Thread: Compare function for qsort

While looking for information on how to use the qsort function here on the board, I came across this:

Code:

int compare(void const *name1, void const *name2)
{
    char *const *x = (char *const *)name1;
    char *const *y = (char *const *)name2;

    return strcmp(*x, *y);
}

I know the local variables are there just to make the function more readable but what I don't understand is why x and y are declared as char *const *. I know strcmp takes two const *char as its parameters but when I tried to declare x and y as const *char, the program won't sort the lines correctly. Thanks.

swoopy, it won't work if I change the code to what you suggested

Code:

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

int compare(const void *name1, const void *name2)
{
    char *const *x = (char *const *)name1;
    char *const *y = (char *const *)name2;

    return strcmp(*x, *y);
}

int main(void)
{
    char *names[] = {"Meg", "John", "Susan", "Charlotte", "Mario"};
    int nelements = sizeof(names) / sizeof(*names);

    char **p = names;

    qsort(names, nelements, sizeof *names, compare);

    while (nelements-- > 0)
        printf("%s\n", *p++);

    return 0;
}

This works perfectly. Now try changing the declarations of x and y in function compare to anything but that...