How two different functions name can have same function signature?

Question

This is the below code

 cast sig "workMyDirefulOwner(uint256,uint256)"                                      
  0xa9059cbb                                                                                                    
 cast sig "transfer(address,uint256)"                 
  0xa9059cbb

Answer 1

Function signatures are deterministically calculated in the context of Solidity contracts. They are nothing but a keccak256 hash of the function name along with the parameter types.

For ex, if you function is transfer(address _recipient, uint256 amount), the signature would be keccak256 of transfer(address,uint256) which would result in the following hash:

a9059cbb2ab09eb219583f4a59a5d0623ade346d962bcd4e46b11da047c9049b

Whereas, during run time the EVM uses function selectors to invoke a function call to a specific method. Function selectors are first 4 bytes (0xa9059cbb from the above example) of the function signature. So the can be function selectors can collide, but function signatures can never be the same for 2 functions.

Some useful resources:

• https://emn178.github.io/online-tools/keccak_256.html
• https://www.4byte.directory/

Answer 2

The function signature is just the beginning of a hash of the function name and its parameters - as you can see.

If you perform the hashing manually, the actual hash is a lot longer. The hashes would then not be the same. But Ethereum uses only the first 8 characters of the full hash. That means that (hash) collisions are a lot more probable. This is also why your two function signatures result in the same signature hash.