Using unsigned int instead of unsigned short changes behaviour

Question

I am attempting the htoi(char*) function from The C Programming Language by K&R (Excercise 2-3, pg. 43).

The function is meant to convert a hexadecimal string to base 10.

I believe I have it working. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

enum {hexbase = 16};
typedef enum{false, true} bool;

unsigned int htoi(char* s);
bool hasHexPrefix(char* s);

int main(int argc, char** argv) {   

    if(argc <= 1) {
        printf("Error: Not enough arguments.\n");
        return EXIT_FAILURE;
    }else {
        for(int i = 1; i < argc; i++) {
            unsigned int numericVal = htoi(argv[i]);
            printf("%s => %u\n",argv[i],numericVal);
        }
    }
}

unsigned int htoi(char* s) {
    unsigned int output = 0;
    unsigned int len = (unsigned int)(strlen(s));

    unsigned short int firstIndex = hasHexPrefix(s) ? 2 : 0;

    /* start from the end of the str (least significant digit) and move to front */
    for(int i = len-1; i >= firstIndex; i--) {
        int currentChar = s[i];
        unsigned int correspondingNumericVal = 0;
        if(currentChar >= '0' && currentChar <= '9') {
            correspondingNumericVal = currentChar - '0';
        }else if(currentChar >= 'a' && currentChar <= 'f') {
            correspondingNumericVal = (currentChar - 'a') + 10;
        }else if(currentChar >= 'A' && currentChar <= 'F') {
            correspondingNumericVal = (currentChar - 'A') + 10;
        }else {
            printf("Error. Invalid hex digit: %c.\n",currentChar);
        }
        /* 16^(digitNumber) */
        correspondingNumericVal *= pow(hexbase,(len-1)-i);
        output += correspondingNumericVal;
    }

    return output;
}

bool hasHexPrefix(char* s) {
    if(s[0] == '0')
        if(s[1] == 'x' || s[1] == 'X')
            return true;

    return false;
}

My issue is with the following line from the htoi(char*) function:

unsigned short int firstIndex = hasHexPrefix(s) ? 2 : 0;

When I remove short to make firstIndex into an unsigned int rather than an unsigned short int, I get an infinite loop.

So when I start from the back of s in htoi(char* s), i >= firstIndex never evaluates to be false.

Why does this happen? Am I missing something trivial or have I done something terribly wrong to cause this undefined behavior?

Answer 1

When firstIndex is unsigned int, in i >= firstIndex then i is converted to unsigned int because of the usual arithmetic conversions. So if i is negative it becomes a large integer in the comparison expression. When firstIndex is unsigned short int in i >= firstIndex, firstIndex is promoted to int and two signed integers are compared.

You can change:

for(int i = len-1; i >= firstIndex; i--)

to

for(int i = len-1; i >= (int) firstIndex; i--)

to have the same behavior in both cases.