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01.WhatIsABI.md

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What is Solidity ABI

ABI = Application Binary Interface.

Smart contract ABIs enable communication and interaction with external apps and other contracts.

img

  1. Bytecode is not human readable.
  2. ABIs can be understood by humans and external apps.
  3. Web3 ABI == Web2 API .

ABI with code

Example online:

  1. https://etherscan.io/address/0x838A5832eB19CfCe692809f5b2DE44b15380CDA2#code
  2. https://etherscan.io/token/0xdac17f958d2ee523a2206206994597c13d831ec7#code

How to use ABI

Generation

  1. tooling: Truffle、Hardhat、Foundry、solcjs
  2. IDE: Remix

Specification

doc: https://docs.soliditylang.org/en/v0.8.23/abi-spec.html#json

{
    "inputs": [
      {
        "internalType": "uint256",
        "name": "value",
        "type": "uint256"
      }
    ],
    "name": "setOrGetNumber",
    "outputs": [
      {
        "internalType": "uint256",
        "name": "newNumber",
        "type": "uint256"
      }
    ],
    "stateMutability": "nonpayable",
    "type": "function"
  }

Function Selector

function setNumber(uint256 newNumber) public

graph LR
A("hash=Keccak-256('setNumber(uint256)')")
--> C["selector=hash[:4]"]
Loading 
cast keccak "setNumber(uint256)"
0x3fb5c1cb9d57cc981b075ac270f9215e697bc33dacd5ce87319656ebf8fc7b92

cast sig "setNumber(uint256)"
0x3fb5c1cb

Keccak-256 vs Sha3:

Event

event Transfer(address indexed from, address indexed to, uint value);

  • address: the address of the contract (intrinsically provided by Ethereum);
  • topics[0]: keccak(EVENT_NAME+"("+EVENT_ARGS.map(canonical_type_of).join(",")+")") (canonical_type_of is a function that simply returns the canonical type of a given argument, e.g. for uint indexed foo, it would return uint256). This value is only present in topics[0] if the event is not declared as anonymous;
  • topics[n]: abi_encode(EVENT_INDEXED_ARGS[n - 1]) if the event is not declared as anonymous or abi_encode(EVENT_INDEXED_ARGS[n]) if it is (EVENT_INDEXED_ARGS is the series of EVENT_ARGS that are indexed);
  • data: ABI encoding of EVENT_NON_INDEXED_ARGS (EVENT_NON_INDEXED_ARGS is the series of EVENT_ARGS that are not indexed, abi_encode is the ABI encoding function used for returning a series of typed values from a function, as described above).

Encode

Signature Database: https://www.4byte.directory/

function setNumber(uint256 newNumber) public {
   number = newNumber;
}
cast abi-encode "setNumber(uint256)" 2023
//0x00000000000000000000000000000000000000000000000000000000000007e7
cast calldata "setNumber(uint256)" 2023 
//0x3fb5c1cb00000000000000000000000000000000000000000000000000000000000007e7

ABI encode the given function argument, excluding the selector

Usage: cast abi-encode <SIG> [ARGS]...

Arguments:
  <SIG>      The function signature
  [ARGS]...  The arguments of the function

How to encode with solidity ❓

Generate JS or ** language files using ABI ❓

Decode

  1. cast calldata-decode
  2. cast abi-decode

how to decode with solidity ❓

Exercise

Decode data

Tx1 calldata

0xa9059cbb0000000000000000000000005494befe3ce72a2ca0001fe0ed0c55b42f8c358f000000000000000000000000000000000000000000000000000000000836d54c

Tx2 calldata

0xb24614f2000000000000000000000000dac17f958d2ee523a2206206994597c13d831ec70000000000000000000000007c92b8efbcf4ca58a6d877f35ab78aaa0d3d13bf0000000000000000000000000000000000000000000000000000000017d7840000000000000000000000000000000000000000000000000000000000000000800000000000000000000000000000000000000000000000000000000000000000