How can geth access contract variables through the console?

Question

I'm using the Mist wallet just to display the contents of the contract.
Bellow, you can see the code to access the contract function (submitUnits) and the result of the LoadScript when I run in the console.

call_function.js
In the last commented row, you can see the value I want to print (100).

Mist wallet

However it always returns this hex number. How can i get the Total Units that is mentioned in the wallet screenshot?

The code of the contract is this:

contract UCounter {
mapping (address => uint256) public UnitsFrom;
uint256 public TotalUnits;

function submitUnits(uint256 Units) returns (uint ){
    UnitsFrom[msg.sender] = Units;
    TotalUnits = Units;
    return TotalUnits;
}

}

Hope someone could help. Thanks.

Answer 1

The function getData return function call parameters encoded together with the function signature.

From the snippet of your abi submitUnits is not constant has input parameter of type uint256. The output type is cropped, but in this case since submitUnits is a transaction not a call, and for transaction we can recover the return value.

From the documentation to execute a transaction we have to do:

var txid = MyContractInstance.submitUnits.sendTransaction(parameter, { from: eth.accounts[3] });
console.log(txid);

And you have to wait until the transaction is mined in the blockchain.

To read a variable, or call a constant method you have to make a call

MyContractInstance.TotalUnits.call(function (value) {
    console.log(value);
});

Answer 2

web3.eth.getStorageAt (address, position)

Here address is address of your smart contract, and position is state variable position, in your case position=1 because TotalUnits is the second variable in your contract, and variables are numbered starting from zero.