Debugger

Forge ships with an interactive debugger.

The debugger is accessible on forge run and on forge test.

Using forge test:

$ forge test --debug $FUNC

Where $FUNC is the signature of the function you want to debug. For example:

$ forge test --debug "testSomething"

If you have multiple contracts with the same function name, you need to limit the matching functions down to only one case using --match-path and --match-contract.

If the matching test is a fuzz test, the debugger will open the first failing fuzz scenario, or the last successful one, whichever comes first.

Using forge run:

$ forge run --debug $FILE --sig $FUNC

Where $FILE is the path to the contract you want to debug, and $FUNC is the signature of the function you want to debug. For example:

$ forge run --debug src/SomeContract.sol --sig "myFunc(uint256,string)" 123 "hello"

You can also specify raw calldata using --sig instead of a function signature.

If your source file contains more than one contract, specify the contract you want to debug using the --target-contract flag.

Debugger layout

An image of the debugger UI

When the debugger is run, you are presented with a terminal divided into four quadrants:

  • Quadrant 1: The opcodes in the debugging session, with the current opcode highlighted. Additionally, the address of the current account, the program counter and the accumulated gas usage is also displayed
  • Quadrant 2: The current stack, as well as the size of the stack
  • Quadrant 3: The source view
  • Quadrant 4: The current memory of the EVM

As you step through your code, you will notice that the words in the stack and memory sometimes change color.

For the memory:

  • Red words are about to be written to by the current opcode
  • Green words were written to by the previous opcode
  • Cyan words are being read by the current opcode

For the stack, cyan words are either being read or popped by the current opcode.

General

  • q: Quit the debugger
  • 0-9 + k: Step a number of times backwards (alternatively scroll up with your mouse)
  • 0-9 + j: Step a number of times forwards (alternatively scroll down with your mouse)
  • g: Move to the beginning of the transaction
  • G: Move to the end of the transaction
  • c: Move to the previous call-type instruction (i.e. CALL, STATICCALL, DELEGATECALL, and CALLCODE).
  • C: Move to the next call-type instruction
  • a: Move to the previous JUMP or JUMPI instruction
  • s: Move to the next JUMPDEST instruction
  • Ctrl + j: Scroll the memory view down
  • Ctrl + k: Scroll the memory view up
  • m: Show memory as UTF8
  • J: Scroll the stack view down
  • K: Scroll the stack view up
  • t: Show labels on the stack to see what items the current op will consume