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Threads & Atomics

The threads proposal adds shared linear memory and atomic operations to WebAssembly, enabling safe multi-threaded programming. Memory is declared with the shared keyword so it can be accessed from multiple threads via SharedArrayBuffer on the host side. All atomic operations provide sequential consistency and require naturally aligned addresses (e.g., 4-byte alignment for i32 operations). Thread synchronization is supported through memory.atomic.wait32/memory.atomic.wait64 to suspend a thread until a memory location changes, and memory.atomic.notify to wake waiting threads. An atomic.fence instruction ensures memory ordering between atomic and non-atomic accesses.

(module
  ;; Shared memory: 1 page minimum, 4 pages maximum
  (memory (export "memory") 1 4 shared)


  ;; Atomically increment a counter at address 0, return the old value
  (func (export "atomicAdd") (param $value i32) (result i32)
    (i32.atomic.rmw.add
      (i32.const 0)           ;; address of counter
      (local.get $value)))    ;; value to add


  ;; Wait for a signal at address 0, then notify other threads
  (func (export "waitAndNotify") (param $addr i32) (result i32)
    ;; Wait until the value at $addr is no longer 0
    (drop
      (memory.atomic.wait32
        (local.get $addr)     ;; address to wait on
        (i32.const 0)         ;; expected value (wait while equal)
        (i64.const -1)))      ;; timeout (-1 = infinite)
    ;; Wake up to 1 other thread waiting on this address
    (memory.atomic.notify
      (local.get $addr)       ;; address to notify
      (i32.const 1)))         ;; max waiters to wake
)

Instruction Reference

Section titled “Instruction Reference”