How to load the first 4 bytes from a bytes calldata var?

Question

It is common for calldata to be encoded as bytes calldata when developing contracts in Solidity.

I need to extract the function selector from such a bytes var. What's the best way to load the first 4 bytes?

Answer 1

function func(bytes calldata data) external {
    bytes4 selector = bytes4(data[:4]);
}

Also works for calldata, if you don't feel like using Yul

Answer 2

Jan 2023

As per the bug report in this GitHub issue, it is more secure to use Ryan Sea's answer, which uses high-level Solidity to access the first 4 bytes in the data bytes array.

Oct 2021

Most gas-efficient way is to use inline assembly:

function foo(bytes calldata data) external payable {
    bytes4 selector;
    assembly {
        selector := calldataload(data.offset)
    }
    // ...
}

The offset suffix is a special property that we need to use in inline assembly to be able to read high-level storage variables.

Answer 3

There's an even faster way to get the first 4 bytes of calldata: accessing the global variable msg.sig will return the first 4 bytes of calldata and give you access to the function selector.

This is the safest, easiest, and cleanest way to retrieve function selectors in solidity.

Have fun coding!