Detailed explanation of call function in solidity

Catalog

Call the external contract code

call function

call Basic usage of function

 call Function return value

call Functions and callback functions

call Function and msg.data

 call Function to modify the state variable of the external contract

dalegatecall

 call Function transfer and callback function details

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Call the external contract code

We have seen it before , Use interface、library To call the code of the external contract .
Next , We will add the third form for you ：
In the following code , Deploy cat After the contract , For example, the address is  0x345678..  In the deployment animal At the time of the contract , Pass this cat Contract address . Thus, the reference of the contract can be stored . call test Method can call the method of the external contract .

call function

Whether it's interface、library Or the form seen above , To call external code , It's all the bottom calls call Or is it delecall function .

call Basic usage of function

call How to use the function , First, you need the address of the external contract . In the following example animalCall contract , When deploying contracts , Delivered external contracts cat The address of  0x345678.. , Stored in address c among .
By contract address .call( Function flags ) To invoke the contract . The function identifier is the first... After the hash of the function declaration 4 Bytes of data .
In the following cases ,c.call(bytes4(keccak256(“eat()”))) Will call cat In the contract eat Method

 call Function return value

call The return value of the function is true perhaps false. Only when this method can be found and executed successfully , Returns the true, If the function cannot be found or the execution fails, it will return false. So call test1 Method will return true, call test2 Method will return false, Because the function cannot be found .

call Functions and callback functions

call Function if the function cannot be found , The callback function will be called by default .
Callback functions are special functions , It has no function name .
In the form of ： function(){}

For the following cat contract . Write the callback function . Assume that the contract address is c. Then call... Externally c.call(“abc”); This function will not be found , The callback function will be executed by default . Therefore, it is called externally c.call(“abc”) Will make cat The state variable of the contract becomes 999. and call Function will return true.

contract cat{
    uint public a = 5;
    function ear() public returns(uint){
        a = 256;
        return a;
    }
    
    function () {
        a = 999;
    }
}

call Function and msg.data

Callback functions are very useful , For example, when the external call fails , Perform some operation .
For the following cat contract . Write the callback function . Assume that the contract address is c. Then call... Externally c.call(“abc”); This function will not be found , The callback function will be executed by default . In the callback function , take msg.data The value of is assigned to fail Variable . adopt getfail Function can be viewed call The complete data passed by the function .fail The value of the variable is 32 Bytes 0x6162630000000000000000000000000000000000000000000000000000000000, front 3 Bytes are parameter letters a、b、c Of ASCII code .61、62、63.

contract cat{
    bytes fail;
    function (){
        fail = msg.data;
    }
    
    function getfail() returns(bytes){
        return fail;
    }
}

 call Function to modify the state variable of the external contract

In the following example ,cat Contract and animalcall There are state variables in the contract. Let's deploy first cat contract , Get the address 0x3456.., Next , Deploy the contract address as a parameter anumalCall contract .
call test2 Method , The call cat The contract eat Method , Revised cat In contract a The value of is 256. call Function call external contract , Modify the state variables in the external contract .

dalegatecall

delegatecall Function and call The function is the same , By contract address .delegatecall( Function flags ) To invoke the contract . The function identifier is the first... After the hash of the function declaration 4 Bytes of data .
library The remote call of the library is exactly used delegatecall function .delegatecall And call The difference is ,delegatecall State variables in external contracts will not be modified , It seems that the code of the external function is loaded into the local contract for execution . The value of the local contract status variable will be modified .
For example, the following code , Deploy first cat contract , Get the address 0x3456.., Next , Deploy the contract address as a parameter anumalCall contract .
call test2 Method , The call cat The contract eat Method , But it was modified animalcall State variables in contracts a. Therefore, it is found that ,cat In the contract a There is no change ,animalCall Contract variables a Changed into 256.

 call Function transfer and callback function details

call Function can transfer money , And is transfer And send The underlying function of .call The use method of function transfer is
Address .call.value( Transfer amount )()

It should be noted that , When performing transfer , If the address of the transfer is the contract , And there is a callback function in the transfer contract . Then the callback function will be executed by default .
But Ethereum in order to avoid reentry attacks , about transfer And send Function is restricted . When using transfer And send function , The maximum number of operations performed in the callback function cannot exceed 2300gas. This means that the transfer cannot be performed 、 Assignment, etc , It can only perform operations such as event triggering .
For example, the following code ： Deploy first Receiver contract , Get the address 0x3456.., Pass it on again Receiver Address deployment Sender contract . When calling sendMoney Method time , Is the contract address 0x3456.. The transfer operation will trigger the callback function , Change state variables balance Increase in the number of . But because the operation of modifying state variables exceeds the maximum 2300gas The limitation of , So the following operations will not succeed .

call Function can make the above operation successful .call Function can specify gas The limitation of , exceed 2300gas The constraints of limits .
As shown in the following example ：
Deploy first Receiver contract , Get the address 0x3456.., Pass it on again Receiver Address deployment Sender contract . When calling sendMoney Method transfer 100wei When , Is the contract address 0x3456.. The transfer operation will trigger the callback function , Change state variables balance Increase in the number of . because call Function specifies the maximum gas Limit to 20317, So trigger the callback function to balance The amount of is revised to 100. But be careful , Because of this ,call Functions are dangerous underlying functions , The problem of reentry attacks cannot be avoided .