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 wether the char type is signed or unsigned, and the only character constant guaranteed to have the same value across platforms, is '\0'.

Answer 2

Have that function return

• the line number the word was found in
• or -1 in case the end of the input has been reached

Problem solved, without a need for relying on any EOF values. The caller can easily test for greater-or-equal-to-zero for a successful call, and assume EOF/IO-error otherwise.

Answer 3

EOF is a condition, rather than a value. The exact value of this sentinel is implementation defined. In a lot of cases, it is a negative number.

Answer 4

From wikipedia :

The actual value of EOF is a system-dependent negative number, commonly -1, which is guaranteed to be unequal to any valid character code.

But no references ...

From Secure Coding : Detect and handle input and output errors EOF is negative but only when sizeof(int) > sizeof(char).

Answer 5

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 ...

Answer 6

From the online draft n1256, 17.9.1.3:

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;

EOF is always negative, though it may not always be -1.

For issues like this, I prefer separating error conditions from data by returning an error code (SUCCESS, END_OF_FILE, READ_ERROR, etc.) as the function's return value, and then writing the data of interest to separate parameters, such as

int getNextWord (FILE *stream, char *buffer, size_t bufferSize, int *lineNumber)
{
  if (!fgets(buffer, bufferSize, stream))
  {
    if (feof(stream)) return END_OF_FILE; else return READ_ERROR;
  }
  else
  {
    // figure out the line number
    *lineNumber = ...;
  }
  return SUCCESS;
}