Language: C

#include <sys/types.h>
#include <stdlib.h>
#define MAX_INPUT 100000
/*??????????±???????????????°??????*/
typedef struct point_t{
  int x;
  int y;
} ;
typedef point_t* point_ptr_t;

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

int main(void)
{
    ssize_t npoints = 0; 
    int i;
    point_t aPoints[MAX_INPUT];
    point_ptr_t** outhull = NULL;
    ssize_t 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, outhull, &h);

    printf("%d\n", &h);
    for(i = 0; i < h -1; i++){
         printf("%d %d\n", *(outhull+ 2 * i * sizeof(point_t)), *(outhull+ 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, ssize_t npoints, point_ptr_t** out_hull, ssize_t* out_hullsize)
{
  point_ptr_t* hull;
  ssize_t 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:8:1: warning: useless storage class specifier in empty declaration
    8 | } ;
      | ^
main.c:9:9: error: unknown type name 'point_t'
    9 | typedef point_t* point_ptr_t;
      |         ^~~~~~~
main.c:12:16: error: unknown type name 'point_t'
   12 | static int ccw(point_t*, point_t*, point_t*);
      |                ^~~~~~~
main.c:12:26: error: unknown type name 'point_t'
   12 | static int ccw(point_t*, point_t*, point_t*);
      |                          ^~~~~~~
main.c:12:36: error: unknown type name 'point_t'
   12 | static int ccw(point_t*, point_t*, point_t*);
      |                                    ^~~~~~~
main.c:13:18: error: unknown type name 'point_t'
   13 | void convex_hull(point_t*, ssize_t, point_ptr_t**, ssize_t*);
      |                  ^~~~~~~
main.c: In function 'main':
main.c:19:5: error: unknown type name 'point_t'; use 'struct' keyword to refer to the type
   19 |     point_t aPoints[MAX_INPUT];
      |     ^~~~~~~
      |     struct 
main.c:26:19: error: request for member 'x' in something not a structure or union
   26 |         aPoints[i].x = NULL;
      |                   ^
main.c:27:19: error: request for member 'y' in something not a structure or union
   27 |         aPoints[i].y = NULL;
      |                   ^
main.c:31:5: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   31 |     scanf("%d", &npoints);
      |     ^~~~~
main.c:3:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
    2 | #include <stdlib.h>
  +++ |+#include <stdio.h>
    3 | #define MAX_INPUT 100000
main.c:31:5: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   31 |     scanf("%d", &npoints);
      |     ^~~~~
main.c:31:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:33:35: error: request for member 'x' in something not a structure or union
   33 |         scanf("%d %d", &aPoints[i].x, &aPoints[i].y);
      |                                   ^
main.c:33:50: error: request for member 'y' in something not a structure or union
   33 |         scanf("%d %d", &aPoints[i].x, &aPoints[i].y);
      |                                                  ^
main.c:36:37: error: 'point_t' undeclared (first use in this function)
   36 |     qsort(&aPoints, npoints, sizeof(point_t), compare);
      |                                     ^~~~~~~
main.c:36:37: note: each undeclared identifier is reported only once for each function it appears in
main.c:39:5: error: implicit declaration of function 'convex_hull' [-Wimplicit-function-declaration]
   39 |     convex_hull(aPoints, npoints, outhull, &h);
      |     ^~~~~~~~~~~
main.c:41:5: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   41 |     printf("%d\n", &h);
      |     ^~~~~~
main.c:41:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:41:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:41:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c: In function 'compare':
main.c:51:5: error: unknown type name 'point_t'; use 'struct' keyword to refer to the type
   51 |     point_t t1 = *(point_t *)c1;
      |     ^~~~~~~
      |     struct 
main.c:51:20: error: 'point_t' undeclared (first use in this function)
   51 |     point_t t1 = *(point_t *)c1;
      |                    ^~~~~~~
main.c:51:29: error: expected expression before ')' token
   51 |     point_t t1 = *(point_t *)c1;
      |                             ^
main.c:52:12: error: expected ';' before 't2'
   52 |     point_t t2 = *(point_t *)c2;
      |            ^~~
      |            ;
main.c:54:10: error: request for member 'y' in something not a structure or union
   54 |     if(t1.y < t2.y){            /*y??§?¨???§?????????*/
      |          ^
main.c:54:15: error: 't2' undeclared (first use in this function); did you mean 't1'?
   54 |     if(t1.y < t2.y){            /*y??§?¨???§?????????*/
      |               ^~
      |               t1
main.c:56:16: error: request for member 'y' in something not a structure or union
   56 |     }else if(t1.y > t2.y){
      |                ^
main.c: At top level:
main.c:64:16: error: unknown type name 'point_t'
   64 | static int ccw(point_t* p1, point_t* p2, point_t* p3)
      |                ^~~~~~~
main.c:64:29: error: unknown type name 'point_t'
   64 | static int ccw(point_t* p1, point_t* p2, point_t* p3)
      |                             ^~~~~~~
main.c:64:42: error: unknown type name 'point_t'
   64 | static int ccw(point_t* p1, point_t* p2, point_t* p3)
      |                                          ^~~~~~~
main.c:72:18: error: unknown type name 'point_t'
   72 | void convex_hull(point_t* points, ssize_t npoints, point_ptr_t** out_hull, ssize_t* out_hullsize)
      |                  ^~~~~~~