What does "intrinsic gas too low" mean?

Question

I'm running the Mist wallet, and I've deployed this contract to testnet (from https://ethereum.stackexchange.com/a/179/520) :

contract Notary{
    struct Document {
        uint timestamp;
        bytes ipfs_hash;
        address[] signatures;
    }
    mapping(address => bytes[]) public users; //maps addresses to ipfs document hashes
    mapping(bytes32 => Document) public documents; //maps sha3(ipfs) hashes to documents

    function addDocument(bytes ipfs) public {
        users[msg.sender].push(ipfs); //Add document to users's "signed" list
        address[] memory sender;
        sender[0] = msg.sender;
        documents[sha3(ipfs)] = Document(block.timestamp, ipfs, sender);
    }

    function signDocument(bytes ipfs) public {
        users[msg.sender].push(ipfs);
        documents[sha3(ipfs)].signatures.push(msg.sender);
    }

}

When I try to execute the addDocument function though, I get "intrinsic gas too low". What does this mean? Also before confirming execution, it says: "Data can't be executed, so it will use all provided gas."

Answer 1

The intrinsic gas for a transaction is the amount of gas that the transaction uses before any code runs. It is a constant “transaction fee” (currently 21000 gas) plus a fee for every byte of data supplied with the transaction (4 gas for a zero byte, 68 gas for non-zeros). These constants are all currently defined for geth in params/protocol_params.go. Presumably they are also coded into the source for the other node/client implementations as well.

Answer 2

You need to allocate more gas to your transaction with the addDocument function. Mist is looking ahead and is able to tell that the transaction will fail because you haven't given the transaction enough gas to execute with.

Answer 3

The keyword payable is needed on a function. See solidity docs:

Payable for functions: Allows them to receive Ether together with a call.

So, this doesn't work:

function transfer(address _to, uint256 _value) {

This does work:

function transfer(address _to, uint256 _value) payable {