> For the complete documentation index, see [llms.txt](https://chihaolu.gitbook.io/all-in-one-solidity/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://chihaolu.gitbook.io/all-in-one-solidity/part-i-basic/chapter-7-jue-se-he-quan-ju-xun-xi-global-variables/msg.md).

# 【Msg】

`msg` 代表這個 contracts 當前收到什麼訊息，

* `msg.data (bytes)`: 完整的 `calldata`
* `msg.sender (address)`: 誰傳遞了這個訊息，或指訊息的來源地址是什麼
* `msg.value (uint)`: 傳遞來的訊息，傳遞了多少 wei
* `msg.gas`: 剩餘的 `gas` 是多少，已被 `gasleft()` 取代

> 在 Solidity 中，要執行一個函數通常會需要外部有一個 caller。如果今天沒有人呼叫這個函數，這個函數就會一直靜靜地躺在區塊鏈之中什麼都不做。反之，`msg.sender` 就會一直存在。

```solidity
pragma solidity ^0.8.11;

contract myContract {
    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];
    }
}
```

`msg` 有兩個屬性，一個是 `msg.sender`，另一個是 `msg.value`，這兩個值可以被任何 `external` 函數調用，包含庫裏面的函數。

`calldata` 為 `msg.data` 和 `external` 函數的參數們。

```solidity
pragma solidity ^0.8.11;

contract SendMoney{
    
    uint public balanceReceived;
    
    function receiveMoney() public payable {
        balanceReceived += msg.value; // amount in wei
    }
    
    function getBalance() public view returns(uint){
        return address(this).balance;
    }
    
    function withdrawMoney() public {
        address payable to = msg.sender;
        
        to.transfer(this.getBalance()); // withdraw all money
        
        // here, you sholud avoid store balance amount in a variable
        // beacuse after withdrawMoney work, the balanceReceived won't change
        // but this.getBalance() return the real balance == 0;
        
        
        // if we transfer 1 ether, finally the to account get 100.999999...
    }
    
    function withdrawMoneyTo(address payable _to) public {
        _to.transfer(this.getBalance());
        
        // if we transfer 1 ether, finally the to account get finally 101!
        // because the gas was paid by the contract deployer
    }
}
```