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Overview

Opcode: 0x33 Introduced: Frontier (EVM genesis) CALLER pushes the address of the immediate caller onto the stack. This is the address that directly invoked the current execution context, changing with each call in the call chain.

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Specification

Stack Input: 
[]
Stack Output: 
caller (uint160 as uint256)
Gas Cost: 2 (GasQuickStep) Operation: 
stack.push(execution_context.caller)

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Behavior

CALLER provides the address that made the current call. Unlike ORIGIN which remains constant, CALLER changes with each contract call in the execution chain. Key characteristics:
  • Changes with each call (CALL, STATICCALL, DELEGATECALL)
  • Can be either EOA or contract address
  • Used for authentication and access control
  • Safe for authorization checks

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Examples

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Basic Usage

import { caller } from '@tevm/voltaire/evm/context';
import { createFrame } from '@tevm/voltaire/evm/Frame';
import * as Address from '@tevm/voltaire/primitives/Address';

// Immediate caller address
const callerAddr = Address('0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb');
const frame = createFrame({
  caller: callerAddr,
  stack: []
});

const err = caller(frame);

console.log(frame.stack[0]); // 0x742d35Cc6634C0532925a3b844Bc9e7595f0bEbn

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Access Control

contract Ownable {
    address public owner;

    constructor() {
        owner = msg.sender;  // Uses CALLER opcode
    }

    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");  // SAFE
        _;
    }

    function restricted() public onlyOwner {
        // Only owner can call
    }
}

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Call Chain Tracking

contract ContractC {
    function whoCalledMe() public view returns (address) {
        return msg.sender;  // Returns ContractB's address
    }
}

contract ContractB {
    function callC(ContractC c) public returns (address) {
        return c.whoCalledMe();  // msg.sender in C = address(this)
    }
}

// User (0xAAA) → ContractB (0xBBB) → ContractC (0xCCC)
// In ContractC: msg.sender = 0xBBB

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Gas Cost

Cost: 2 gas (GasQuickStep) Same cost as other environment access opcodes:
  • ADDRESS (0x30): 2 gas
  • ORIGIN (0x32): 2 gas
  • CALLVALUE (0x34): 2 gas

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Common Usage

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Ownership Pattern

contract Owned {
    address public owner;

    constructor() {
        owner = msg.sender;
    }

    function transferOwnership(address newOwner) public {
        require(msg.sender == owner, "Not owner");
        require(newOwner != address(0), "Invalid address");
        owner = newOwner;
    }
}

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Access Control Lists

contract ACL {
    mapping(address => bool) public authorized;

    modifier onlyAuthorized() {
        require(authorized[msg.sender], "Not authorized");
        _;
    }

    function grantAccess(address account) public onlyAuthorized {
        authorized[account] = true;
    }

    function revokeAccess(address account) public onlyAuthorized {
        authorized[account] = false;
    }
}

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Payment Tracking

contract PaymentTracker {
    mapping(address => uint256) public payments;

    receive() external payable {
        payments[msg.sender] += msg.value;
    }

    function refund() public {
        uint256 amount = payments[msg.sender];
        require(amount > 0, "No payment");

        payments[msg.sender] = 0;
        payable(msg.sender).transfer(amount);
    }
}

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Delegation Pattern

contract Delegator {
    mapping(address => address) public delegates;

    function setDelegate(address delegate) public {
        delegates[msg.sender] = delegate;
    }

    function actAsDelegate(address principal) public view returns (bool) {
        return delegates[principal] == msg.sender;
    }
}

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Security

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CALLER vs ORIGIN

SAFE pattern - use msg.sender (CALLER): 
contract Safe {
    address public owner;

    function withdraw() public {
        require(msg.sender == owner, "Not owner");  // ✓ SAFE
        payable(owner).transfer(address(this).balance);
    }
}
UNSAFE pattern - use tx.origin (ORIGIN): 
contract Unsafe {
    address public owner;

    function withdraw() public {
        require(tx.origin == owner, "Not owner");  // ✗ DANGEROUS
        payable(owner).transfer(address(this).balance);
    }
}

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DELEGATECALL Context Preservation

contract Implementation {
    address public owner;

    function whoIsOwner() public view returns (address) {
        return msg.sender;  // Returns caller in delegatecall context
    }
}

contract Proxy {
    address public implementation;

    fallback() external payable {
        address impl = implementation;
        assembly {
            calldatacopy(0, 0, calldatasize())
            let result := delegatecall(gas(), impl, 0, calldatasize(), 0, 0)
            returndatacopy(0, 0, returndatasize())
            switch result
            case 0 { revert(0, returndatasize()) }
            default { return(0, returndatasize()) }
        }
    }
}

// User calls Proxy.whoIsOwner() via delegatecall
// msg.sender in Implementation = User's address (not Proxy)

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Reentrancy Protection

contract ReentrancyGuard {
    mapping(address => bool) private locked;

    modifier nonReentrant() {
        require(!locked[msg.sender], "Reentrant call");
        locked[msg.sender] = true;
        _;
        locked[msg.sender] = false;
    }

    function withdraw() public nonReentrant {
        // Protected from reentrancy
        uint256 amount = balances[msg.sender];
        balances[msg.sender] = 0;
        payable(msg.sender).transfer(amount);
    }
}

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Authorization Checks

contract MultiSig {
    mapping(address => bool) public isSigner;
    mapping(bytes32 => mapping(address => bool)) public approved;

    function approve(bytes32 txHash) public {
        require(isSigner[msg.sender], "Not a signer");  // ✓ SAFE
        approved[txHash][msg.sender] = true;
    }

    function execute(bytes32 txHash) public {
        require(approved[txHash][msg.sender], "Not approved");
        // Execute transaction
    }
}

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Implementation

import { consumeGas } from "../Frame/consumeGas.js";
import { pushStack } from "../Frame/pushStack.js";
import { toU256 } from "../../primitives/Address/AddressType/toU256.js";

/**
 * CALLER opcode (0x33) - Get caller address
 *
 * Stack: [] => [caller]
 * Gas: 2 (GasQuickStep)
 */
export function caller(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, 2n);
  if (gasErr) return gasErr;

  const callerU256 = toU256(frame.caller);
  const pushErr = pushStack(frame, callerU256);
  if (pushErr) return pushErr;

  frame.pc += 1;
  return null;
}

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Edge Cases

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Contract as Caller

// Caller can be a contract address
const contractCaller = Address('0xContractAddress...');
const frame = createFrame({ caller: contractCaller });

caller(frame);
console.log(frame.stack[0]); // Contract address as u256

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Stack Overflow

const frame = createFrame({
  caller: callerAddr,
  stack: new Array(1024).fill(0n)
});

const err = caller(frame);
console.log(err); // { type: "StackOverflow" }

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Out of Gas

const frame = createFrame({
  caller: callerAddr,
  gasRemaining: 1n
});

const err = caller(frame);
console.log(err); // { type: "OutOfGas" }

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Best Practices

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✅ DO: Use for access control

require(msg.sender == owner);

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✅ DO: Track caller identity

mapping(address => uint256) public balances;
balances[msg.sender] += amount;

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✅ DO: Validate caller

require(authorizedCallers[msg.sender], "Unauthorized");

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❌ DON’T: Confuse with tx.origin

// WRONG
require(tx.origin == owner);

// CORRECT
require(msg.sender == owner);

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References