Language: C

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#define max(a,b) a<b?b:a
#define min(a,b) a<b?a:b
#define POW2(a) a*a
#define ROOP(i,n) for(i = 0;i < n;i++)
#define _USE_MATH_DEFINES

typedef struct POINT{
double x;
double y;
}POINT;
typedef struct LINE{
POINT s;//start point
POINT e;//end point
}LINE;
typedef struct COMPLEX{
double r;//?????¢
double q;//????§???????theta
}COMPLEX;

//POINT????????????
void print_p(POINT a){
printf("(%lf,%lf)\n",a.x,a.y);
}

//LINE????????????
void print_l(LINE a){
printf("(%lf,%lf) -> (%lf,%lf)\n",(a.s).x,(a.s).y,(a.e).x,(a.e).y);
}

//COMPLEX????????????
void print_c(COMPLEX a){
printf("(%lf,%lf)\n",a.r,a.q);
}

//????????????????????¢?????????
double norm2(POINT a){
return POW2(a.x)+POW2(a.y);
}

//????????????????????¢
double norm(POINT a){
return sqrt(norm2(a));
}

//a->b?????´??§???????????????
POINT make_vec(POINT a,POINT b){
b.x -= a.x;
b.y -= a.y;
return b;
}

//2???a,b???????????¢
double distance(POINT a,POINT b){
return norm(make_vec(a,b));
}

//??´??§????????????
POINT sum_vec(POINT a,POINT b){
a.x += b.x;
a.y += b.y;
return a;
}

//??´??§????????????
//mkae_vec??¨?????£??????????????¨???????????????
//??????????????????
POINT dif_vec(POINT a,POINT b){
a.x -= b.x;
a.y -= b.y;
return a;
}

//??´??§????????????????????????
POINT scalar_vec(POINT a,double k){
a.x *= k;
a.y *= k;
return a;
}

//???????????????
POINT minus_vec(POINT a){
return scalar_vec(a,-1.0);
}

//??????
double dot(POINT a,POINT b){
return a.x*b.x + a.y*b.y;
}

//??????
double cross(POINT a,POINT b){
return a.x*b.y - a.y*b.x;
}

//AP:PB=t:s??¨?????????????±???????
//k, 0:??????, 1:??????
//t:(1-t)??§???????????£??????????????£????????????????????????????????????????????????????????¨?????£???
POINT division(POINT a,POINT b,double t,double s,int k){
double tmp = t + s * (1 - k*2);
a.x += (make_vec(a,b)).x / tmp * t;
a.y += (make_vec(a,b)).y / tmp * t;
return a;
}

//???p????????´???a?????????????????????????¶??????????
POINT perpendicular(LINE a,POINT p){
POINT ps = make_vec(p,a.s);
POINT pe = make_vec(p,a.e);
double t = norm2(ps) - dot(ps,pe);
double s = norm2(pe) - dot(ps,pe);
return division(a.s,a.e,t,s,0);
}

//??´???a???????????????p??¨????§°???????????????
POINT reflection(LINE a,POINT p){
POINT h = perpendicular(a,p);
return sum_vec(h,make_vec(p,h));
}

// ????§???¢?????????
int inclusion(POINT a[],int n,POINT p){
int ans = 0;
int i;
POINT g[n+1];
double w,rad = 0.0;

for(i = 0;i < n;i++){
g[i] = a[i];
}
g[n] = g[0];

for(i = 0;i < n;i++){
if(g[i].x == p.x && g[i].y == p.y){
return ans = 1;
}
}
for(i = 0;i < n;i++){
w = atan2(cross(make_vec(p,g[i]),make_vec(p,g[i+1])),
dot(make_vec(p,g[i]),make_vec(p,g[i+1])));
rad += w;
if(fabs(w - M_PI) <= 0.000001 ||
fabs(w + M_PI) <= 0.000001)
return ans = 1;
}
if(fabs(rad - M_PI*2)<=0.000001){//2??
return ans = 2;//in!
}
return ans;
}

// a.s, a.e, p?????????????????????
// ???????¨??????????????????????????????? 1
// ????¨?????????????????????????????????? -1
// p, a.s, a.e????????????????????? 2
// a.s, a.e, p?????????????????????-2
// ???p?????´???a???????????????????????? 0
int ccw(LINE a,POINT p){
int cls = cross(make_vec(a.s,a.e),make_vec(a.s,p));
if(cls > 0){
return 1;
}else if(cls < 0){
return -1;
}else if(dot(make_vec(a.s,a.e),make_vec(a.s,p)) < 0.0){
return 2;
}else if(dot(make_vec(a.e,a.s),make_vec(a.e,p)) < 0.0){
return -2;
}else{
return 0;
}
}

// ??????????????????
int parallel(LINE a,LINE b){
if(!cross(make_vec(a.s,a.e),make_vec(b.s,b.e)))
return 1;
else
return 0;
}

// ?????´????????????
int orthogonal(LINE a,LINE b){
if(!dot(make_vec(a.s,a.e),make_vec(b.s,b.e)))
return 1;
else
return 0;
}

// ??????a,b???????????????????????????
int intersection(LINE a,LINE b){
if(ccw(a,b.s)*ccw(a,b.e) <= 0 &&
ccw(b,a.s)*ccw(b,a.e) <= 0)
return 1;
return 0;
}

