Generic quicksort not working for some reason - c

Question

I'm trying to make a generic quicksort function, and I fail to understand what's wrong with what I'm doing, because it's not working properly.
Here is my code:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <stdbool.h>
#include <assert.h>
typedef bool (*CmpFunction)(void*, void*);
int cmp(const void *c1, const void *c2)
{
assert(c1 && c2);
int a = *(const int*)c1;
int b = *(const int*)c2;
if (a > b) return 1;
if (a < b) return -1;
return 0;
}
void swap(void *a, void *b, size_t size) {
char tmp[size];
memcpy(tmp, a, size);
memcpy(a, b, size);
memcpy(b, tmp, size);
}
void quick_sort(void* a, int n, int size, CmpFunction cmp)
{
int b = 1, t = n - 1;
if (n < 2)
return;
swap((char*)a, (char*)a+(n/2)*size, size);
char p = *(char*)a;
while(b <= t) {
while(t >= b && cmp((char*)a + t*size, &p) >= 0)
t--;
while(b <= t && cmp((char*)a + b*size, &p) < 0)
b++;
if (b < t)
swap((char*)a+size*(b++), (char*)a+size*(t--), size);
}
swap((char*)a, (char*)a+t*size, size);
quick_sort(a, t, size, cmp);
n=n-t-1;
quick_sort((char*)a + (t + 1)*size, n, size, cmp);
}
While the original quicksort function, without me trying to make it generic is:
void quick_sort(int a[], int n)
{
int p, b = 1, t = n - 1;
if (n < 2)
return;
swap(&a[0], &a[n/2]);
p = a[0];
while(b <= t) {
while(t >= b && a[t] >= p )
t--;
while(b <= t && a[b] < p)
b++;
if ( b < t)
swap(&a[b++], &a[t--]);
}
swap(&a[0], &a[t]);
quick_sort(a, t);
n=n-t-1;
quick_sort(a + t + 1, n);
}
void swap(int *c1, int *c2)
{
int c = *c1;
*c1 = *c2;
*c2 = c;
}
I'm using this main():
int main(){
char b[] = {'a','t','b','c','y','s'};
int c[] = {1,4,6,3,5,7};
quick_sort(c, 6, sizeof(c[0]), &cmp);
for (int i=0;i<6;i++)
printf("%d | ", c[i]);
return 0;
}
Now we all agree that the output should be:
1, 3, 4, 5, 6, 7
which is indeed what I get when running the NOT generic function.
When I run my generic(upper) function I get this:
5 | 1 | 4 | 7 | 6 | 3 |
You all have any ideas where I'm wrong? :)

Answer 1

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int integerCompare(const void *c1, const void *c2) // use this to compare int
{
int a = *(const int*) c1;
int b = *(const int*) c2;
if (a > b) {
return 1;
} else if(a < b) {
return -1;
} else {
return 0;
}
}
int characterCompare(const void *c1, const void *c2) // use this to compare char
{
char a = *(const int*) c1;
char b = *(const int*) c2;
if (a > b) {
return 1;
} else if(a < b) {
return -1;
} else {
return 0;
}
}
void swap(void *a, void *b, size_t size) {
// instead of using an array, use a void pointer and malloc
void* tmp = malloc(size);
memcpy(tmp, a, size);
memcpy(a, b, size);
memcpy(b, tmp, size);
free(tmp); // free the allocated space
}
void quick_sort(void* a, int n, int size, int (*cmp)(const void*, const void*))
{
int b = 1;
int t = n - 1;
if (n < 2) {
return;
}
swap(a, a + (n/2) * size, size); // &a[0] == a
// you do not need that p variable .. its value is not being modified
while(b <= t) {
while(t >= b && cmp(a + t*size, a) >= 0)
t--;
while(b <= t && cmp(a + b*size, a) < 0)
b++;
if (b < t)
swap(a + size * (b++), a + size * (t--), size);
}
swap(a, a + t * size, size);
quick_sort(a, t, size, cmp);
n= n - t - 1;
quick_sort(a + (t + 1)*size, n, size, cmp);
}
int main()
{
char b[] = {'a','t','b','c','y','s'};
int c[] = {1,4,6,3,5,7};
quick_sort(c, 6, sizeof(c[0]), &integerCompare);
for (int i=0;i<6;i++) {
printf("%d | ", c[i]);
}
printf("\n");
quick_sort(b, 6, sizeof(b[0]), &characterCompare);
for (int i=0;i<6;i++) {
printf("%c | ", b[i]);
}
return 0;
}