JSON RPC – How to Get ‘data’ for JSON RPC eth_call

contract-invocationjson-rpc

Is there a more convenient way to construct the value of data for an eth_call JSON-RPC API call, than the "regular" way of first 4 bytes of web3.sha3 of canonical function signature followed by encoded and padded arguments? I know that the web3 Javascript getData method can be used but, if access via web3 is available, then one could just as well call the function directly, instead of just getting data and using JSON-RPC API. Suppose that only JSON-RPC API access is available in some situation. Is constructing data by hand for a contract function call the only option?

EDIT (6/21/2017): It occurred to me that browser-solidity might display the data for an eth_call or eth_sendTransaction, since it does display compiled contract bytecode and interface. That could have been copy-pasted. However, I checked and do not see the data needed for a function call displayed when I call a function in the right panel.

EDIT #2 (6/21/2017): Maybe my question was not clear as posted above. I will try putting it differently. Say, we have the following Solidity contract (named simple) deployed at some address, e.g. at 0xfbb5fa2ea8c5fc6f492c0795564352f262f49f50

pragma solidity ^0.4.9;

contract owned {
    address owner;
    function owned() {
        owner = msg.sender;
    }
    function getOwner() constant returns(address) {
        return owner;
    }
}

contract simple is owned {
    function twice(int a) constant returns(int) {
        return 2*a;
    }
}

The function twice can be called from Javascript code, using web3.js, as follows:

var simple = eth.contract(<ABI>).at(0xfbb5fa2ea8c5fc6f492c0795564352f262f49f50);
var result = simple.twice(7);

However, to call this function using JSON-RPC, an HTTP post request needs to be sent to the RPC port of a node, with the following body:

{
    "jsonrpc": "2.0",
    "method": "eth_call",
    "params": [
        {
            "from": "0xccf9d7d2f8be1f821cb8d9ec9553ffa92aa8fc4d",
            "to": "0xfbb5fa2ea8c5fc6f492c0795564352f262f49f50",
            "data": "0x6ffa1caa0000000000000000000000000000000000000000000000000000000000000007"
        },
        "latest"
    ],
    "id": 1
}

The value of data element is calculated as first 4 bytes of { the Keccak hash of [ the ASCII encoding of ( a canonical form of the function signature ) ] } followed by the arguments encoded in a particular way, as specified at https://github.com/ethereum/wiki/wiki/Ethereum-Contract-ABI. NOTE: Added 3 types of brackets to remove ambiguity 🙂 (This is a very simple function with only 1 int argument but this can quickly get very nasty and error-prone with more complex arguments.

There is a way to easily obtain the appropriate value of data for a function call using a web3.js function, as follows:

var callData = simple.twice.getData(7);

But suppose the function is to be called from a remote machine and also that Javascript is not an option. Is there a convenient way, (i.e. not manually doing so as per the complex rules in the wiki linked above) to compute the correct value of the data element of the JSON-RPC API request?

Best Answer

I found just the tool that I was looking for - ethabi from Parity.

https://github.com/paritytech/ethabi

Summary

Your original contract code would not work correctly until I added a constant to the function definition to indicate that this function does not modify the blockchain.
I had to use the method signature of double(int256) instead of double(int) to get the eth_call JSON-RPC to work.

The Details

Run Your `geth` Instance

You can use the Testnet blockchain that is synchronised with other peers over the Internet, or you can use a private Dev blockchain that is only available on your computer and faster to develop with. The --rpc parameter will allow you to use curl to communicate with your geth instance.

Testnet Blockchain

You will need to create an account and mine some coins as you need the coins to insert your code into the blockchain or to send transactions to the blockchain:

geth --testnet account new
Your new account is locked with a password. Please give a password. Do not forget this password.
Passphrase: 
Repeat Passphrase: 
Address: {aaaa725f2f36a28c01bc47f883534990c9c8bbbb}

Your geth instance will have to synchronise your Testnet blockchain with other peers on the Internet. This may take and hour or more. Once synchronised, your mining operation will commence and you should mine some coins in 10 or 20 minutes. Start geth with the mining parameters:

geth --testnet --rpc --mine --minerthreads 1 console

To check if you have coins, run the command:

> web3.fromWei(eth.getBalance(eth.accounts[0]), "ether");
25

If you want to exit from geth, press Control-D.

