Gas required exceeds block gas limit fallback function

Question

I'm working on a smart contract and followed this video here: https://www.youtube.com/watch?v=s677QFT6e4U&t=911s. I copied the code exactly, but when I try to call the fallback function I get the following error: Gas required exceeds block gas limit: 300000000. Even though the fallback function is as follows (it does nothing):

function () payable {

}

How could this be using too much gas?

CONTRACT CODE:

pragma solidity ^0.4.11;

import './IERC20.sol';
import './SafeMath.sol';

contract AToken is IERC20 {
    
    using SafeMath for uint256;
    
    uint256 public _totalSupply = 0;
    uint256 public constant hardLimit = 45000000;
    string public constant symbol = "ABC";
    string public constant name = "Alphabet";
    uint8 public constant decimals = 18;
    
    //1 ETH = 25000 Alphabet
    uint256 public constant RATE = 25000;
    
    address public owner;
    
    mapping(address => uint256) balances;
    mapping(address => mapping(address => uint256)) allowed;
    
    function () payable {
        createTokens();
    }
    
    function SnapToken() {
        owner = msg.sender;
    }
    
    function createTokens() payable {
        //require(msg.value > 0);
        //uint256 tokens = msg.value.mul(RATE);
        //require(tokens.add(_totalSupply) <= hardLimit);
        //balances[msg.sender] = balances[msg.sender].add(tokens);
        //_totalSupply = _totalSupply.add(tokens);
        //owner.transfer(msg.value);
    }
    
    function totalSupply() constant returns (uint256 totalSupply) {
        return _totalSupply;
    }
    
    function balanceOf(address _owner) constant returns (uint256 balance) {
        return balances[_owner];
    }
    
    function transfer(address _to, uint256 _value) returns (bool success) {
        require(balances[msg.sender] >= _value && _value > 0);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(_value);
        Transfer(msg.sender, _to, _value);
        return true;
    }
    
    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
        require(allowed[_from][msg.sender] >= _value && balances[_from] >= _value && _value > 0);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(_value);
        allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
        Transfer(_from, _to, _value);
        return true;
    }
    
    function approve(address _spender, uint256 _value) returns (bool success) {
        //allowed[msg.sender][_spender] = allowed[msg.sender][_spender].add(_value);
        Approval(msg.sender, _spender, _value);
        return true;
    }
    
    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
        return allowed[_owner][_spender];
    }
    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 value);
}

I commented out some stuff to see if this would reduce the gas requirement but unfortunately not. Have you faced this before?

Thanks

Answer 1

The contract, as posted, does execute when calling the fallback function (Tested in Remix). However, it will fail once you uncomment the logic in createTokens()

Fallback functions have low gas limits (2300) and, therefore, are very limited in what they can do. You can't do things like write to storage, call external functions, or send ether out as you will instantly hit the limit. It should primarily be used to enable your contract to receive ether and maybe log an event.

In the example you posted above, remove the call to createTokens() in your fallback function and just call that function directly from your client.

Documentation on Fallback Functions

Example client code:

const abiDefinition = ...;
const contractAddress = ...;
const account = ...;
const amountInEther = ...;

const contract = web3.eth.contract(abiDefinition);
const contractInstance = contract.at(contractAddress);

const transactionObj = {
  from: account,
  value: web3.toWei(amountInEther, 'ether'),
};

contractInstance.createTokens.sendTransaction(transactionObj, (error, result) = {
  ...
};

Also, as a side note, your value calculations are incorrect. msg.value is in Wei, not ether. Sending in 1 ether causes you to go well above your hardlimit. It's recommended to work with Wei in your contracts, so you should adjust your RATE.