getchar() and EOF

K&R Exercise 1-6: show that getchar() != EOF is only ever 0 or 1. Type a character and it is 1. Send end-of-file and it is 0.

This is the first character-input exercise. Later programs loop on that same test; here you prove the test itself.

What the expression means

getchar() waits for one character from standard input and returns it as an int. EOF is a special value from <stdio.h> that means “no more input.”

getchar() != EOF compares those two. In C, a comparison is an integer: 1 when the character is not end-of-file, 0 when it is. It does not print the letter you typed.

Write the program

Create a file called getchar-and-eof.c:

#include <stdio.h>

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

One printf. The interesting work is the argument: read a character, compare it to EOF, print the 0 or 1.

Compile and run

In Terminal, from the folder that holds getchar-and-eof.c:

gcc getchar-and-eof.c -o getchar-and-eof
./getchar-and-eof

Type a letter (for example t) and press Return. You should see:

Value of expression = 1

Run it again:

./getchar-and-eof

This time do not type a letter. Press Ctrl+D at the start of the line. That is end-of-file on macOS. You should see:

Value of expression = 0

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

How to send EOF

On macOS, Ctrl+D with an empty line is EOF. If a character is already waiting, Ctrl+D flushes that line instead — so press it before you type anything.

The program reads one character and exits. That is enough for this exercise. The next lesson prints the numeric value of EOF itself.

What you should take away

A comparison in C is a number. getchar() != EOF is 1 for a real character and 0 at end-of-file — the same test a copy loop will use, over and over.

Next

→ The value of EOF — Exercise 1-7