What is “-1L” in C?

Question

What do \"-1L\", \"1L\" etc. mean in C ?

For example, in ftell reference, it says

... If an error occurs, -1L is returned ...

Answer 1

ftell() returns type long int, the L suffix applied to a literal forces its type to long rather than plain int.

NULL would be wholly incorrect because it is a macro representing a pointer not an integer. Its value, when interpreted and an integer may represent a valid file position, while -1 (or any negative value) cannot.

For all intents and purposes you can generally simply regard the error return as -1, the L suffix is not critical to correct operation in most cases due to implicit casting rules