msg.sender 在solidity中有什么用？

Question

In this codpiece, I am finding it hard to figure out what is msg.sender is and how it works internally.

What I am understanding is, we have a mapping favoriteNumber, and the key is an address and the value is a uint.

What is the meaning of comment - "Update our favoriteNumber mapping to store _myNumber under msg.sender , I am understanding that we are updating favoriteNumber, but what does it mean that under msg.sender. What is the role of this method, how it's working?

    mapping (address => uint) favoriteNumber;

function setMyNumber(uint _myNumber) public {
  // Update our `favoriteNumber` mapping to store `_myNumber` under `msg.sender`
  favoriteNumber[msg.sender] = _myNumber;
  // ^ The syntax for storing data in a mapping is just like with arrays
}

function whatIsMyNumber() public view returns (uint) {
  // Retrieve the value stored in the sender's address
  // Will be `0` if the sender hasn't called `setMyNumber` yet
  return favoriteNumber[msg.sender];
}

Answer 1

Every smart contract invocation has a caller address. Each EVM (Ethereum Virtual Machine that executes the code) knows which account carries out each action. In Solidity, you can access the calling account by referencing msg.sender

So when you call a function of solidity contract, your contract already gets the information of your account, so your account is the msg.sender

favoriteNumber is a mapping. think it like a javascript object. It maps the account addresses to their favourite number.

 0x9C6520Dd9F8d0af1DA494C37b64D4Cea9A65243C -> 10 

So when you call setMyNumber(_myNumber) , you are passing your favourite number. so this number will be stored in favoriteNumber mapping like this:

 yourAccountAdress -> yourFavouriteNumber

So when you call whatIsMyNumber function, since EVM already gets your account number, checks in the mappings and returns you your favourite number.

Answer 2

In solidity exists 3 types of variables: state, local and global. example of global variables:

• msg.sender (sender of the message)
• msg.value (number of wei sent with the message)

pseudocode from favoriteNumber[msg.sender] = _myNumber;

• given a favoriteNumber list,
• select the address of the account calling this function,
• assign _myNumber to that address

note: global variables are available in all contracts by default. see more info here: solidity docs - global variable