any
  ;; export can be called -- initialization without a host contract.
  (global $seeded (mut i32) (i32.const 0))
  (memory 1)

  (func $init
    (i32.store (i32.const 0) (i32.const 1408))
    (global.set $seeded (i32.const 1)))
  (start $init)

  (func (export "elevation") (result i32)
    (i32.load (i32.const 0)))
  (func (export "ready") (result i32)
    (global.get $seeded)))

How it works

1. (start $init) nominates one function to run at instantiation.
2. It seeds memory and flips a (mut i32) flag.
3. By the time the host calls elevation, the store already happened.

Keywords and builtins used here

• export
• func
• global
• i32
• memory
• module
• mut
• result
• start

The run, in numbers

Lines

15

Characters to type

419

Tokens

85

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 78 seconds.

Type this snippet

Step 2 of 3 in Globals & startup, step 21 of 28 in Language basics.

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