Debugging a Transaction

The goal of this tutorial is to explain how to debug transaction using Remix.

Start debugging

There are two different ways to start debugging, each way correspond to a different use case.

From the Transaction GUI

We will not explain in detail here how to write or deploy contract. Let us start with a basic contract (replace this one by yours):

contract Donation {
    address owner;
    event fundMoved(address _to, uint _amount);
    modifier onlyowner { if (msg.sender == owner) _; }
    address[] _giver;
    uint[] _values;

    function Donation() {
        owner = msg.sender;
    }

    function donate() payable {
        addGiver(msg.value);
    }

    function moveFund(address _to, uint _amount) onlyowner {
        uint balance = this.balance;
        uint amount = _amount;
        if (_amount <= this.balance) {
            if (_to.send(this.balance)) {
                fundMoved(_to, _amount);
            } else {
                throw;
            }
        } else {
            throw;
        }
    }

    function addGiver(uint _amount) internal {
        _giver.push(msg.sender);
        _values.push(_amount);
    }
}

For the purpose of this tutorial we will run the JavaScript VM (that’s the default mode if you don’t use Remix with Mist or Metamask) which simulates a custom blockchain. You could do the same using a proper backend node.

Let’s deploy the contract:

Right panel / Red button Create

_images/remix1.png

Then we should call the Donate function (that will send Ether to the contract).

Let’s set the amount of Ether:

Right panel / second tab from the left - fill in the ´´value´´ input (´1 ether´ for instance)

_images/remix_valueinput.png

Then click on Donate. As we are using the JavaScript VM, everything goes almost instantly.

Remix displays also some information related to each transaction result. On the right of each transaction result you can start debugging the corresponding transaction.

_images/remix_startdebugging.png

From the Debugger

The debugger can be found in the right panel / 5th tab from the left.

You can start a debug session by providing either a transaction hash or a block number and transaction index

_images/remix3.png

Click the play button to start debugging.

Using the debugger

The debugger allows to see detailed informations about the transaction execution. It uses the editor (left panel) to display the location in the source code where the current execution is.

First the transaction panel display basic and simple information about the current transaction.

_images/remix_debugtransactioninfo.png

The navigation part contains a slider and buttons that can be used to step through the transaction execution.

From the left to the right:

step over back, step into back, step into forward, step over forward, jump out (jump out of the current call), jump to the previous breakpoint, jump to the next breakpoint.

_images/remix_navigation.png

11 panels give detailed informations about the execution:

Instructions

_images/remix_debuginstructions.png

This panel displays the bytecode of the current executing contract, the current step is highlighted.

Important note: When this panel is hidden, the slider will have a courser granularity and only stop at expression boundaries, even if they are compiled into multiple EVM instructions. When the panel is diplayed, it will be possible to step over every instruction, even those that refers to the same expression.

Solidity Locals

_images/remix_soliditylocals.png

That panel displays local variables associated with the current context.

Solidity State

_images/remix_soliditystate.png

That panel displays state variables of the current executing contract.

Low level panels

These panels display low level informations about the execution:

  • Stack
  • Storages Changes
  • Memory
  • Call Data
  • Call Stack
  • Return Value (only if the current step is a RETURN opcode)
  • Full Storages Changes (only at the end of the execution - display every storage change of every modified contract)

Reverted Transaction

A transaction could be reverted (either because of out of gas exception, Solidity throw or low level exception).

In that case it is important to be aware of the exception and to locate where the exception is in the source code.

Remix will warn you when the execution throws an exception. The warning button will jump to the last opcode before the exception happened.

_images/remix_executionexception.png

Breakpoints

The two last buttons from the navigation area are used to jump either back to the previous breakpoint or forward to the next breakpoint.

Breakpoints can be added and removed by clicking on the line number.

_images/remix_breakpoint.png

When a debug session is started, the execution will jump to the first encountered breakpoint.

Important note: If you add a breakpoint to a line that declares a variable, it might be triggered twice: Once for initializing the variable to zero and second time for assigning the actual value. As an example, assume you are debugging the following contract:

contract ctr {
    function hid () {
        uint p = 45;
        uint m;
        m = 89;
        uint l = 34;
    }
}

And let’s says that breakpoints are set for the lines

uint p = 45;

m = 89;

uint l = 34;

then clicking on Jump to next breakpoint will stop at the following lines in the given order:

uint p = 45; (declaration of p)

uint l = 34; (declaration of l)

uint p = 45; (45 assigned to p)

m = 89; (89 assigned to m)

uint l = 34; (34 assigned to l)