CallDataDecoded - Voltaire

CallDataDecoded represents the structured form of calldata after parsing the function selector and ABI-decoding the parameters.

Structure

export type CallDataDecoded = {
  selector: [4]u8,
  signature: ?[]const u8,
  parameters: []AbiValue,
}

Fields

selector - 4-byte function identifier

First 4 bytes of keccak256 hash of function signature
Required for all function calls
Immutable once created

signature - Optional function signature string

Human-readable like "transfer(address,uint256)"
Useful for debugging and logging
Not required for execution (EVM only uses selector)

parameters - Decoded ABI parameter values

Array of structured ABI values
Preserves type information
Enables type-safe parameter access

Decoding CallData

With ABI
Without ABI

import { CallData, Abi } from '@tevm/voltaire';

const abi = Abi([{
  name: "transfer",
  type: "function",
  inputs: [
    { name: "to", type: "address" },
    { name: "amount", type: "uint256" }
  ]
}]);

const calldata = CallData("0xa9059cbb...");
const decoded = CallData.decode(calldata, abi);

console.log(decoded.selector); // [0xa9, 0x05, 0x9c, 0xbb]
console.log(decoded.signature); // "transfer(address,uint256)"
console.log(decoded.parameters[0]); // Address
console.log(decoded.parameters[1]); // Uint256

import { CallData } from '@tevm/voltaire';

const calldata = CallData("0xa9059cbb...");

// Can still extract selector
const selector = CallData.getSelector(calldata);
console.log(selector); // [0xa9, 0x05, 0x9c, 0xbb]

// But parameters remain encoded
const rawParams = calldata.slice(4); // Everything after selector

Encoding to CallData

TypeScript
Zig

import { CallData, Address, TokenBalance } from '@tevm/voltaire';

const decoded: CallDataDecoded = {
  selector: [0xa9, 0x05, 0x9c, 0xbb],
  signature: "transfer(address,uint256)",
  parameters: [
    Address("0x70997970C51812dc3A010C7d01b50e0d17dc79C8"),
    TokenBalance.fromUnits("1", 18)
  ]
};

const calldata = CallData.encode(decoded);
console.log(CallData.toHex(calldata));
// "0xa9059cbb00000000000000000000000070997970..."

const std = @import("std");
const CallData = @import("primitives").CallData;

pub fn example(allocator: std.mem.Allocator) !void {
    var decoded = CallDataDecoded{
        .selector = [_]u8{ 0xa9, 0x05, 0x9c, 0xbb },
        .signature = "transfer(address,uint256)",
        .parameters = &[_]AbiValue{
            .{ .address = address_value },
            .{ .uint256 = amount_value },
        },
        .allocator = allocator,
    };

    const calldata = try decoded.encode(allocator);
    defer allocator.free(calldata.data);
}

Working with Parameters

Type-Safe Access

import { CallData, Abi } from '@tevm/voltaire';

const abi = Abi([{
  name: "swap",
  type: "function",
  inputs: [
    { name: "tokenIn", type: "address" },
    { name: "tokenOut", type: "address" },
    { name: "amountIn", type: "uint256" },
    { name: "minAmountOut", type: "uint256" }
  ]
}]);

const calldata = CallData("0x...");
const decoded = CallData.decode(calldata, abi);

// Type-safe parameter access
const tokenIn: Address = decoded.parameters[0];
const tokenOut: Address = decoded.parameters[1];
const amountIn: Uint256 = decoded.parameters[2];
const minAmountOut: Uint256 = decoded.parameters[3];

Dynamic Parameters

const abi = Abi([{
  name: "multicall",
  type: "function",
  inputs: [
    { name: "calls", type: "bytes[]" }
  ]
}]);

const decoded = CallData.decode(calldata, abi);

// Access dynamic array
const calls: CallData[] = decoded.parameters[0];
for (const call of calls) {
  console.log(CallData.toHex(call));
}

Selector Operations

Matching Selectors

import { CallData } from '@tevm/voltaire';

const TRANSFER_SELECTOR = [0xa9, 0x05, 0x9c, 0xbb];
const APPROVE_SELECTOR = [0x09, 0x5e, 0xa7, 0xb3];

function routeCall(calldata: CallData) {
  const selector = CallData.getSelector(calldata);

  if (equals(selector, TRANSFER_SELECTOR)) {
    return handleTransfer(calldata);
  } else if (equals(selector, APPROVE_SELECTOR)) {
    return handleApprove(calldata);
  } else {
    throw new Error("Unknown function");
  }
}

Computing Selectors

import { Keccak256 } from '@tevm/voltaire';

function computeSelector(signature: string): [4]u8 {
  const hash = Keccak256.hashString(signature);
  return hash.slice(0, 4);
}

const selector = computeSelector("transfer(address,uint256)");
// [0xa9, 0x05, 0x9c, 0xbb]

Memory Management (Zig)

In Zig, CallDataDecoded owns allocated memory:

pub fn processCallData(
    allocator: std.mem.Allocator,
    calldata: CallData,
) !void {
    var decoded = try calldata.decode(allocator);
    defer decoded.deinit(); // Free allocated memory

    // Use decoded parameters
    for (decoded.parameters) |param| {
        std.debug.print("Param: {}\n", .{param});
    }
}

Signature Resolution

Known Signatures

const KNOWN_SIGNATURES = {
  "0xa9059cbb": "transfer(address,uint256)",
  "0x095ea7b3": "approve(address,uint256)",
  "0x70a08231": "balanceOf(address)",
};

function resolveSignature(calldata: CallData): string | null {
  const selector = CallData.getSelector(calldata);
  const selectorHex = Hex.fromBytes(selector);
  return KNOWN_SIGNATURES[selectorHex] ?? null;
}

4byte.directory API

async function lookupSelector(selector: [4]u8): Promise<string[]> {
  const selectorHex = Hex.fromBytes(selector);
  const response = await fetch(
    `https://www.4byte.directory/api/v1/signatures/?hex_signature=${selectorHex}`
  );
  const data = await response.json();
  return data.results.map(r => r.text_signature);
}

Documentation Index

​Structure

​Fields

​Decoding CallData

​Encoding to CallData

​Working with Parameters

​Type-Safe Access

​Dynamic Parameters

​Selector Operations

​Matching Selectors

​Computing Selectors

​Memory Management (Zig)

​Signature Resolution

​Known Signatures

​4byte.directory API

​See Also