title: return type while using GMP.h header file
body: <p>While I'm using gmp.h header file. I need a function which takes inputs of type mpz_t and return mpz_t type too.
  I'm very beginner of using gmp.h
  So, Here is snaps follows of my approached code...</p>
  
  <pre><code>mpz_t sum_upto(mpz_t max)  
  {    
      mpz_t sum;  
      mpz_init(sum);    
      mpz_init(result);  
      for(int i=0;i&lt;=max-1;i++)    
          mpz_add_ui(sum,sum,pow(2,i));   
      return sum;   
  }
  </code></pre>
  
  <p>but it will show error:</p>
  
  <ol>
  <li>pow has been not used in this scope.", although I have added math.h at the very beginning of the file.  </li>
  <li>sum_upto declared as function returning an array...</li>
  </ol>