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I have created a contract using Remix.

When I run the function burnFrom(), it shows:

gas required exceeds allowance or always failing transaction.

What am I doing wrong?

function burnFrom(address _from, uint256 _value) onlyOwner public returns 
(bool success) {
    require(balanceOf[_from] >= _value);                
    require(_value <= allowance[_from][msg.sender]);    
    balanceOf[_from] -= _value;                         
    allowance[_from][msg.sender] -= _value;             
    totalSupply -= _value;                              
    emit Burn(_from, _value);
    return true;
}
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  • As the error says, the transaction always fails. You need to check that all require conditions are fullfilled. Jul 4 '18 at 15:51
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Check that you are either using the javascript VM or that you have metamask unlocked and connected to a test net if using injected web3.

Make sure you have an allowance mapping that looks like this:

mapping(address => mapping(address => uint256)) public allowance;

And that your Burn event looks like this:

event Burn(address indexed from, uint256 value);

If you have both of these correct there is probably something wrong with the onlyOwner modifier.

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  • OP must have declared these mappings else he would be getting compile time error in remix. The code compiled successfully means there is no cases that he missed declarations. Jul 5 '18 at 0:13

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