zebra

Strings in EVM

Example:- We are returning a string hello using inline assembly in the below function

function hello() public pure returns(string memory){
        assembly{
            mstore(0x00,0x20)
            mstore(0x20,0x5)
            mstore(0x40,0x68656c6c6f000000000000000000000000000000000000000000000000000000)
            return(0x00,0x60)
        }
    }

Basic things to know before starting

• Assembly language is used for low level access to the EVM .
• Strings are dynamic types .
• Memory layout in solidity is as follows:-

{
   "0x00": "0000000000000000000000000000000000000000000000000000000000000000",
   "0x20": "0000000000000000000000000000000000000000000000000000000000000000",
   "0x40": "0000000000000000000000000000000000000000000000000000000000000080"
}

First 2 slots are reserved for hashing methods, 0x40 stores the address that points to an area that is not used a.k.a Free memory Pointer .

Strings are dynamic types , they are stored as follows:

• First we store the pointer to the start of the dynamic data i.e offset
• Then store the length of the string in the next slot .
• In the next slot, we store the string in Hex format right padded .

Example - Store "hello"

1. If we want to start the data at 0x20, We will first store the pointer at 0x00 ,

assembly{
  mstore(0x00, 0x20)
}

Memory : -

"0x00" 0000000000000000000000000000000000000000000000000000000000000020
"0x20": "0000000000000000000000000000000000000000000000000000000000000000",
"0x40": "0000000000000000000000000000000000000000000000000000000000000080"

2. Now we store the length of the string at the pointer we declared above:

hello is of length 5 . In hex 5 = 0x5

assembly{
  mstore(0x00,0x20)
  mstore(0x20,0x5)
}

Memory :-

"0x00" 0000000000000000000000000000000000000000000000000000000000000020
"0x20": "0000000000000000000000000000000000000000000000000000000000000005",
"0x40": "0000000000000000000000000000000000000000000000000000000000000080"

3. Next, we store the actual string in hex format in the next memory slot

hello in hex -> 0x0568656c6c6f

All the values cover full 32 bytes , hence remaining values will be 0 padded upto 32 bytes.

assembly{
mstore(0x00,0x20)
mstore(0x20,0x5)
mstore(0x40,0x0568656c6c6f0000000000000000000000000000000000000000000000000000)
}

Memory :-

"0x00" 0000000000000000000000000000000000000000000000000000000000000020
"0x20": "0000000000000000000000000000000000000000000000000000000000000005",
"0x40": "0568656c6c6f0000000000000000000000000000000000000000000000000000"

Now we return the data using the return opcode

assembly{
mstore(0x00,0x20)
mstore(0x20,0x5)
mstore(0x40,0x0568656c6c6f0000000000000000000000000000000000000000000000000000)
return(0x00,0x60)
}

But if you view the result it will return only hell instead of hello

That is because hello in hex -> 0x0568656c6c6f , this hex code has 05 in the beginning , which implies the length of the string i.e 5 .

Therefore , there are 2 options to return hello correctly

• Either enter length in the slot 0x20 as 6 instead of 5.
• Or , remove 05 from the 0x0568656c6c6f .

Final code is :

function hello() public pure returns(string memory){
        assembly{
            mstore(0x00,0x20)
            mstore(0x20,0x5)
            mstore(0x40,0x68656c6c6f000000000000000000000000000000000000000000000000000000)
            return(0x00,0x60)
        }
    }

OR

function hello() public pure returns(string memory){
        assembly{
            mstore(0x00,0x20)
            mstore(0x20,0x6)
            mstore(0x40,0x0568656c6c6f000000000000000000000000000000000000000000000000000000)
            return(0x00,0x60)
        }
    }