MsgSender.sol

pragma solidity ^0.4.24;

contract A {
    address public sender;
    
    function setMsgSender() public {
        sender = msg.sender;
    }
}

contract B {
    address public sender;
    A a;

    constructor(address _a) public {
        sender = msg.sender;
        a = A(_a);
    }

    function callA() public {
        a.setMsgSender();
    }
}