Thread: warning: comparison between signed and unsigned - K&R program

Hi!

When I try to compile the program from K&R (using -Werror -Wall -Wextra -pedantic), I get this warning (the full program is below):
warning: comparison between signed and unsigned

which refers to this line:

Code:

for (n = 0; n < NKEYS; n++)

I see that I have declared n as an int, and NKEYS is an unsigned type.
But I can't change n for sure, because binsearch function might
return -1, hence I need signed int.
As to NKEYS, sizeof returns size_t value, which is, as said in K&R,
is an unsigned type.

I wonder why this error occurs in code from K&R and they do not refer
to it. Is there a way to overcome this issue?

Code:

/* Program to count the occurences of each C keyword */
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stddef.h>

struct key
{
    char *word;
    int count;
} keytab[] =
{
    {"auto", 0},
    {"break", 0},
    {"case", 0},
    {"char", 0},
    {"const", 0},
    {"continue", 0},
    {"default", 0},
    /* ... */
    {"unsigned", 0},
    {"void", 0},
    {"volatile", 0},
    {"while", 0}
};

#define MAXWORD 100
#define NKEYS (sizeof keytab / sizeof(struct key))
/* same as #define NKEYS (sizeof keytab / sizeof keytab[0]) */

int getword(char *, int);
int binsearch(char *, struct key *, int);

/*count C keywords */
int main(void)
{
    int n;
    char word [MAXWORD];
    
    while (getword(word, MAXWORD) != EOF)
    {
        if (isalpha(word[0]))
        {
            if ((n = binsearch(word, keytab, NKEYS)) >= 0)
            {
                keytab[n].count++;
            }
        }
    }
    
    for (n = 0; n < NKEYS; n++)
    {
        if (keytab[n].count > 0)
        {
            printf("%4d %s\n",
                   keytab[n].count, keytab[n].word);
        }
    }
    return 0;
}
/* GETCH and UNGETCH functions */

#define BUFSIZE 100

/* buffer for ungetch */
static char buf[BUFSIZE];

/* next free position in buf */
static int bufp = 0;

/* get a (possibly pushed back) character */
int getch (void)
{
    return (bufp > 0) ? buf[--bufp] : getchar();
}

/* push character back on input */
void ungetch (int c)
{
    if (bufp >= BUFSIZE)
    {
        printf("ungetch: too many characters\n");
    }
    else
    {
        buf[bufp++] = c;
    }
}

/* getword: get next word or characer from input */
int getword (char *word, int lim)
{
    int c, getch(void);
    void ungetch(int);
    char *w = word;
    
    while (isspace(c = getch ()))
        ;
    if (c != EOF)
    {
        *w++ = c;
    }
    if (!isalpha(c))
    {
        *w = '\0';
        return c;
    }
    for ( ; --lim > 0; w++)
    {
        if (isalnum(*w = getch()))
        {
            ungetch(*w);
            break;
        }
    }
    *w = '\0';
    return word[0];
}

/* binsearch: find word in tab[0] ... tab[n-1] */
int binsearch(char *word, struct key tab[], int n)
{
    int cond;
    int low, high, mid;
    
    low = 0;
    high = n - 1;
    while (low <= high)
    {
        mid = (low + high) / 2;
        if ((cond = strcmp(word, tab[mid].word)) < 0)
        {
            high = mid - 1;
        }
        else if (cond > 0)
        {
            low = mid + 1;
        }
        else
            return mid;
    }
    return -1;
}

Thank you!

Originally Posted by laserlight

You can still use an unsigned integer type because -1 will be converted to the largest value for the given unsigned integer type. So, you could change the count member to be a size_t, change the n local variable to be a size_t, and change binsearch to return a size_t.


It is not an error; it is a warning indicating that there could be a bug. Or not.

Thank you for your reply. Unfortunately, this doesn't solve the issue. Yes, before posting my question, I have tried to change types to unsigned and size_t.
Both didn't work, because compiler produces an understandable warning:
warning: comparison of unsigned expression >= 0 is always true

I am new to programming, and might miss important details.

Originally Posted by laserlight

That warning is a valid one: you need to change how to detect "not found" when you change the function's return type to size_t.

I see, I have to change return -1 to some value appropriate for size_t. Thank you!