Get token balance with Ethereum RPC?

Question

how display balance of token through Ethereum RPC?

$id = 0;
$data = array();
$data['jsonrpc'] = '2.0';
$data['id'] = $id++;
$data['method'] = 'eth_call';
$data['params'] = [['from' => '0x0...', 'to' => '0x0...', 'data' => 'contract byte code here 0x0...'], 'latest'];

$ch = curl_init();
...

Return: 

{"jsonrpc":"2.0","id":0,"result":"0x"}

What to do next? Call contract method balanceOf? How to do that?

Answer 1

For fetching the token balance with eth_call, you need 'to' and 'data' parameters. 

To refers to the contract address. And here we need to generate the data parameter. 

data: DATA - (optional) Hash of the method signature and encoded parameters. 

Take this EOS token transaction as an example.

Contract address:0x86fa049857e0209aa7d9e616f7eb3b3b78ecfdb0

Token Holder address:0x0b88516a6d22bf8e0d3657effbd41577c5fd4cb7

You can see the contract code here.

contract ERC20 {

    function totalSupply() constant returns (uint supply);

    function balanceOf( address who ) constant returns (uint value);

    function allowance( address owner, address spender ) constant returns (uint _allowance);

    function transfer( address to, uint value) returns (bool ok);

    function transferFrom( address from, address to, uint value) returns (bool ok);

    function approve( address spender, uint value ) returns (bool ok);

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

    event Approval( address indexed owner, address indexed spender, uint value);

}

Now comes the Function Selector

>>> from web3 import Web3

>>> Web3.sha3("balanceOf(address)")

HexBytes('0x70a08231b98ef4ca268c9cc3f6b4590e4bfec28280db06bb5d45e689f2a360be')

Take the first four bytes 70a08231

Argument Encoding

address: equivalent to uint160, except for the assumed interpretation and language typing.

int: enc(X) is the big-endian two's complement encoding of X, padded on the higher-order (left) side with 0xff for negative X and with zero bytes for positive X such that the length is a multiple of 32 bytes.

Padding the 20 bytes token address to 32 bytes with 0 to token holder address:

0000000000000000000000000b88516a6d22bf8e0d3657effbd41577c5fd4cb7

Then concat the function selector and encoded parameter, we get data parameter:

0x70a082310000000000000000000000000b88516a6d22bf8e0d3657effbd41577c5fd4cb7

Make the request with:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_call","params":[{"to": "0x86fa049857e0209aa7d9e616f7eb3b3b78ecfdb0", "data":"0x70a082310000000000000000000000000b88516a6d22bf8e0d3657effbd41577c5fd4cb7"}, "latest"],"id":67}' -H "Content-Type: application/json" http://127.0.0.1:8545/

here is the curl result (You may get different answer here, as there may be some transaction with this address done after my polling the request)

{"jsonrpc":"2.0","id":67,"result":"0x00000000000000000000000000000000000000000000014a314d9ff9b20b9800"}

You can change convert hex format balance to decimal

>>> 0x00000000000000000000000000000000000000000000014a314d9ff9b20b9800

6090978215900000000000

Check the result,