Language: C

#define MAX_INPUT 100000
/*??????????±???????????????°??????*/
typedef struct {
  int x;
  int y;
} point_t;

int compare(const void*, const void*);
static int ccw(point_t*, point_t*, point_t*);
void convex_hull(point_t*, int, point_t***, int*);

int main()
{
    int npoints; 
    int i;
    point_t aPoints[MAX_INPUT];
    point_t*** out_hull;
    int* h;

/*??§?¨??????????????????????*/
    for(i = 0; i < MAX_INPUT; i++){
        aPoints[i].x = NULL;
        aPoints[i].y = NULL;
    }


    scanf("%d", npoints);
    for (i = 0; i < npoints; i++){
        scanf("%d %d", &aPoints[i].x, &aPoints[i].y);
    }
/*y??§?¨???§?????????*/
    qsort(&aPoints, npoints, sizeof(point_t), compare);
/*??????????±???????*/
    convex_hull(aPoints, npoints, out_hull, h);

    printf("%d\n", &h);
    for(i = 0; i < *h -1; i++){
         printf("%d %d\n", *(out_hull+ 2 * i * sizeof(point_t)), *(out_hull+ 2 * i * sizeof(point_t) + 2));
    }

    return 0;
}
/* ????????¢??° */
int compare(const void* c1, const void* c2)
{
    point_t t1 = *(point_t *)c1;
    point_t t2 = *(point_t *)c2;

    if(t1.y < t2.y){            /*y??§?¨???§?????????*/
        return -1;
    }else if(t1.y > t2.y){
        return 1;
    }else return 0;
}
/* Three points are a counter-clockwise turn if ccw > 0, clockwise if
 * ccw < 0, and collinear if ccw = 0 because ccw is a determinant that
 * gives the signed area of the triangle formed by p1, p2 and p3.
 */
static int ccw(point_t* p1, point_t* p2, point_t* p3)
{
    return (p2->x - p1->x)*(p3->y - p1->y) - (p2->y - p1->y)*(p3->x - p1->x);
}

/* Returns a list of points on the convex hull in counter-clockwise order.
 * Note: the last point in the returned list is the same as the first one.
 */
void convex_hull(point_t* points, int npoints, point_t*** out_hull, int* out_hullsize)
{
    point_t** hull;
    int i, t, k = 0;

    hull = *out_hull;

/* lower hull */
    for (i = 0; i < npoints; ++i) {
      while (k >= 2 && ccw(hull[k-2], hull[k-1], &points[i]) <= 0) --k;
      hull[k++] = &points[i];
    }

/* upper hull */
    for (i = npoints-2, t = k+1; i >= 0; --i) {
      while (k >= t && ccw(hull[k-2], hull[k-1], &points[i]) <= 0) --k;
      hull[k++] = &points[i];
    }

    *out_hull = hull;
    *out_hullsize = k;
}

Compiler output:
main.c: In function 'main':
main.c:22:24: error: 'NULL' undeclared (first use in this function)
   22 |         aPoints[i].x = NULL;
      |                        ^~~~
main.c:1:1: note: 'NULL' is defined in header '<stddef.h>'; this is probably fixable by adding '#include <stddef.h>'
  +++ |+#include <stddef.h>
    1 | #define MAX_INPUT 100000
main.c:22:24: note: each undeclared identifier is reported only once for each function it appears in
   22 |         aPoints[i].x = NULL;
      |                        ^~~~
main.c:27:5: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   27 |     scanf("%d", npoints);
      |     ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | #define MAX_INPUT 100000
main.c:27:5: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   27 |     scanf("%d", npoints);
      |     ^~~~~
main.c:27:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:32:5: error: implicit declaration of function 'qsort' [-Wimplicit-function-declaration]
   32 |     qsort(&aPoints, npoints, sizeof(point_t), compare);
      |     ^~~~~
main.c:36:5: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   36 |     printf("%d\n", &h);
      |     ^~~~~~
main.c:36:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:36:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:36:5: note: include '<stdio.h>' or provide a declaration of 'printf'