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

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 …