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Version: 4.x

Contract Debugging

There are three ways to debug your ink! contract currently:

  • You can write tests using one of the mechanisms described on the Contract Testing page.
  • You can interact with your contract via a UI or command-line. This is described on the Call Your Contract page.
  • You can print debug statements in your contract. Those will appear on the Substrate node's stdout. This is described on this page.

How do I print to the terminal console from ink!?​

You can use those two macros:

There are things you could do to enable debug messages on the client console:

  1. Enable the feature ink-debug for the ink_env crate.
    cargo-contract does this automatically for you (for versions >= 0.13.0), except if you compile a contract in --release mode.

  2. Set the log level of your node to runtime::contracts=debug.
    For example, to have only errors and debug output show up for the substrate-contracts-node:

substrate-contracts-node -lerror,runtime::contracts=debug
  1. Set the log level of your node to runtime::contracts::strace to trace host function calls.
    These function calls logs will also be displayed in the Debug message panel of Contracts UI. For example, to view these traces in addition to the logs described above:
substrate-contracts-node -lerror,runtime::contracts=debug,runtime::contracts::strace=trace

Example​

The following code depicts how to print debug statements from a message or constructor.

#[ink(constructor)]
fn new() -> Self {
    ink::env::debug_println!("created new instance at {}", Self::env().block_number());
    Self { }
}

#[ink(message)]
fn print(&self) {
   let caller = self.env().caller();
   let message = ink_prelude::format!("got a call from {:?}", caller);
   ink::env::debug_println!(&message);
}
note

Debug output is not printed for transactions!

It is only printed for RPC calls or off-chain tests.

Decode Data Payload

You can use a block explorer or an app like PolkadotJs to retrieve the data payload of a contract transaction, and then use cargo-contract to decode it.

# From your contract directory
cargo contract decode message -d fe5bd8ea01000000

This command will output the method name and parameters encoded in the data payload:

Decoded data: inc_by { n: 1 }

If the contract was called through a cross-contract interaction, the payload will not be available in the transaction. In such cases, you can use the approach described in the next section to access it.

Replay and Debug a Block

To replay a transaction, you can use Chopstick to create a local fork of the chain and replay the block with trace-level logging.

Assuming you have a node that you can connect to at $ENDPOINT and the transaction you want to replay is in block $BLOCK_HASH, you can use the following command:

npx @acala-network/chopsticks@latest run-block \
--endpoint $ENDPOINT \
--block $BLOCK_HASH \
--runtime-log-level 5 \
| grep runtime::contracts

This command replays the block with trace-level logging enabled. By filtering the output with runtime::contracts, you can view all the contract calls in the block:

runtime::contracts           TRACE: call ExportedFunction::Call account: , input_data: [246, 118, 44, 201]
runtime::contracts           TRACE: call ExportedFunction::Call account: , input_data: [254, 91, 216, 234, 1, 0, 0, 0]
runtime::contracts           TRACE: call result ExecReturnValue { flags: (empty), data: [0] }
runtime::contracts           TRACE: call result ExecReturnValue { flags: (empty), data: [0] }

From here, you can identify the call you are interested in and decode the data payload:

echo 254, 91, 216, 234, 1, 0, 0, 0 \
| tr ',' ' ' \
| xargs printf '%02x' \
| xargs cargo contract decode message -d

This command will output the following:

Decoded data: inc_by { n: 1 }

Fork Node and Replay Transactions

You can also use Chopstick to start a local fork of your chain.

This command starts a fork beginning at block $BLOCK_HASH. You can connect to this fork using ws://localhost:8000 to submit extrinsics via PolkadotJs or cargo contract:

npx @acala-network/chopsticks@latest \
--endpoint $ENDPOINT \
--block $BLOCK_HASH \
--runtime-log-level 5

Here, for example, you can re-run the transaction that we decoded in the previous section:

cargo contract call \
--contract $CONTRACT_ADDR \
--message inc_by --args 1 \
--suri //Alice \
--url ws://localhost:8000

Since trace-level logging is used, you will receive detailed information about all the host functions called during the execution of the contract:

runtime::contracts           TRACE: call ExportedFunction::Call account: , input_data: [254, 91, 216, 234, 2, 0, 0, 0]
runtime::contracts           TRACE: call result ExecReturnValue { flags: (empty), data: [0] }
runtime::contracts           DEBUG: Execution finished with debug buffer: seal0::value_transferred(out_ptr: 65488, out_len_ptr: 65516) = Ok(())
seal0::input(out_ptr: 65536, out_len_ptr: 65524) = Ok(())
seal1::get_storage(key_ptr: 65536, key_len: 4, out_ptr: 65540, out_len_ptr: 65524) = Ok(Success)
seal2::set_storage(key_ptr: 65536, key_len: 4, value_ptr: 65540, value_len: 4) = Ok(4)
seal0::seal_return(flags: 0, data_ptr: 65536, data_len: 1) = Err(TrapReason::Return(ReturnData { flags: 0, data: [0] }))