> ## Documentation Index
> Fetch the complete documentation index at: https://voltaire.tevm.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# Selectors & Signatures

> Function selectors, event topic hashes, and signature generation

<Card title="Try it Live" icon="play" href="https://playground.tevm.sh?example=primitives/abi.ts">
  Run ABI examples in the interactive playground
</Card>

<Info>
  **Conceptual guide** - For encoding rules, see [ABI Encoding](/primitives/abi/encoding).
</Info>

## Overview

Selectors identify functions, events, and errors in Ethereum. They are derived from canonical signatures using keccak256.

## Quick Start

<Tabs>
  <Tab title="Function">
    ```typescript theme={null}
    import { Abi } from '@tevm/voltaire/Abi';

    const transfer = {
      type: 'function',
      name: 'transfer',
      inputs: [
        { type: 'address', name: 'to' },
        { type: 'uint256', name: 'amount' }
      ],
      outputs: [{ type: 'bool' }]
    } as const;

    const selector = Abi.Function.getSelector(transfer);
    // 0xa9059cbb
    ```
  </Tab>

  <Tab title="Event">
    ```typescript theme={null}
    import { Abi } from '@tevm/voltaire/Abi';

    const Transfer = {
      type: 'event',
      name: 'Transfer',
      inputs: [
        { type: 'address', name: 'from', indexed: true },
        { type: 'address', name: 'to', indexed: true },
        { type: 'uint256', name: 'value' }
      ]
    } as const;

    const topic0 = Abi.Event.getSelector(Transfer);
    // 0xddf252ad...
    ```
  </Tab>

  <Tab title="Error">
    ```typescript theme={null}
    import { Abi } from '@tevm/voltaire/Abi';

    const InsufficientBalance = {
      type: 'error',
      name: 'InsufficientBalance',
      inputs: [
        { type: 'uint256', name: 'balance' },
        { type: 'uint256', name: 'required' }
      ]
    } as const;

    const selector = Abi.Error.getSelector(InsufficientBalance);
    // first 4 bytes of keccak256(signature)
    ```
  </Tab>
</Tabs>

## Canonical Signatures

Signatures are generated from the **canonical** type list:

* No whitespace
* Use full size types (`uint256`, `int256`)
* Tuples use parentheses: `foo((uint256,address))`
* Arrays include brackets: `foo(address[])`

```typescript theme={null}
const sig = Abi.Function.getSignature(transfer);
// "transfer(address,uint256)"
```

## Event Topic0 and Anonymous Events

Non-anonymous events include `topic0 = keccak256(signature)`. Anonymous events omit topic0 entirely, so all topics are indexed parameters only.

```typescript theme={null}
const anonymous = {
  type: 'event',
  name: 'Log',
  anonymous: true,
  inputs: [{ type: 'uint256', name: 'id', indexed: true }]
} as const;
```

## Selector Collisions

Function selectors are 4 bytes, so collisions are possible. You can scan an ABI for collisions:

```typescript theme={null}
import { Abi } from '@tevm/voltaire/Abi';

const hasCollisions = Abi.hasSelectorCollisions(erc20Abi);
const collisions = Abi.findSelectorCollisions(erc20Abi);
```

Event selectors are 32 bytes and effectively collision-free.

## Common Selectors

ERC-20 functions:

* `0xa9059cbb` - `transfer(address,uint256)`
* `0x23b872dd` - `transferFrom(address,address,uint256)`
* `0x095ea7b3` - `approve(address,uint256)`
* `0x70a08231` - `balanceOf(address)`

ERC-20 events:

* `0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef` - `Transfer(address,address,uint256)`
* `0x8c5be1e5ebec7d5bd14f71427d1e84f3dd0314c0f7b2291e5b200ac8c7c3b925` - `Approval(address,address,uint256)`

## Custom Hash Functions

Use selector factories when you need a custom keccak256 implementation:

```typescript theme={null}
import { Function } from '@tevm/voltaire/Abi';

const getSelector = Function.GetSelector({
  keccak256String: (value) => myKeccak256(value)
});

const selector = getSelector(transfer);
```

## See Also

* [Function.getSelector](/primitives/abi/function-get-selector)
* [Event.getSelector](/primitives/abi/event-get-selector)
* [Error.getSelector](/primitives/abi/error-get-selector)
* [ABI Encoding](/primitives/abi/encoding)