将额外参数传递给qsort的比较器

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

typedef struct {
    int a;
    int b;
} ExtraParams;

int compare(const void *a, const void *b, void *extra_params) {
    ExtraParams *params = (ExtraParams *)extra_params;
    int diff = ((int *)a)[params->a] - ((int *)b)[params->b];
    return diff;
}

int main() {
    int arr[] = {4, 5, 3, 2, 1};
    ExtraParams params = {0, 1};
    qsort_r(arr, sizeof(arr) / sizeof(arr[0]), sizeof(arr[0]), compare, &params);
    for (int i = 0; i< sizeof(arr) / sizeof(arr[0]); i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

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

int a = 0;
int b = 1;

int compare(const void *a, const void *b) {
    int diff = ((int *)a)[::a] - ((int *)b)[::b];
    return diff;
}

int main() {
    int arr[] = {4, 5, 3, 2, 1};
    qsort(arr, sizeof(arr) / sizeof(arr[0]), sizeof(arr[0]), compare);
    for (int i = 0; i< sizeof(arr) / sizeof(arr[0]); i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}