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

-1 formated in long int is a -1L. Why not simple NULL? Because NULL in this function is a normal result and can't sygnalize error too. Why NULL in this function is a normal result? Because NULL == 0 and ftell returns position in a stream, when you are on start of stream function returns 0 and this is a normal result not error, then if you compare this function to NULL to check error, you will be get error when you will be on start position in stream.