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https://github.com/supernovahs/zebra
https://github.com/supernovahs/zebra
Last synced: about 1 month ago
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- Host: GitHub
- URL: https://github.com/supernovahs/zebra
- Owner: supernovahs
- Created: 2023-01-06T19:26:21.000Z (about 2 years ago)
- Default Branch: main
- Last Pushed: 2023-02-09T18:44:39.000Z (almost 2 years ago)
- Last Synced: 2024-10-29T17:56:21.292Z (3 months ago)
- Language: Solidity
- Size: 8.79 KB
- Stars: 0
- Watchers: 2
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
Awesome Lists containing this project
README
# zebra
## Strings in EVM
Example:-
We are returning a string `hello` using inline assembly in the below function
```solidity
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 ,
```solidity
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
```solidity
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.
```solidity
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
```solidity
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 :
```solidity
function hello() public pure returns(string memory){
assembly{
mstore(0x00,0x20)
mstore(0x20,0x5)
mstore(0x40,0x68656c6c6f000000000000000000000000000000000000000000000000000000)
return(0x00,0x60)
}
}
```
OR
```solidity
function hello() public pure returns(string memory){
assembly{
mstore(0x00,0x20)
mstore(0x20,0x6)
mstore(0x40,0x0568656c6c6f000000000000000000000000000000000000000000000000000000)
return(0x00,0x60)
}
}
```