Comment

Here is the Triangle type, a Triangle variable tri, and a variable triptr that points to a Triangle:

typedef struct
{
  int x, y;
} Point;

typedef struct
{
  int red, green, blue;
} Color;

typedef struct
{
  Point p0, p1, p2;
  Color color;
} Triangle;

int main ()
{
  Triangle tri ;
  
  Triangle *triptr ;
  triptr = (Triangle *)malloc( sizeof(Triangle) );
  
  . . .
  
}

At this point, neither triangle is initialized.

The identifier tri is the name for the memory of a Triangle. That memory can be assigned values by statements like:

  tri.p0.x = 45;
  tri.p0.y = 97; 

The variable triptr holds a pointer to the memory of a Triangle. That memory can be assigned values by statements like:

  triptr->p0.x = 45;
  triptr->p0.y = 97;