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When we make a token tx to a receiver contract, the sender has to use the transfer function in the token contract first before being able to send the tokens to the receiver contract.

Sender --> Token Contract --> Receiver Contract

Does this mean the msg.sender the receiver contract will get is the token address? If so, how can a receiver contract know the original sender address?

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You're correct that msg.sender in the receiver will be the address of a token contract. This is why the first parameter to tokenFallback is the address of the original sender: for a compliant ERC223 token, the receiver knows the original sender by this first parameter.

https://github.com/ethereum/EIPs/issues/223 mentions these points:

Contract to work with tokens

function tokenFallback(address _from, uint _value, bytes _data) A function for handling token transfers, which is called from the token contract, when a token holder sends tokens. _from is the address of the sender of the token,_value is the amount of incoming tokens, and _data is attached data similar to msg.data of Ether transactions. It works by analogy with the fallback function of Ether transactions and returns nothing.

NOTE: msg.sender will be a token-contract inside the tokenFallback function. It may be important to filter which tokens are sent (by token-contract address). The token sender (the person who initiated the token transaction) will be _from inside the tokenFallback function.

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