If you want to, you can now run your geth instance without the mining parameters as another geth instance somewhere on the Internet will mine your transactions. Use the following command to remove the mining option, or just use the previous geth command:

geth --testnet --rpc console

Dev Blockchain

You will need to create an account and mine some coins:

geth --dev account new
Your new account is locked with a password. Please give a password. Do not forget this password.
Passphrase: 
Repeat Passphrase: 
Address: {cccc725f2f36a28c01bc47f883534990c9c8dddd}

Start geth with the --miner and --minerthreads parameters as your geth instance will need to mine your private blockchain:

geth --dev --rpc --mine --minerthreads 1 console

Flatten Your Code And Assign To A Variable

I had to modify your code to get it to run correctly by adding the word constant to your function definition as this function does not modify the blockchain when it is run.

In your example, your code will normally be formatted for easy reading:

contract Test { 
    function double(int a) constant returns(int) {
        return 2*a;
    } 
}

You can use a service like Line Break Removal Tool to strip out your line breaks, or see How to load Solidity source file into geth for some alternatives.

Your flattened code will look like the example you posted:

contract Test { function double(int a) constant returns(int) { return 2*a; } }

You will need to assign your code to a string variable:

var testSource='contract Test {  function double(int a) constant returns(int) { return 2*a; } }'

Note that // comments will cause some problems when your code if flattened as all text after the // comment will be treated as a comment. Use /* ... */ instead if you need to add comments to your code.

Compile Your Code

In your geth instance, type in your flattened code with the assignment to the variable. Don't worry about the undefined result as this is a normal response from geth:

> var testSource='contract Test {  function double(int a) constant returns(int) { return 2*a; } }'
undefined

Compile your code:

> var testCompiled = web3.eth.compile.solidity(testSource);
I0503 09:04:15.907715    3190 solidity.go:114] solc, the solidity compiler commandline interface
Version: 0.3.2-0/Release-Linux/g++/Interpreter

path: /usr/bin/solc
undefined

If you want to see the binary form of your code:

testCompiled
{
  Test: {
    code: "0x6060604052602a8060106000396000f3606060405260e060020a60003504636ffa1caa8114601a575b005b6002600435026060908152602090f3",
    info: {
      abiDefinition: [{...}],
      compilerOptions: "--bin --abi --userdoc --devdoc --add-std --optimize -o /tmp/solc497335011",
      compilerVersion: "0.3.2",
      developerDoc: {
        methods: {}
      },
      language: "Solidity",
      languageVersion: "0.3.2",
      source: "contract Test {  function double(int a) constant returns(int) { return 2*a; } }",
      userDoc: {
        methods: {}
      }
    }
  }
}

You will need the Application Binary Interface (ABI) signature for your contract if you want to access run your function outside this geth session:

> testCompiled.Test.info.abiDefinition
[{
    constant: true,
    inputs: [{
        name: "a",
        type: "int256"
    }],
    name: "double",
    outputs: [{
        name: "",
        type: "int256"
    }],
    type: "function"
}]

Note: This is the second hint as to why your JSON RPC call did not work - the method signature has int256 for the a parameter instead of int.

Insert Your Code Into The Blockchain

Run the following commands to insert your code into the blockchain:

> var testContract = web3.eth.contract(testCompiled.Test.info.abiDefinition);
undefined
> var test = testContract.new({
    from:web3.eth.accounts[0], 
    data: testCompiled.Test.code, gas: 2000000}, 
    function(e, contract) {
      if (!e) {
        if (!contract.address) {
          console.log("Contract transaction send: TransactionHash: " +
            contract.transactionHash + " waiting to be mined...");
        } else {
          console.log("Contract mined! Address: " + contract.address);
          console.log(contract);
        }
    }
})
I0503 09:09:39.632499    3190 xeth.go:1026] Tx(0x8c15e8b8bb593d4a680b084a455ba82b103e60204638a3674ef1fb90ca0ad4f0) created: 0x7a2d8fa11aa28097135eb514f6a23ba12501191e
Contract transaction send: TransactionHash: 0x8c15e8b8bb593d4a680b084a455ba82b103e60204638a3674ef1fb90ca0ad4f0 waiting to be mined...
undefined

Wait up to a few minutes and you will see the following output:

I0503 09:10:34.319772    3190 worker.go:569] commit new work on block 11122 with 0 txs & 0 uncles. Took 431.081µs
Contract mined! Address: 0x7a2d8fa11aa28097135eb514f6a23ba12501191e
[object Object]

You should save the address if you want to run this contract from outside your geth session.

