Is EOF guaranteed to be -1?

Question

When writing code in C, is it good style to use -1 and EOF interchangeably? Does the standard guarantee that EOF is -1? Or is the value implementation defined?

For example, if a function returns EOF in certain cases, is it good style to test if the function has returned -1?

If a function returns -1 in certain cases, is it good style to test if the function has returned EOF?

Answer 1

No, EOF most certainly is not guaranteed to be -1.

It's a macro, defined in <stdio.h>,

which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream

(quoting from section 7.21.1 of the ISO C standard).

I've never seen an implementation where EOF has a value other than -1 -- but then I haven't bothered to check.

Using EOF rather than a literal -1 avoids having your code break on an implementation that chooses a different negative value, but far more importantly it's clear to the reader. If I see -1 in your code, I won't be sure why you chose that particular value. If I see EOF, I know exactly what you meant. (This would apply even if the standard required EOF to have the value -1.)

As for "magic numbers" in general, there are some times when they're unavoidable. An example from the comments: the getopt function is documented to return -1 in certain circumstances, so you should compare its result to -1, not to EOF. Avoid magic numbers when you can, but if the documentation uses them, you're pretty much stuck with them.