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;
typedef struct CIRCLE{
	double r;//??????
	POINT p;//??????
}CIRCLE;

//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);
}


//* ?????§??????
int is_convex(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){
			return 0;
		}			
	}
	return 1;
}

//* ??????????????????
//4
//3
//2
//1
//0
//??\?????°?????????
int inter_c(CIRCLE a,CIRCLE b){
	double tmp = distance(a.p,b.p);
	if(tmp > a.r+b.r)
		return 4;
	else if(fabs(tmp-(a.r+b.r)) <= 0.00000001)
		return 3;
	else if(fabs(tmp-fabs(a.r-b.r)) <= 0.00000001)
		return 1;
	else if(tmp < fabs(a.r-b.r))
		return 0;
	else return 2;
}

//?????¨??´????????????
POINT *cross_pointCL(CIRCLE a,LINE b){
	double k,p = distanceLP(b,a.p);
	static POINT ans[2];
	POINT q,c= perpendicular(b,a.p);
	if(fabs(p-a.r) <= 0.000001){
		ans[0] = c;
		ans[1] = c;
		return ans;
	}else if(p < a.r){
		k = a.r*sin(acos(p/a.r));
		q = scalar_vec(make_vec(b.s,b.e),1/norm(make_vec(b.s,b.e)));
		q = scalar_vec(q,k);
		ans[0] = sum_vec(c,q);
		q = scalar_vec(q,-k);
		ans[1] = sum_vec(c,q);
	}else{
		return -1;//??¨??????
	}
} 

int main(){
	CIRCLE b;
	LINE a;
	int k;
	POINT ans[2];
	scanf("%lf%lf%lf",&b.p.x,&b.p.y,&b.r);
	scanf("%d",&k);
	while(1){
		scanf("%lf%lf%lf%lf",&a.s.x,&a.s.y,&a.s.x,&a.s.y);
		ans = cross_pointCL(b,a);
		if(ans[0].y<ans[1].y)
			printf("%.10lf %.10lf %.10lf %.10lf\n",ans[0].x,ans[0].y,ans[1].x,ans[1].y);
		else
			printf("%.10lf %.10lf %.10lf %.10lf\n",ans[1].x,ans[1].y,ans[0].x,ans[0].y);
	}
	return 0;
}

Compiler output:
main.c: In function 'cross_pointCL':
main.c:312:24: error: returning 'int' from a function with return type 'POINT *' makes pointer from integer without a cast [-Wint-conversion]
  312 |                 return -1;//??¨??????
      |                        ^
main.c: In function 'main':
main.c:325:21: error: assignment to expression with array type
  325 |                 ans = cross_pointCL(b,a);
      |                     ^