solidity – How to Return Error Message from Smart Contract

ethereumjsquorumremixsolidityweb3js

I am trying to return error messages from solidity. I have this contract:

pragma solidity ^0.4.23;
contract Simple {
    function arithmetics(uint a, uint b){           
                     require(a % 2 == 0, "Even value required.");             
                    b = b * 3; 
            }
}

When I test in remix, it's returning me the following error message:

transact to Simple.arithmetics errored: VM error: revert.
revert The transaction has been reverted to the initial state.
Reason provided by the contract: "Even value required.". Debug the transaction to get more information.

However, when I tried calling it from my dApp using web3.js I am not getting an error message, only this:

Transaction ran out of gas.

Best Answer

You can find the error message tracing the transaction. For example, using geth and I assume your node is running at http://127.0.0.1:8545:

1- Open a console

geth attach http://127.0.0.1:8545

2- Trace transaction

debug.traceTransaction(<your TX hash here>)

You should get a transaction trace. If your geth node wasn't started with debug enabled, you can just do an HTTP request like:

curl -X POST --data '{"jsonrpc":"2.0","method":"debug_traceTransaction","params":["<your TX hash here>"],"id":1}' http://127.0.0.1

Now that you have the trace, you can prettify the JSON to see it better. At the very bottom you will find the last instruction and the EVM state (and other things). The last instruction should be a REVERT opcode. Then, go to the MEMORY section, and you can convert it to ascii to find the error message. For example, in bash this command would convert from hex to ascii:

echo <your HEX here> | xxd -r -p

Related Solutions

Troubleshooting Transaction Reverts in Remix for Solidity

Problem: You are trying to send Ethers with the command receiver.transfer(amount); and specifying the amount as a parameter. Since your contract do not have enough ether balance it reverts the transaction.

Solution: You need to have enough ethers in your contract or you should send ethers along with the function call to solve the problem.

Change your smart contract as follows if you need to send _amount to _receiver from sender's address

pragma solidity ^0.4.14;
contract Sample {

  address private receiver;
  uint public amount;

  function pay(address _receiver) payable public returns (bool) {
    receiver = _receiver;
    receiver.transfer(msg.value);
    return true;
  }
}

And your caller function as follows

sampleContract.pay(_receiver, {value : <value in wei>});

[Ethereum] what are the diffrences between
.call and
.

As already said by smarx you are calling two different functions, therefore they behave differently.

About the question in the title:

You can get insight into the difference between <address>.call and <address>.<function> from the documentation of Solidity, in Section 4.4.4 (page 68) and in Section 4.4.5 (pages 70-71), respectively.

`<address>.call`

<address>.call(bytes memory) returns (bool): issue low-level CALL with the given payload, returns false on failure, forwards all available gas, adjustable

`<address>.function`

Function calls cause exceptions if the called contract does not exist (in the sense that the account does not contain code) or if the called contract itself throws an exception or goes out of gas

Conclusion

So the former is low-level in the sense that you have to check the return value manually, while the second let the exception bubble up (is more high-level).

Best Answer

Related Solutions

Troubleshooting Transaction Reverts in Remix for Solidity

[Ethereum] what are the diffrences between .call and .

<address>.call

<address>.function

Conclusion

Related Topic

[Ethereum] what are the diffrences between
.call and
.

`<address>.call`

`<address>.function`