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wanderer / wasm-metering


wanderer authored Update README.mdLatest: 248f5d0 on 12/22/2016, 7:16:03 PM


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Injects metering into webassembly binaries. The metering counts computation time for a given program in units of gas. The metered wasm binary expects an import that functions as the gas counter. This works for binary version 0xD. For a more detailed description of how this works see


npm install wasm-metering


const fs = require('fs')
const metering = require('wasm-metering')

const wasm = fs.readFileSync(`fac.wasm`)
const meteredWasm = metering.meterWASM(wasm, {
  meterType: 'i32'

const limit = 90000000
let gasUsed = 0

const mod = WebAssembly.Module(meteredWasm.module)
const instance = WebAssembly.Instance(mod, {
  'metering': {
    'usegas': (gas) => {
      gasUsed += gas
      if (gasUsed > limit) {
        throw new Error('out of gas!')

const result = instance.exports.fac(6)
console.log(`result:${result}, gas used ${gasUsed}`) // result:720, gas used 4177





Injects metering into a JSON output of wasm2json


Returns Object the metered json



Injects metering into a webassembly binary


Returns Buffer


The costTable option defines the cost of each of the operations. Cost Tables consist of an object whos keys are section in a wasm binary. For example

module.exports = {
  'start': 1,
  'type': {
    'params': {
      'DEFAULT': 1
    'return_type': {
      'DEFAULT': 1
  'import': 1,
  'code': {
    'locals': {
      'DEFAULT': 1
    'code': {
      'DEFAULT': 1
  'memory': (entry) => {
    return entry.maximum * 10
  'data': 5

Keys can either map to a function which will be given that sections entries or an interger which will be used as the cost for each entry or an object who's keys are matched againt the JSON repesentation of the code. The default cost table used is from here

The cost table can use a special key 'DEFAULT' that will be used as the cost value for any fields in a section that are not defined.

Initial Cost

The Intial cost for instantation for the module is calculated from all the sections other than the code section (which is metered at runtime). This infromation is stored as a custom section
that is inserted directly after the peramble. It use the the name initCost and its payload contains the initial cost encoded as an unsigned leb128 interger.



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