I was asked to validate a smart contract for a token and a function that puzzled me looks like that:

     function transferFrom(address _from, address _to, uint tokens)

            require(balanceOf[_from] >= tokens);

            if (_from != msg.sender && allowance[_from][msg.sender] != uint(-8523)) {
                require(allowance[_from][msg.sender] >= tokens);
                allowance[_from][msg.sender] -= tokens;

        _transfer(_from, _to, tokens);


So the logic is: If and only if both the sender is different than _from and the approval is for anything other than -8523 then make sure _from has approved message sender to transfer the specified amount of tokens and also subtract them from the allowance.

Which of course leads to the question: is it technically possible for uint to be negative and can someone specify allowance for negative uint which is namely -8523? What if uint is send in hexadecimal or so? Thanx!

  • "is it technically possible for uint to be negative" - No, uint is by definition non-negative. Commented Nov 18, 2019 at 20:21
  • thought so....yet how is this possible: ethereum.stackexchange.com/questions/69404/…
    – Robert Ggg
    Commented Nov 18, 2019 at 20:52
  • maybe he gets HUGE positive number but sends in negative....
    – Robert Ggg
    Commented Nov 18, 2019 at 20:52
  • Read the first comment to that question (which happens to be mine, I just noticed, but it summarizes pretty well what I was thinking of writing to you when I read the title of that question). Commented Nov 18, 2019 at 21:02
  • Yeah OK so do you mean by "off chain" a client call? For example what will happen if I call it vis web3 say using python and simply send -1 to the blockchain or to the testnet (ropsten)? I guess it will be converted to the 2^256-1 or so...
    – Robert Ggg
    Commented Nov 18, 2019 at 21:12

1 Answer 1


Some tokens use a special value in allowance to minimize gas fees when used in combination with approve/transferFrom.

If allowance is that special value it will skip updating it, saving one storage modification. If you perform multiple transactions for example in decentralized exchanges you will have important savings over time.

It is a "hackish" way to minimize gas usage. I don't like it I'd prefer to have a separate flag and for the user to explicitely enable it. Otherwise you have to make sure the value will never be generated under normal circumstances.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.