Language: C

#include <sys/types.h>
#include <stdlib.h>
#define MAX_INPUT 100000

typedef struct point_t{
  int x;
  int y;
}point_t ;

typedef point_t* point_ptr_t;

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

int main(void)
{
    size_t npoints = 0; 
    int i;
    struct point_t aPoints[MAX_INPUT];
    point_ptr_t *ppHull;
    size_t hullsize = 0;

    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);
    }

    qsort(aPoints, npoints, sizeof(point_t), compare);

        if(aPoints[0].y == aPoints[1].y && aPoints[0].x > aPoints[1].x)
        {
            point_t temp;
            temp = aPoints[0];
            aPoints[0] = aPoints[1];
            aPoints[1] = temp;
        }
        if(aPoints[1].y == aPoints[2].y && aPoints[1].x > aPoints[2].x)
        {
            point_t temp;
            temp = aPoints[1];
            aPoints[1] = aPoints[2];
            aPoints[2] = temp;
        }
        if(aPoints[0].y == aPoints[2].y && aPoints[0].x > aPoints[2].x)
        {
            point_t temp;
            temp = aPoints[0];
            aPoints[0] = aPoints[2];
            aPoints[2] = temp;
        }
    for(i = 2; i < npoints - 1; i++)
    {
        if(aPoints[i].y == aPoints[i+1].y && aPoints[i].x > aPoints[i+1].x)
        {
            point_t temp;
            temp = aPoints[i];
            aPoints[i] = aPoints[i+1];
            aPoints[i+1] = temp;
            i -= 2;
        }
    }    

    ppHull = (point_ptr_t*)calloc(npoints, sizeof(int));
    if (ppHull == NULL)
    {
        printf("memory error");
    }

    for(i = 0; i < npoints; i++)
    {
        ppHull[i] = NULL;
    }
    convex_hull(aPoints, npoints, &ppHull, &hullsize);

    printf("%d\n", hullsize);
    for(i = 0; i < hullsize; i++){
         printf("%d %d\n", ppHull[i]->x, ppHull[i]->y);
    }

    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){
        return -1;
    }else if(t1.y > t2.y){
        return 1;
    }else return 0;
}

inline static double 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);
}

nvex_hull(point_t* points, size_t npoints, point_ptr_t** out_hull, size_t* out_hullsize)
{
    point_ptr_t* hull;
    int i, t;
    int k = 0;

    hull = *out_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];
    }

    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 - 1;

}

Compiler output:
main.c: In function 'main':
main.c:25:22: error: assignment to 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]
   25 |         aPoints[i].x = NULL;
      |                      ^
main.c:26:22: error: assignment to 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]
   26 |         aPoints[i].y = NULL;
      |                      ^
main.c:29:5: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
   29 |     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:29:5: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
   29 |     scanf("%d", &npoints);
      |     ^~~~~
main.c:29:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:72:9: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   72 |         printf("memory error");
      |         ^~~~~~
main.c:72:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:72:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:72:9: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:81:5: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   81 |     printf("%d\n", hullsize);
      |     ^~~~~~
main.c:81:5: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c: At top level:
main.c:106:1: error: return type defaults to 'int' [-Wimplicit-int]
  106 | nvex_hull(point_t* points, size_t npoints, point_ptr_t** out_hull, size_t* out_hullsize)
      | ^~~~~~~~~