Bubble sorting an array with nulls in C

Question

I'm trying to create a bubble sort with nulls in the middle in C.
The code works ok when the array is ordered in a way so the nulls are at the end of the array (hense the "continue" condition works).

My array looks like this: [John,David,NULL,Grace,NULL,NULL] on which I run this function:

void SortContacts(char * people[]) {
    int i,j;
    char *tempswap;
    for (i=0; i<storage-1; i++) {
        for (j=0; j<storage-i-1; j++) {
                if (people[j+1]==NULL) {
                    continue;
                }
            if (strcmp(people[j],people[j+1]) > 0) {
                tempswap = people[j];
                people[j] = people[j+1];
                people[j+1] = tempswap;
            }
        }
    }
}

When executing with the NULL in the middle of the array the exe crashes.

Answer 1

You cannot strcmp a NULL value. Although you are guarding against a strcmp of people[j+1] being NULL, you don't check people[j].

Try the following (untested), which simply provides a strcmp function which treats a NULL like "".

int
strcmpwithnull(const char *a, const char *b)
{
    return strcmp(a?a:"", b?b:"");
}

void SortContacts(char * people[]) {
    int i,j;
    char *tempswap;
    for (i=0; i<storage-1; i++) {
        for (j=0; j<storage-i-1; j++) {
        if (strcmpwithnull(people[j],people[j+1]) > 0) {
                tempswap = people[j];
                people[j] = people[j+1];
                people[j+1] = tempswap;
            }
        }
    }
}

If you want a NULL to be treated as greater than any other string, then try (again untested):

int
strcmpwithnull(const char *a, const char *b)
{
    if (a == b)
        return 0; /* handles 2 NULLs and two strings at the same location */
    else if (!a)
        return 1;
    else if (!b)
        return -1;
    else
        return strcmp(a, b);
}

If you want them to be less than any other string (including the empty string), swap the return 1 and return -1.