The value of EOF

K&R Exercise 1-7: write a program that prints the value of EOF.

getchar() and EOF showed that getchar() != EOF is 0 or 1. This page asks the next question: what number is EOF?

EOF is a macro, not a character

EOF lives in <stdio.h>. It is a negative integer that means “end of file” — not a key you type. The standard does not pin it to one number, but on the machines you will use it is almost always -1. Printing it is how you check.

getchar returns an int so it can hold every character and this extra value. That is why the last exercise compared against EOF instead of a char.

Write the program

Create a file called value-of-eof.c:

#include <stdio.h>

int main(void)
{
    printf("Value of EOF is %d\n", EOF);
    return 0;
}

One printf. The argument is the macro itself, not a call to getchar.

Compile and run

In Terminal, from the folder that holds value-of-eof.c:

gcc value-of-eof.c -o value-of-eof
./value-of-eof

You should see:

Value of EOF is -1

If gcc is missing, finish Getting your macOS ready for C first.

A different negative number is still valid — the library picked another value. What matters is that it is negative, so it cannot collide with a real character (0–255).

Keep the name

Do not replace EOF with -1 in your programs. The macro is the portable name; the number is an implementation detail this exercise lets you peek at.

You still send end-of-file with Ctrl+D on macOS. That is a signal to the terminal. This program does not wait for it — it only prints the constant.

What you should take away

EOF is a negative integer from <stdio.h>, usually -1. A tiny printf is enough to see it. Keep writing EOF in code, not a hard-coded number.

Next

→ Count blanks, tabs, and newlines — Exercise 1-8