Solidity

What are Global Variables?

Global (special) variables and functions always exist in the global namespace. They give information about the current call (msg), the current block (block) and the transaction (tx), and provide utility functions. You do not declare them – you simply use them.

Global Variable Chart

msg.sender vs tx.origin

Never use tx.origin for authorisation. If Alice is tricked into calling a malicious contract, that contract can call your contract and tx.origin will still be Alice. Use msg.sender.

Units and Built-in Functions

Example : Using Global Variables

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

contract GlobalsDemo {
    address public owner;
    uint public deployedAt;
    uint public constant LOCK_TIME = 1 days;   // time unit

    constructor() {
        owner = msg.sender;                     // who deployed the contract
        deployedAt = block.timestamp;           // when it was deployed
    }

    function deposit() external payable {
        require(msg.value >= 0.01 ether, "Send at least 0.01 ether");
    }

    function info() external view returns (address caller, uint blockNo, uint chainId, uint balance) {
        return (msg.sender, block.number, block.chainid, address(this).balance);
    }

    function isUnlocked() external view returns (bool) {
        return block.timestamp >= deployedAt + LOCK_TIME;
    }

    function hashOf(string calldata text) external pure returns (bytes32) {
        return keccak256(abi.encodePacked(text));
    }
}

Key Takeaways

The PDF E-Notes are the earlier printable version of this chapter. The notes on this page are the latest, revised version.

Question / Answer (15 questions)

MCQ Test : Check Your Understanding (5 questions)

Select an option and press Check Answer, or answer all questions and press Submit Test to see your score. Each answer comes with an explanation.

Bibliography/References

The notes, questions and MCQs on this page are original explanations written for this course. The works listed below were consulted and are cited for attribution and further reading. Product names, logos and trademarks belong to their respective owners. Short quotations, where used, are marked and attributed.

1. Solidity Team. (n.d.). Units and Globally Available Variables. Solidity Documentation. https://docs.soliditylang.org/en/latest/units-and-global-variables.html
2. Solidity Team. (n.d.). Security Considerations. Solidity Documentation. https://docs.soliditylang.org/en/latest/security-considerations.html
3. Ethereum Improvement Proposals. (2021). EIP-4399: Supplant DIFFICULTY opcode with PREVRANDAO. https://eips.ethereum.org/EIPS/eip-4399
4. ethereum.org. (n.d.). Transactions. Ethereum Developer Documentation. https://ethereum.org/en/developers/docs/transactions/
5. Chainlink. (n.d.). Chainlink VRF (Verifiable Random Function) documentation. https://docs.chain.link/vrf
6. Modi, R. (2018). Solidity Programming Essentials: A beginner’s guide to build smart contracts for Ethereum and blockchain. Packt Publishing. ISBN 978-1-78883-138-3.