How to customize your Orbit chain's precompiles

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caution

The guidance in this document will work only if you use eth_call to call the new precompiles. If you're calling from other contracts or adding non-view/pure methods, this approach will break block validation.

To support these additional use-cases, follow the instructions described in How to customize your Orbit chain's behavior.

There are two ways to customize your chain's precompiles:

1. Add new methods to an existing precompile.
2. Create a new precompile.

Prerequisites

Clone the Nitro repository before you begin:

git clone --branch v2.1.1 <https://github.com/OffchainLabs/nitro.git>
cd nitro
git submodule update --init --recursive --force

Option 1: Add new methods to an existing precompile

Using your favorite code editor, open an existing precompile from the precompiles implementation directory, /precompiles. We'll use ArbSys.go as an example. Open the corresponding Go implementation file (ArbSys.go) and add a simple SayHi method:

func (con *ArbSys) SayHi(c ctx, evm mech) (string, error) {
    return "hi", nil
}

Then, open the corresponding Solidity interface file (ArbSys.sol) from the precompiles interface directory, /src/precompiles, and add the required interface. Ensure that the method name on the interface matches the name of the function you introduced in the previous step, camelCased:

function sayHi() external view returns(string memory);

Next, follow the steps in How to customize your Orbit chain's behavior to build a modified Arbitrum Nitro node docker image and run it.

info

Note that the instructions provided in How to run a full node will not work with your Orbit node. See Command-line options (Orbit) for Orbit-specific CLI flags.

Once your node is running, you can call ArbSys.sol either directly using curl, or through Foundry's cast call.

Call your function directly using curl

curl Your_IP_Address:8547\
-X POST \
-H "Content-Type: application/json" \
--data '{"method":"eth_call","params":[{"from":null,"to":"0x0000000000000000000000000000000000000064","data":"0x0c49c36c"}, "latest"],"id":1,"jsonrpc":"2.0"}'

You should see something like this:

{"jsonrpc":"2.0","id":1,"result":"0x000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000026869000000000000000000000000000000000000000000000000000000000000"}

0x6869 is the hex-encoded utf8 representation of hi, which you'll see embedded in the result hex string.

Call your function using Foundry's cast call

cast call 0x0000000000000000000000000000000000000064 "sayHi()(string)”

You should see something like this:

hi

Option 2: Create a new precompile

First, navigate to the precompiles implementation directory, /precompiles, and create a new precompile implementation file called ArbHi.go. We'll define a new method, and we'll give it an address:

package precompiles

// ArbGasInfo provides insight into the cost of using the rollup.
type ArbHi struct {
    Address addr // 0x11a, for example
}

func (con *ArbHi) SayHi(c ctx, evm mech) (string, error) {
    return "hi", nil
}

Then, update precompile.go to register the new precompile under the Precompiles() method:

insert(MakePrecompile(templates.ArbHiMetaData, &ArbHi{Address: hex("11a")})) // 0x011a here is an example address

Navigate to the precompiles interface directory, /src/precompiles, create ArbHi.sol, and add the required interface. Ensure that the method name on the interface matches the name of the function you introduced in the previous step, camelCased:

pragma solidity >=0.4.21 <0.9.0;

/// @title Say hi.
/// @notice just for test
/// This custom contract will set on 0x000000000000000000000000000000000000011a since we set it in precompile.go.
interface ArbHi {
    function sayHi() external view returns(string memory);
}

Next, follow the steps in How to customize your Orbit chain's behavior to build a modified Arbitrum Nitro node docker image and run it.

info

Note that the instructions provided in How to run a full node will not work with your Orbit node. See Command-line options (Orbit) for Orbit-specific CLI flags.

Once your node is running, you can call ArbHi.sol either directly using curl, or through Foundry's cast call.

Call your function directly using curl

curl Your_IP_Address:8547 \
-X POST \
-H "Content-Type: application/json" \
--data '{"method":"eth_call","params":[{"from":null,"to":"0x000000000000000000000000000000000000011a","data":"0x0c49c36c"}, "latest"],"id":1,"jsonrpc":"2.0"}'

You should see something like this:

{"jsonrpc":"2.0","id":1,"result":"0x000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000026869000000000000000000000000000000000000000000000000000000000000"}

Call your function using Foundry's cast call

cast call 0x000000000000000000000000000000000000011a "sayHi()(string)”

You should see something like this:

hi