How to extract int128 from msg.data as first argument?

Question

Let's say we have Ethereum smart contract with fallback function:

event onDeposit(int128 indexed key, address from, uint amount);
... 
function () payable public {
  require(msg.value > 0);

  int128 key = ... // how to extract it from msg.data? (0x123)
  address from = msg.sender;
  uint amount = msg.value;

  // fire event
  emit onDeposit(key, from, amount);
}

and taking into account key is entered in myetherwallet.com like that:

Is it correct that i should skip 4 first bytes in msg.data (function address) and get the next 4 bytes (int128 size)? How can i do it (slice bytes and cast them as int128)?

Answer 1

Is it correct that i should skip 4 first bytes in msg.data (function address)

No, because no function selector is being included in the data.

and get the next 4 bytes (int128 size)?

An int128 is 16 bytes wide (128 bits).

I believe just doing int128(msg.data) will work, assuming the data is properly formatted (16 bytes wide, left-padded with zeros).