//??????a,b?????????
POINT cross_point(LINE a,LINE b){
double c1 = cross(make_vec(a.s,b.s),make_vec(a.s,b.e));
double c2 = cross(make_vec(a.e,b.s),make_vec(a.e,b.e));
return division(a.s,a.e,fabs(c1),fabs(c2),0);
}

//??´???a??¨???p????????¢
double distanceLP(LINE a,POINT p){
if(dot(make_vec(a.s,a.e),make_vec(a.s,p)) < 0.0)
return norm(make_vec(a.s,p));
if(dot(make_vec(a.e,a.s),make_vec(a.e,p)) < 0.0)
return norm(make_vec(a.e,p));
return norm(make_vec(p,perpendicular(a,p)));
}

//??´???a??¨??´???b????????¢
double distanceLL(LINE a,LINE b){
double a1,a2,a3,a4;
if(intersection(a,b) == 1)
return 0.0000000000;
a1 = distanceLP(a,b.s);
a2 = distanceLP(a,b.e);
a3 = distanceLP(b,a.s);
a4 = distanceLP(b,a.e);
//a1 = min(a1,min(a2,min(a3,a4)));
a1 = min(a1,a2);
a3 = min(a3,a4);
a1 = min(a1,a3);
return a1;
}

//????§???¢?????¢???
double area(POINT a[],int n){
double ans = 0.0;
int i;
POINT g[n+1];
double w,rad = 0.0;

for(i = 0;i < n;i++){
g[i] = a[i];
}
g[n] = g[0];

for(i = 1;i < n-1;i ++){
ans += cross(make_vec(g[0],g[i]),make_vec(g[0],g[i+1]));
}
return fabs(ans)/2;
}

// ????§???¢?????????
POINT *convex_hull(POINT a[],int n){
	int ans = 0;
	int i,j;
	POINT g[n+1];
	double w,rad = 0.0;

	for(i = 0;i < n;i++){
		g[i] = a[i];
	}
	g[n] = g[0];

	for(i = 1;i < n;i++){
		w = cross(make_vec(g[i],g[i-1]),make_vec(g[i],g[i+1]))/norm(make_vec(g[i],g[i-1]))/norm(make_vec(g[i],g[i+1]));
		if(w > 0.00000000){
			for(j = i;j < n;j++){
				g[j] = g[j+1];
			}
			g[n]=NULL;
			n--;
			i--;
		}			
	}
	return g;
}


int main(){
LINE a,b;
int k;
int i;
POINT p[101];
POINT *q;
scanf("%d",&k);
for(i =0 ;i < k;i++){
scanf("%lf%lf",&p[i].x,&p[i].y);
}
q=area(p,k);
for(i =0 ;q[i]!=NULL;i++);
k=i;
printf("%d\n",k);
for(i =0 ;i < k;i++){
printf("%d %d\n",q[i].x,q[i].y);
}
/*
POINT a,b,q;
double t,s;

scanf("%lf%lf%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y,&t,&s);

q = division(a,b,t,s,0);
printf("%lf %lf\n",q.x,q.y);
*/
/*
LINE a;
POINT b;
int k;
char s[5][100]={{"ONLINE_FRONT"},{"CLOCKWISE"},{"ON_SEGMENT"},{"COUNTER_CLOCKWISE"},{"ONLINE_BACK"}};

scanf("%lf%lf%lf%lf%d",&a.s.x,&a.s.y,&a.e.x,&a.e.y,&k);
while(k--){
scanf("%lf%lf",&b.x,&b.y);
printf("%s\n",s[ccw(a,b)+2]);
}
*/
/*
POINT a,b;
int k;
while(1){
scanf("%lf%lf%lf%lf%d",&a.x,&a.y,&b.x,&b.y,&k);
print_p(sum_vec(a,b));
print_p(dif_vec(a,b));
print_p(scalar_vec(a,k));
print_p(minus_vec(a));
}
*/
/*
POINT a,b;
while(1){
scanf("%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y);
printf("%lf\n",distance(a,b));
}
*/
/*
int i,m,n;
POINT g[100];//n
POINT q;//m
int ans = 0;

scanf("%d",&n);
for(i = 0;i < n;i++){
scanf("%lf%lf",&g[i].x,&g[i].y);
}
scanf("%d",&m);
for(i = 0;i < m;i++){
scanf("%lf%lf",&q.x,&q.y);
ans = inclusion(g,n,q);
printf("%d\n",ans);
}
*/
return 0;
}

Compiler output:
In file included from /usr/include/stdlib.h:32,
                 from main.c:3:
main.c: In function 'convex_hull':
main.c:267:30: error: incompatible types when assigning to type 'POINT' from type 'void *'
  267 |                         g[n]=NULL;
      |                              ^~~~
main.c:272:16: warning: function returns address of local variable [-Wreturn-local-addr]
  272 |         return g;
      |                ^
main.c: In function 'main':
main.c:286:3: error: incompatible types when assigning to type 'POINT *' from type 'double'
  286 | q=area(p,k);
      |   ^~~~
main.c:287:15: error: invalid operands to binary != (have 'POINT' and 'void *')
  287 | for(i =0 ;q[i]!=NULL;i++);
      |           ~~~~^~
      |            |
      |            POINT