What is an ABI? Smart Contracts ABI Explained

Last Updated:
May 19, 2023
What is an ABI of a Smart Contract - Feature Image

In the world of blockchain and smart contracts, there are numerous technical terms that might seem daunting to the uninitiated. One such term is 'ABI', an acronym for 'Application Binary Interface'. Learn more.

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In the world of blockchain and smart contracts, there are numerous technical terms that might seem daunting to the uninitiated. One such term is 'ABI', an acronym for 'Application Binary Interface'. This article will delve into what an ABI is, why it's important, and how it functions in the context of smart contracts.

First, let's break down the term 'Application Binary Interface'. It might sound complicated, but once we understand each word, the concept becomes clearer. An 'application' is a program designed for end users. 'Binary' refers to the basic language of computers, composed of 1s and 0s. 'Interface', in this context, is a point where two systems meet and interact.

So, an Application Binary Interface is a defined way for an application to interact with the system at a binary level. It's like a contract specifying how software components should interact with each other, providing a predictable environment for this interaction.

In the context of Ethereum and other smart contract platforms, the ABI is a critical piece of the puzzle. It's the standard way to interact with contracts in the Ethereum ecosystem, specifying how to convert function calls into data that Ethereum's EVM (Ethereum Virtual Machine) can understand, and vice versa.

Why is an ABI important?

The ABI is essential for interacting with smart contracts on the Ethereum blockchain. When you create a smart contract, it's compiled into bytecode that can be executed by the Ethereum Virtual Machine. But this bytecode isn't human-readable, making it difficult for developers to interact with the contract directly.

This is where the ABI comes in. It's a kind of user manual that tells you how to interact with the contract. It defines the contract's functions and the inputs they accept. It also specifies how the function's output will be formatted when it's returned to you. Without an ABI, it would be near impossible to interact effectively with a smart contract.

ABI in action

Let's consider a simple example to better understand how ABIs work. Suppose you have a smart contract for a simple voting system. This contract might have functions like ‘voteForCandidate’, which takes a candidate's name as input, and getVotes, which returns the total votes a candidate has received.

When this contract is compiled, these human-readable function names and arguments are turned into bytecode. For instance, the voteForCandidate function might be represented in bytecode as 0xabcd1234.

The ABI for this contract would look something like this:

Here, the ABI is a JSON array containing two objects, each representing a function. Each object specifies the function's name, its input and output types, and whether it alters the state of the contract (constant: false means it changes the state, constant: true means it doesn't).

When you call voteForCandidate("Alice"), your Ethereum client uses the ABI to figure out how to translate that into bytecode the EVM can understand. It finds the voteForCandidate function in the ABI, sees that it takes a single string input, and uses this information to construct the bytecode needed for the call.

Now, the voteForCandidate("Alice") function call is translated into something like 0xabcd1234 + encode("Alice"), where encode is a function that turns the string "Alice" into a format the EVM can understand. This entire encoded string is what actually gets sent to the EVM.

Conversely, when the contract returns data from a function call, the ABI is used to decode the returned bytecode back into a human-readable format. If you call ‘getVotes("Alice")’ and the contract returns 0x000000000000000000000000000000000000000000000000000000000000000a, the ABI tells your Ethereum client to decode that as a uint256, giving you the result 10.

This process of encoding and decoding is made possible through the use of tools like web3.js, which make it easy to interact with smart contracts on Ethereum. When you create an instance of a contract in web3.js, you provide the contract's ABI and address. Web3.js then uses this information to generate an object that exposes all the contract's functions for you to call in JavaScript​​.

Generating an ABI

When you write a smart contract in a high-level language like Solidity, the compiler not only generates the bytecode for the contract but also the ABI. This is usually done using the Solidity compiler solc. When you compile your contract using solc, it will output a JSON file containing the ABI for that contract​​.


In conclusion, the Application Binary Interface (ABI) plays a crucial role in the interaction between smart contracts and the outside world. It acts as a translator, converting high-level language function calls into a format that the Ethereum Virtual Machine can understand, and vice versa.

Understanding the ABI is key to effective Ethereum development. It allows developers to interact with smart contracts, call their functions, and interpret the data they return.

With the rise of blockchain technology and smart contracts, understanding concepts like the ABI becomes even more significant. It opens the door to the development of more complex and powerful decentralized applications, paving the way for a future where blockchain technology is increasingly integrated into our digital lives​