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Selector

A Selector is a 4-byte identifier used to uniquely identify Ethereum function calls. It’s computed as the first 4 bytes of the Keccak-256 hash of the function signature.

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Type Definition

const primitives = @import("primitives");
// Selector is a 4-byte array: [4]u8
const Selector = primitives.Selector.Selector;

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Creating Selectors

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From Signature String

The most common way to create a selector is from a function signature: 
const primitives = @import("primitives");
const sel = primitives.Selector.fromSignature("transfer(address,uint256)");
const hex = primitives.Selector.toHex(sel); // "0xa9059cbb"

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From Hex String

const primitives = @import("primitives");
const selector = try primitives.Selector.fromHex("0xa9059cbb");

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From Bytes

const primitives = @import("primitives");
const raw = [_]u8{ 0xa9, 0x05, 0x9c, 0xbb };
const selector = try primitives.Selector.fromBytes(&raw);

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Operations

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Convert to Hex

const primitives = @import("primitives");
const hex = primitives.Selector.toHex(selector); // "0xa9059cbb"

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Compare Selectors

const primitives = @import("primitives");
const sel1 = primitives.Selector.fromSignature("transfer(address,uint256)");
const sel2 = primitives.Selector.fromSignature("approve(address,uint256)");
const equal = primitives.Selector.equals(sel1, sel2); // false

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Common Selectors

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ERC-20

  • transfer(address,uint256) → 0xa9059cbb
  • approve(address,uint256) → 0x095ea7b3
  • balanceOf(address) → 0x70a08231
  • totalSupply() → 0x18160ddd

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ERC-721

  • transferFrom(address,address,uint256) → 0x23b872dd
  • safeTransferFrom(address,address,uint256) → 0x42842e0e
  • ownerOf(uint256) → 0x6352211e

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Signature Format

Function signatures must use canonical type names: ✅ Correct:
  • transfer(address,uint256)
  • swap(uint256,uint256,address,bytes)
  • execute((address,uint256,bytes)[])
❌ Incorrect:
  • transfer(address, uint256) (has spaces)
  • transfer(address,uint) (should be uint256)
  • Transfer(address,uint256) (wrong capitalization)

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How Selectors Work

  1. Function Signature: Start with the canonical function signature
  2. Hash: Compute keccak256(signature)
  3. Truncate: Take the first 4 bytes
transfer(address,uint256)
  ↓ keccak256
0xa9059cbb2fead7aba11f5e0a11af...
  ↓ slice(0, 4)
0xa9059cbb

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Selector Collisions

While rare, different function signatures can theoretically produce the same selector (collision). In practice, this is extremely unlikely due to the cryptographic properties of Keccak-256. If a collision occurs:
  1. One function will override the other in the contract
  2. The EVM will use whichever function is defined first
  3. This is considered a critical contract bug

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Usage in Contracts

Selectors are used in:
  1. Function Calls: First 4 bytes of transaction data identify the function
  2. Low-level Calls: address.call(abi.encodeWithSelector(selector, args))
  3. Function Routing: Contracts use selectors to route calls to the correct function

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API Reference

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Constructors

  • fromBytes(bytes: []const u8) !Selector - Create from raw bytes
  • fromHex(hex: []const u8) !Selector - Create from hex string
  • fromSignature(signature: []const u8) Selector - Compute from function signature

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Operations

  • toHex(selector: Selector) [10]u8 - Convert to hex string (0x + 8 hex chars)
  • equals(a: Selector, b: Selector) bool - Compare selectors

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See Also