Solidity – What Happens if Calling Non-existent Function Without Fallback?

Any message sent to a contract, even if it is empty or just an ether transfer will invoke the code of the contract. The fallback function is run if the data sent does not start with a known function identifier (e.g. if a non-existent function is called or if the message data is empty as is the case for a simple ether transfer).

So if a contract has a fallback function, there is no way to prevent it being called (except by calling an actually existing function or not sending enough gas but no ether).

In Solidity, both .call and .send compile to the CALL opcode, just their expected arguments are different.

Functions can be invoked in two different ways:

1. Via a transaction, which is broadcast to the network and changes blockchain state.
2. Via a call, which is performed locally on a single Ethereum network and does not change any blockchain state.

It's actually up to the client to decide which to do, but if a function is marked view, the default behavior of most clients (e.g. Remix, web3.js, etc.) is to use a call. That basically means running the transaction but not recording any state changes.

So in the above case, if you call B(), it will in effect give you back the value counter + 1. (During execution, it will increment the counter, but that change will simply be discarded.)

That said, you could invoke B() via a transaction, in which case the counter would truly be incremented in a persistent way. In essence, view is a hint to clients that they should use a call rather than a transaction.