Use Your Contract Within The Same `geth` Session

Before adding the constant to the double function, I was getting the following results:

> test.double(5)
invalid address

Note: This is the first hint as to why your JSON RPC call failed.

To use your contract within your geth session, AFTER adding constant to the double function:

> test.double(5)
10
> test.double(55)
110

Use Your Contract Using `curl` And JSON-RPC

And now we get to the tricky part.

Your function double(int) is really double(int256). From Solidity - Types:

int• / uint•: Signed and unsigned integers of various sizes. Keywords uint8 to uint256 in steps of 8 (unsigned of 8 up to 256 bits) and int8 to int256. uint and int are aliases for uint256 and int256, respectively.

In geth, we run the following command to find the signature of the double(...) function:

> web3.sha3('double(int256)')
"6ffa1caacdbca40c71e3787a33872771f2864c218eaf6f1b2f862d9323ba1640"

And take the first 4 bytest to create the data parameter

0x6ffa1caa0000000000000000000000000000000000000000000000000000000000000005

And using the address from the "Contract mined! parameter", we build the following curl command:

curl localhost:8545 -X POST --data '{"jsonrpc":"2.0", "method":"eth_call", "params":[{"from": "eth.accounts[0]", "to": "0x65da172d668fbaeb1f60e206204c2327400665fd", "data": "0x6ffa1caa0000000000000000000000000000000000000000000000000000000000000005"}], "id":1}'

And run this command from the command line:

user@Kumquat:~$ curl localhost:8545 -X POST --data '{"jsonrpc":"2.0", "method":"eth_call", "params":[{"from": "eth.accounts[0]", "to": "0x65da172d668fbaeb1f60e206204c2327400665fd", "data": "0x6ffa1caa0000000000000000000000000000000000000000000000000000000000000005"}], "id":1}'
{"id":1,"jsonrpc":"2.0","result":"0x000000000000000000000000000000000000000000000000000000000000000a"}

The result of 0x0000...000a is the value 10.

Update Oct 18 2016

As documented in RPC error "invalid or missing value for params" when calling contract constant void functions, you may have to add a block parameter to the params list. Your command with the block parameter will be:

user@Kumquat:~$ curl localhost:8545 -X POST --data '{"jsonrpc":"2.0", "method":"eth_call", "params":[{"from": "eth.accounts[0]", "to": "0x65da172d668fbaeb1f60e206204c2327400665fd", "data": "0x6ffa1caa0000000000000000000000000000000000000000000000000000000000000005"}, "latest"], "id":1}'
{"id":1,"jsonrpc":"2.0","result":"0x000000000000000000000000000000000000000000000000000000000000000a"}

You may also have to leave out the from parameter.

References:

Your Other Question

Also, are there any way to access already-existing methods within the Go Ethereum library that would free me from having to implement for example methods that to the padding myself?

Could you please ask this in another question as this answer is already too long.

RPC Error – Fixing ‘Invalid or Missing Value for Params’ When Calling Contract Constant Void Functions

Summary

Adding the block number to the eth_call JSON-RPC call parameters as suggested by the closed bug eth_call requires block as last parameter #2472 will work.

Details

I created a contract using Browser Solidity as shown in the screen shot below:

And here is my result:

Iota:ESE bok$ curl localhost:8545 -X POST --header 'Content-type: application/json' --data '{"jsonrpc":"2.0", "method":"eth_call", "params":[{"to": "0xb82020341122e7c8c4ba6551fd25950681af3570", "data": "0x0127efc5"}, "latest"], "id":1}'
{"jsonrpc":"2.0","id":1,"result":"0x000000000000000000000000000000000000000000000000000000000000007b"}

Reference The default block parameter and eth_call.

eth_call without the block parameter worked previously. I've updated my old ESE Q&As on eth_call usage to add a note about this new requirement.

Best Answer

Related Solutions

JSON-RPC API – How to Call a Contract Method Using eth_call

Summary

The Details

Run Your geth Instance

Testnet Blockchain

Dev Blockchain

Flatten Your Code And Assign To A Variable

Compile Your Code

Insert Your Code Into The Blockchain

Use Your Contract Within The Same geth Session

Use Your Contract Using curl And JSON-RPC

Update Oct 18 2016

Your Other Question

RPC Error – Fixing ‘Invalid or Missing Value for Params’ When Calling Contract Constant Void Functions

Summary

Details

Related Topic

Run Your `geth` Instance

Use Your Contract Within The Same `geth` Session

Use Your Contract Using `curl` And JSON-RPC