In the asteroids game, when you press the s button to put up a shield, it draws a circle.  I must confess, I didn’t know how to draw a cuircle so looked it up and found an example on StackOverflow. You can use code from StackOverflow, licensed under a MIT license.  I include the link to StackOverflow in the game code (in the chapter 48 zip file) in a comment.

Here’s the code in the game.

void DrawCircle(SDL_Renderer *Renderer, int _x, int _y, int radius)
{
	int x = radius - 1;
	int y = 0;
	int tx = 1;
	int ty = 1;
	int err = tx - (radius << 1); // shifting bits left by 1 effectively
								  // doubles the value. == tx - diameter
	while (x >= y)
	{
		//  Each of the following renders an octant (1/8th) of the circle
		SDL_RenderDrawPoint(Renderer, _x + x, _y - y);
		SDL_RenderDrawPoint(Renderer, _x + x, _y + y);
		SDL_RenderDrawPoint(Renderer, _x - x, _y - y);
		SDL_RenderDrawPoint(Renderer, _x - x, _y + y);
		SDL_RenderDrawPoint(Renderer, _x + y, _y - x);
		SDL_RenderDrawPoint(Renderer, _x + y, _y + x);
		SDL_RenderDrawPoint(Renderer, _x - y, _y - x);
		SDL_RenderDrawPoint(Renderer, _x - y, _y + x);

		if (err <= 0)
		{
			y++;
			err += ty;
			ty += 2;
		}
		else 
		{
			x--;
			tx += 2;
			err += tx - (radius << 1);
		}
	}
}

it’s as simple as that! To make it more interesting, it is called each frame with the shield throbbing  by increasing  the radius from 38 to 46 pixels by 2 then restarting at 38 again. Here’s the code for that. Note that when the shield energy is below 10, it no longer works.

void DisplayShield(SDL_Rect * target) {
	if (shieldFlag && shieldStrength >10) {
		SDL_SetRenderDrawColor(renderer, 0xff, 0xff, 0xff, 0xff);
		DrawCircle(renderer, target->x + (SHIPWIDTH/2), target->y + (SHIPHEIGHT/2), shieldRadius);
		shieldRadius += 2;
		if (shieldRadius == 46) {
			shieldRadius = 38;
		}
	}
	if (shieldStrength < 100) {
		TextAt(target->x + 10, target->y + 58, sltoa(shieldStrength), 0.67f);
	}
}

The number under the player ship is the shield energy which drains while the shield is being displayed and recharges back up to 100 when you take your finger off the shield button. The number is only shown when the value is less than 100.