> For the complete documentation index, see [llms.txt](https://calnix.gitbook.io/eth-dev/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calnix.gitbook.io/eth-dev/compendium/abi-and-debugging.md).

# ABI & Debugging

ABI -> Application Binary Interface

The ABI contains all teh Functions, parameters, return values of the contract.

#### Example

```solidity
//SPDX-License-Identifier: MIT

pragma solidity 0.8.3;

contract MyContract {
    string public myString = 'hello world';
}
```

In remix, go to compilation tab and select compilation details:

![](/files/LMubR3OwjM0S8IvNZRhb)      ![](/files/HoG0JkOMURq3Gu2AcV1l)

We see that there is one single element under the ABI array section . This is because all we did was declare a public state variable -> for which solidity created a getter function. This getter function is the single element.

inputs -> input parameters (none here)

output -> output variables (a single string as output)

### Function Hashes

Function hashes are the way how you interact with a Smart Contract on the blockchain. When a SC gets compiled, you will no longer see the readable cleartext version of code "`myString()`", rather you would see the hash `492bfa18`

![](/files/930sBhN0e46bUoxm9kGr)

note to self: this doesn't match up exactly. could be version difference?

What matches up:

* on deploying the contract, the value in the input field matchs the opcode in debugger:

![in javaVM](/files/ZboRhDtHXfqKoJozQhdT)

{% hint style="warning" %}
<https://ethereum.stackexchange.com/questions/135205/what-is-a-function-signature-and-function-selector-in-solidity-and-evm-language>
{% endhint %}

## Function Signature

A function signature is a combination of a function and the types of parameters it takes, combined together as a string with no spaces.

For example, let's say you have a function in solidity where the method looks like this:

```javascript
function transfer(address sender, uint256 amount) public {
  // Some code here
}
```

This function would have a function signature of:

```
transfer(address,uint256)
```

These are important because we use function signatures to get the next part, function selectors. Additionally, you'll want to use `uint256` instead of `uint` [for computing a function signature](https://docs.soliditylang.org/en/develop/abi-spec.html#types) (and function selector).

## Function Selector

A function selector is the first 4 bytes of the call data for a function call that specifies the function to be called. A function selector is the hash of the same function's signature.

Which might be a little confusing, but let's break it down. When someone makes a call to an EVM smart contract, the smart contract needs to know which function it should execute. The piece of code that governs this is known as the function selector, and it might look like this:

```javascript
0xa9059cbb //this is the function selector for the transfer function signature above. 
```

You can get the function selector by hashing the string of the function signature in solidity.

```javascript
bytes4(keccak256(bytes(function_signature)))
```

### Bonus

There are [EVM Signature Databases](https://sig.eth.samczsun.com/) that make it easier to find out the function signature of a selector. Understanding function signatures and selectors allow developers to [call functions of any contract without having an ABI](https://twitter.com/PatrickAlphaC/status/1517156225670078465).

## abi.encode variants

* <https://ethereum.stackexchange.com/questions/129642/what-is-the-difference-between-encodewithselector-and-encode>
* <https://ethereum.stackexchange.com/questions/91826/why-are-there-two-methods-encoding-arguments-abi-encode-and-abi-encodepacked>
* <https://solidity-by-example.org/abi-encode/>
*


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