The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Exercise 1.11 on page 21

How would you test the word count program? What kinds of input are most likely to uncover bugs if there are any?

Solution by Dann Corbit

It sounds like they are really trying to get the programmers to learn how to do a unit test. I would submit the following:

0. input file contains zero words
1. input file contains 1 enormous word without any newlines
2. input file contains all white space without newlines
3. input file contains 66000 newlines
4. input file contains word/{huge sequence of whitespace of different kinds}/word
5. input file contains 66000 single letter words, 66 to the line
6. input file contains 66000 words without any newlines
7. input file is /usr/dict contents (or equivalent)
8. input file is full collection of moby words
9. input file is binary (e.g. its own executable)
10. input file is /dev/null (or equivalent)

66000 is chosen to check for integral overflow on small integer machines.

Dann suggests a followup exercise 1-11a: write a program to generate inputs (0,1,2,3,4,5,6)

I guess it was inevitable that I'd receive a solution for this followup exercise! Here is Gregory Pietsch's program to generate Dann's suggested inputs:

#include <assert.h>
#include <stdio.h>

int main(void)
{
    FILE *f;
    unsigned long i;
    static char *ws = " \f\t\v";
    static char *al = "abcdefghijklmnopqrstuvwxyz";
    static char *i5 = "a b c d e f g h i j k l m "
                      "n o p q r s t u v w x y z "
                      "a b c d e f g h i j k l m "
                      "n o p q r s t u v w x y z "
                      "a b c d e f g h i j k l m "
                      "n\n";

    /* Generate the following: */
    /* 0. input file contains zero words */
    f = fopen("test0", "w");
    assert(f != NULL);
    fclose(f);

    /* 1. input file contains 1 enormous word without any newlines */
    f = fopen("test1", "w");
    assert(f != NULL);
    for (i = 0; i < ((66000ul / 26) + 1); i++)
        fputs(al, f);
    fclose(f);

    /* 2. input file contains all white space without newlines */
    f = fopen("test2", "w");
    assert(f != NULL);
    for (i = 0; i < ((66000ul / 4) + 1); i++)
        fputs(ws, f);
    fclose(f);
 
    /* 3. input file contains 66000 newlines */
    f = fopen("test3", "w");
    assert(f != NULL);
    for (i = 0; i < 66000; i++)
        fputc('\n', f);
    fclose(f);
 
    /* 4. input file contains word/ 
     *    {huge sequence of whitespace of different kinds}
     *    /word 
     */
    f = fopen("test4", "w");
    assert(f != NULL);
    fputs("word", f);
    for (i = 0; i < ((66000ul / 26) + 1); i++)
        fputs(ws, f);
    fputs("word", f);
    fclose(f);

    /* 5. input file contains 66000 single letter words,
     *    66 to the line 
     */
    f = fopen("test5", "w");
    assert(f != NULL);
    for (i = 0; i < 1000; i++)
        fputs(i5, f);
    fclose(f);

    /* 6. input file contains 66000 words without any newlines */
    f = fopen("test6", "w");
    assert(f != NULL);
    for (i = 0; i < 66000; i++)
        fputs("word ", f);
    fclose(f);

    return 0;
}