[alkonaut]

How could an api for number parsing ever be designed to return 0 for invalid input, for a function where 0 is also a common (perhaps the most common) return value for a valid input?

This wouldn't even pass a cursory sanity check of the api from a beginner developer, how did it end up in a standard library at all? Was it a mistake and then it was just too late to remove it?

Any function that can either succeed or fail, which is basically every parsing function, must typically indicate success or failure. You can terminate the program or you can return an object that itself indicates failure (such as -1 when finding a positive index) but if ALL values of the return type CAN be valid then the success state must be a separate return value.

What's the purpose of the function atol() if it doesn't have that? Is it "It's still useful for trusted input we know is a string representation of a long" (E.g. for bounded number roundtrip)? That seems awfully limited. But perhaps such a scenario was perhaps more common in 1960?