Getting contract balance over JSON RPC

Question

I'd like to read the contract balance (amount of unspend ETH on the contract) over Geth JSON-RPC API.

E.g. the number displayed here: http://testnet.etherscan.io/address/0xc5910bcb2442e84845aa98b20ca51e8f5d2bee23

What would be a good starting point?

Answer 1

Here is an example using the Linux utility curl to retrieve the balance of the contract over JSON-RPC:

curl -s -X POST --data '{"jsonrpc":"2.0", "method":"eth_getBalance", "params":["0xc5910bcb2442e84845aa98b20ca51e8f5d2bee23", "latest"], "id":1}' http://localhost:8545

You will have to start geth with the --rpc parameter to run the curl program above, e.g.,

geth --testnet --rpc console

The results you receive will be the number of weis in hexadecimal format:

{"jsonrpc":"2.0","id":1,"result":"0x58d15e17628000f"}

Converting this number using a Hexadecimal to Decimal Converter results in a decimal number of 400000000000000015 weis.

To calculate the number of ethers, divide the decimal number by 1e18 and your result is 0.400000000000000015 ethers.

And checking this balance using the geth JavaScript console:

> web3.fromWei(eth.getBalance("0xc5910bcb2442e84845aa98b20ca51e8f5d2bee23"), "ether")
> 0.400000000000000015

Here is my Perl code to convert the hex wei number to a double ether number:

sub hexToDouble{
  my $param = shift;
  $param =~ s/^0x//;
  my $fullnum = 0.0;
  while ($param =~ /(.)/g) {
    my $num = hex($1);
    $fullnum = $fullnum * 16 + $num;
  }
  $fullnum *= 1e-18;
  return $fullnum;
}

Answer 2

Use the official web3 JavaScript library's method web3.eth.getBalance

Example Node.js code:

Web3 = require('web3');

var web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"));

var balance = web3.eth.getBalance("0xc5910bcb2442e84845aa98b20ca51e8f5d2bee23");

console.log("Balance = " + web3.fromWei(balance, "ether"));

You should change http://localhost:8545 to point to your own geth server.

Also the documentation linked above has some best practices for not overriding previously defined web3 objects, but I skipped them for brevity. Consult the docs yourself to see what all you can do with that lib.

If you cannot use Node.JS then you can read the Official Ethereum JSON RPC spec which tells you all of the methods available and how to use them.