Function in C programming xcode "C99" : <p>This is the first time I try to use a function in C. I have checked the internet and do not understand why my compiler (xcode) are complaining</p>

<pre><code>#include &lt;stdio.h&gt;

int upp2(int argc, const char * argv[]) {
   float kronor;
   float valutakurs = 0.1;
   float svar = 0;


   printf("Mata in antal svenska kronor: ");
   scanf("%f",&amp;kronor);

   svar = square(kronor, valutakurs);

   return 0;
}

float square(float x, float y){
   float p ;
   p = x * x ;
   return ( p ) ;
 }
</code></pre>
