How can I restrict a function to make it only callable by one contract?

Question

Assume I have two contracts as follows

contract FirstContract{

function anyFunction(){}

}

contract SecondContract{

  FirstContract x = new FirstContract();

function anotherFunction(){
  x.anyFunction(); ///////
}

}

How can I restrict anyFunction() so that only SecondContract can call it and nothing else?

Answer 1

Use modifiers to restrict access to a function by other contracts/users.

Example:

contract FirstContract {

    address creator; // This will be the address of SecondContract.

    function FirstContract() {
        creator = msg.sender; // Which we set here.
    }

    modifier onlyCreator() {
        require(msg.sender == creator); // If it is incorrect here, it reverts.
        _;                              // Otherwise, it continues.
    } 

    function anyFunction() onlyCreator {} // This can now only be accessed by SecondContract

}

If this is what you are looking for, you may also be interested in researching the more common owned pattern, which works similarly.

Answer 2

Don't have enough rep to add comment. But just confirming the selected answer is correct and Enrique's comment is incorrect. User -> Contract1 ->Contract2, msg.sender IS contract 1 for contract2.

See here: Who is msg.sender when calling a contract from a contract

tx.origin is the EOA, msg.sender works for contract.