Draw a filled circle

[Cybermonkey]

Has anybody a suggestion for an algorithm which draws a filled circle? It's not OpenGL but SDL2 based. So used can be drawing a point at x,y or drawing a line or drawing a filled rectangle.

Ok, I found a solution but it's absolutely slow:

procedure fillcircle (xc,yc,radius:integer);var x,y:integer;
begin
for  y :=-radius to radius do begin
    for x :=-radius to radius do begin
        if (x*x+y*y <= radius*radius) then begin
            dot (xc+x, yc+y);
         end;
     end;
 end;
end;

If anyone knows a faster one I would appreciate some help.