Is EOF always negative?

Question

Is EOF always negative?

I'm thinking of writing a function that reads the next word in the input and returns the line number the word was found in or EOF if the end of the input has been reached. If EOF is not necessarily negative, the function would be incorrect.

Answer 1

EOF is always == EOF. Don't assume anything else.

On a second reading of the standard (and as per some other comments here) it seems EOF is always negative - and for the use specified in this question (line number or EOF) it would work. What I meant to warn against (and still do) is assuming characters are positive and EOF is negative.

Remember that it's possible for a standard conforming C implementation to have negative character values - this is even mentioned in 'The C programming language' (K&R). Printing characters are always positive, but on some architectures (probably all ancient), control characters are negative. The C standard does not specify whether the char type is signed or unsigned, and the only character constant guaranteed to have the same value across platforms, is '\0'.

Answer 2

Yes, EOF is always negative.

The Standard says:

7.19 Input/output
7.19.1 Introduction

3 The macros are [...] EOF 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;

Note that there's no problem with "plain" char being signed. The <stdio.h> functions which deal with chars, specifically cast the characters to unsigned char and then to int, so that all valid characters have a positive value. For example:

int fgetc(FILE *stream)

7.19.7.1
... the fgetc function obtains that character as an unsigned char converted to an int ...