transform a triangle and sort them in C

Question

Given a triangle, I have to transform a triangle into another in the following way: rotate a given angle the vertex with respect to the origin, and then applied a homothety with respect to the origin. And save every triangle in n+1 vector, and then sort the vector according to the argument (angle between 0 and 2π) of the vertex a, with direct insertion.

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
typedef struct{
float x;
float y; }point;
typedef struct {
point a;
point b;
point c; }triangle;

      float angle(triangle *t,int i){

float a,b,c,alpha;
 a=(t[i].b.x-t[i].c.x)*(t[i].b.x-t[i].c.x)+(t[i].b.y-t[i].c.y)*(t[i].b.y-t[i].c.y);
 b=(t[i].a.x-t[i].b.x)*(t[i].a.x-t[i].b.x)+(t[i].a.y-t[i].b.y)*(t[i].a.y-t[i].b.y);
   c=(t[i].a.x-t[i].c.x)*(t[i].a.x-t[i].c.x)+(t[i].a.y-t[i].c.y)*(t[i].a.y-t[i].c.y);
  alpha=acos((b*b+c*c-a*a)/(2*b*c));
 return alpha;} 

   void move(int r,float angle,triangle *t,int i){

float alpha;
 // rotation
     alpha=angle*M_PI/180;
        t[i+1].a.x=t[i].a.x*cos(alpha)-t[i].a.y*sin(alpha);
        t[i+1].a.y=t[i].a.x*sin(alpha)+t[i].a.y*cos(alpha);
    t[i+1].b.x=t[i].b.x*cos(alpha)-t[i].b.y*sin(alpha);
    t[i+1].b.y=t[i].b.x*sin(alpha)+t[i].b.y*cos(alpha);
    t[i+1].c.x=t[i].c.x*cos(alpha)-t[i].c.y*sin(alpha);
    t[i+1].c.y=t[i].c.x*sin(alpha)+t[i].c.y*cos(alpha);

// homotecy
    t[i+1].a.x=t[i+1].a.x*r;
    t[i+1].a.y=t[i+1].a.y*r;
    t[i+1].b.x=t[i+1].b.x*r;
    t[i+1].b.y=t[i+1].b.y*r;
    t[i+1].c.x=t[i+1].c.x*r;
     t[i+1].c.y=t[i+1].c.y*r;
       return;}

void sort(triangle *t,int n){

float a,b;
int k,i;
for(i=1;i<=n-1;i++){
    a=angle(t,i);
    k=i-1;
    b=angle(t,k);
    while(a>=0 && a<=(M_PI*2) && b>a){
        t[k+1]=t[k];
        k--;
    }
    t[k+1]=t[i];
}
return;}

 int main(void){

triangle *t;
float angle;
int r,n,i;
char so[50];
FILE *s;

printf("n?\n");
scanf("%d",&n);
printf("r?\n");
scanf("%d",&r);
printf("angle?\n");
scanf("%f",&angle);
t=(triangle*)malloc((n+1)*sizeof(point));
if(t==NULL) exit(1);
printf("Triangle?\n");
scanf("%f %f %f %f %f %f",&t[0].a.x,&t[0].a.y,&t[0].b.x,&t[0].b.y,&t[0].c.x,&t[0].c.y);
for(i=0;i<n;i++){
    move(r,angle,t,i);
    sort(t,n);
}
printf("file?\n");
scanf("%s",so);
s=fopen(so,"w");
if(s==NULL) exit(1);
for(i=0;i<n;i++){
    fprintf(s,"%9.5f %9.5f %19.5f %9.5f %9.5f %9.5f\n",t[i].a.x,t[i].a.y,t[i].b.x,t[i].b.y,t[i].c.x,t[i].c.y);
}
fclose(s);  
free(t);
   return 0;}

When executing it says:

free(): invalid next size (normal)

Answer 1

Change the following line:

t=(triangle*)malloc((n+1)*sizeof(point)); to

t=(triangle*)malloc((n+1)*sizeof(triangle)); or even

t = malloc((n+1) * sizeof *t); to avoid casting and dealing with the type of t.

You allocate points while you wanted to allocate triangles. Then, you write into memory that you do not own and you corrupt it. This could have lead to a segmentation fault. In your case, you were lucky (or unlucky if you consider that you would have caught the bug before) and you got the error when freeing the allocated memory. From my experience, an error when freeing memory is almost always memory corruption and sometimes a double free if you are not careful.