How do I use a Simplify function in a Rational Structure

#include "stdafx.h"
#include <math.h>
#include <stdlib.h>
 
typedef struct RationalClass
{
	int numPart;
	int denomPart;
}RationalClass;
 
typedef RationalClass* Rational;
Rational initRational( int num, int denom );
Rational cloneRational( Rational orig );
void freeRational( Rational r );
int getNumPart( Rational r );
int getDenomPart( Rational r );
Rational addRational( Rational r1, Rational r2 );
Rational subRational( Rational r1, Rational r2 );
Rational multRational( Rational r1, Rational r2 );
Rational divideRational( Rational r1, Rational r2 );
int gcd( int num, int denom );
Rational simplify( Rational r);
Rational readRational( FILE* f );
void displayRational( Rational r );
 
 
int main()
 
{
	Rational r1, r2, r3, r4, r5, r6, r7;
	r1 = initRational( 2,1 );
	printf( "Enter a rational number\n" );
	r2 = readRational( stdin );
	r3 = addRational( r1, r2 );
	r4 = multRational( r2, r3 );
	r5 = addRational( r1, multRational( r2, r3 ));
	r6 = divideRational( r1, r2 );
	r7 = subRational( r1, r2 );
	
	 
	printf( "(" );
	//displayRational( r1 );			/* r1 + r2 = r3 */
	printf("%d %c %d", r1->numPart, '/', r1->denomPart);
	printf( ") + (" );
	//displayRational( r2 );
	printf("%d %c %d", r2->numPart, '/', r2->denomPart);
	printf( ") = (" );
	//displayRational( r3 );
	printf("%d %c %d", r3->numPart, '/',r3->denomPart);
	printf( ") \n\n" );
	simplify(r3);
	//simplify( r1->numPart, r1->denomPart, r2->numPart, r2->denomPart);
 
	printf( "(" );					/*r2 - r2 = r7 */
	//displayRational( r1 );
	printf("%d %c %d", r1->numPart, '/', r1->denomPart);
	printf( ") - (" );
	//displayRational( r2 );
	printf("%d %c %d", r2->numPart, '/', r2->denomPart);
	printf( ") = (" );
	//displayRational( r7 );
	printf("%d %c %d", r7->numPart, '/', r7->denomPart);
	printf( ") \n\n" );
 
	printf( "(" );					/* r2 * r3 = r4 */
	//displayRational( r2 );
	printf("%d %c %d", r2->numPart, '/', r2->denomPart);
	printf( ") * (" );
	//displayRational( r3 );
	printf("%d %c %d", r3->numPart, '/', r3->denomPart);
	printf( ") = (" );
	//displayRational( r4 );
	printf("%d %c %d", r4->numPart, '/', r4->denomPart);
	printf( ") \n\n" );
 
	printf( "(" );					/* r1 / r2 = r6 */
	//displayRational( r1 );
	printf("%d %c %d", r1->numPart, '/', r1->denomPart);
	printf( ") / (" );		
	//displayRational( r2 );
	printf("%d %c %d", r2->numPart, '/', r2->denomPart);
	printf( ") = (" );
	//displayRational( r6 );
	printf("%d %c %d", r6->numPart, '/', r6->denomPart);
	printf( ") \n\n" );
 
	printf( "(" );
	displayRational( r1 );
	printf( ") + (" );
	displayRational( r2 );
	printf( ") * (" );
	displayRational( r3 );
	printf( ") \n   = (" );
	displayRational( r5 );
	printf( ")\n\n" );
 
	r5 = cloneRational( r1 );
	displayRational( r1 );
	printf( " should be the same as " );
	displayRational( r5 );
	printf( "\n" );
 
	freeRational( r5 );
	printf( "\n\nThis should be a problem." );
	printf( "r5 was freed...\n" );
	printf( "\n" );
	displayRational( r5 );
	printf( "\n");
 
 
 
 
	return 0;
}
Rational initRational( int num, int denom )
{
	Rational r = (Rational) malloc( sizeof( RationalClass ));
	r->numPart = num;
	r->denomPart = denom;
 
	return r;
}
 
Rational cloneRational( Rational orig )
{
	Rational copy = ( Rational ) malloc( sizeof( RationalClass ));
	copy->numPart = orig->numPart;
	copy->denomPart = orig->denomPart;
	return copy;
}
void freeRational( Rational r )
{
	free(r);
}
 
int getNumPart( Rational r )
{
 return r->numPart;
}
 
int getDenomPart( Rational r )
{
	return r->denomPart;
}
 
 
Rational addRational( Rational r1, Rational r2 )
{
	Rational sum = ( Rational ) malloc( sizeof ( RationalClass ));
	sum->numPart = ( r1->numPart  * r2->denomPart ) + ( r1->denomPart * r2->numPart );
	sum->denomPart = ( r1->denomPart * r2->denomPart ); 
	simplify(sum);
	return sum;
	
}
 
Rational subRational( Rational r1, Rational r2 )
{
	Rational sub = ( Rational ) malloc( sizeof ( RationalClass ));
	sub->numPart = ( r1->numPart * r2->denomPart ) - ( r1->denomPart * r2->numPart );
	sub->denomPart = ( r1->denomPart * r2->denomPart );
	return sub;
}
 
Rational multRational( Rational r1, Rational r2 )
{
	Rational mult = ( Rational ) malloc( sizeof ( RationalClass ));
	mult->numPart = ( r1->numPart * r2->numPart );
	mult->denomPart = (r1->denomPart * r2->denomPart );
	return mult;
}
 
Rational divideRational( Rational r1, Rational r2 )
{
	Rational quot = ( Rational ) malloc( sizeof( RationalClass ));
	quot->numPart = ( r1->numPart * r2->denomPart );
	quot->denomPart = (r1->denomPart * r2->numPart );
	return quot;
}
 
int gcd( int num, int denom )
{
	int c;
	do {
	c = num % denom;
	num = denom;
	denom = c;
	} while (denom==0);
	return c;
}
 
Rational simplify( Rational r )
{
	Rational simp = ( Rational ) malloc( sizeof( RationalClass ));
	simp->numPart = r->numPart / gcd(r->numPart, r->denomPart);
	simp->denomPart = r->denomPart / gcd(r->numPart, r->denomPart);
	return simp;
}
 
Rational readRational ( FILE* f )
{
	Rational r = ( Rational ) malloc( sizeof( RationalClass ));
	fscanf( f, "%d %d", &( r->numPart ), &( r->denomPart ));
	return r;
}
 
void displayRational (Rational r )
{
	printf("%d", r->numPart );
	if( r->denomPart < 0 ) printf( "-" );
	else printf( "+" );
	printf( "%d", abs( r->denomPart ));
}