Why this qsort() doesn't work?

Question

Am sorting an array of strings (case insensitive).

qsort causes segmentation fault, probably my casting isn't proper.

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

int compare(const void *string1, const void *string2) {
    char *a = (char*)(string1);
    char *b = (char*)(string2);
    printf("comparing %s     AND    %s\n", a, b);
    return strcasecmp(a,b);
}

void sortListName(char **fileList, int noOfFiles) {
    printf("Sorting\n");
    qsort(fileList, noOfFiles, 260*sizeof(char), compare); 
    return;     
}

**fileList = array of strings (filenames)

P.S. main() is obvious and works fine.

Answer 1

I would adjust things so that you're just sorting a simple array, in this case of pointers to char - qsort will arrange for you to get pointers to two elements in that array (that is, char ** pointers), and some basic dereferencing is needed to get you to the "pointers to char" comparable via strcasecmp. @Mark likely has sussed out the source of the 260 in your unseen calling code, but I'm not a big fan of those kinds of 2d arrays in C.

The following functions for me, with an example main() to exercise it.

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


int compare(const void *v1, const void *v2){
    char *a = *(char **)v1;
    char *b = *(char **)v2;

    printf("comparing %s to %s\n", a, b);

    return strcasecmp(a,b);
}


void sortListName(char **fileList, int noOfFiles){
    printf("Sorting\n");
    qsort(fileList, noOfFiles, sizeof(*fileList), compare); 
    return;

}

int
main(void)
{
    char *filenames[] = {
        "/var/www/icons/comp.gray.png",
        "/var/www/error/HTTP_SERVICE_UNAVAILABLE.html.var",
        "/var/www/icons/right.gif",
        "/var/www/error/HTTP_NOT_IMPLEMENTED.html.var",
        "/var/www/icons/pie3.png",
        "/var/www/icons/pie2.png",
        "/var/www/htdocs/manual/mod/mod_proxy_balancer.html",
        "/var/www/htdocs/manual/programs/rotatelogs.html",
        "/var/www/htdocs/manual/vhosts/mass.html",
        "/var/www/icons/movie.png",
        "/var/www/htdocs/manual/images/caching_fig1.png",
        "/var/www/htdocs/htdig/search.html",
        "/var/www/icons/generic.gif",
        "/var/www/htdocs/manual/mod/quickreference.html",
        "/var/www/icons/small/blank.png",
        "/var/www/icons/image2.gif"
    };

    int i, nf = (int) (sizeof(filenames) / sizeof(filenames[0]));

    puts("Unsorted:");

    for (i = 0; i < nf; i++) {
        puts(filenames[i]);
    }

    sortListName(filenames, nf);

    puts("Sorted:");

    for (i = 0; i < nf; i++) {
        puts(filenames[i]);
    }

    return 0;
}

Answer 2

If this is all the code you have related to the qsort, it looks like you declared the comparePtr function pointer, but it's still not initialized; it's not pointing to your compare function (which is what I assume you wanted it to point to).

After that, a few more things:

1) comparePtr has the correct types, but compare does not. It needs to take in two const void*, but you have two const void**.
2) Once you fix the types, you could just pass compare to qsort, instead of making a function pointer and passing that.
3) I'm not convinced the first argument to qsort is correct. You want to be passing in the pointer to the first element in the array, which ought to just be fileList (I'm assuming it points to the first string in your array).
4) The third argument isn't correct either. fileList is a char**, meaning you're passing in an array of char *s, and hence the third argument should just be sizeof(char*), not the strlens of the strings.