Function Selector.md

82 - Function Selector

The first four bytes of the call Data for a function call specifies the function to be called. 

1. It is the first (left, high-order in big-endian) four bytes of the Keccak256 hash of the signature of the function. 

2. The signature is defined as the canonical expression of the basic prototype without data location specifier, i.e. the function name with the parenthesised list of parameter types. Parameter types are split by a single comma - no spaces are used.

3. Function Arguments: The encoded arguments follow the function selector from the fifth byte onwards.

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• Specifies the exact function that needs to be called
• Selector -> Contract Function
• First 4 bytes -> Keccak256(fn sig)
• Function signature -> Name(param1type,param2type)
• Function Arguments -> Encoded & 5th Byte onwards

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Example

Function: function deposit(address _from, uint256 _amount) {...}

Function selector: deposit(address,uint256)

Keccak256: 0x47e7ef24b3022e382e65b1298581281f0ae273ac4f76464efcf5b06769264f2f

Function Selector/Signature: 0x47e7ef24

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Code Example

# seth sig 'deposit(address,uint256)'
0x47e7ef24

# seth keccak 'deposit(address,uint256)'|cut -c -10
0x47e7ef24

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References

• Youtube Reference
• 4byte Directory