Please clarify my doubt on matrix multiplication

/*Multiplication of two matrices*/
#include<stdio.h>
float product(float[20][20],float[20][20]);
int i,j,k,r1,c1,r2,c2;
main()
{
	/*c1=r2*/
	printf("Enter no.of rows of first matrix:\t");
	scanf("%d",&r1);
	printf("Enter no.of columns of first matrix:\t");
	scanf("%d",&c1);
	printf("Enter no.of rows of second matrix:\t");
	scanf("%d",&r2);
	printf("Enter no.of columns of second matrix:\t");
	scanf("%d",&c2);
	float a[r1][c1];
	float b[r2][c2];

	product(a,b);
	for(i=0;i<r1;i++)
	{
		for(j=0;j<c2;j++)
		{
			printf("%f",c[i][j]);
		}
	}
	
}

float product(float a[20][20],float b[20][20])
{	float c[r1][c2];
	if(c1==r2)
	{
		/*scanning elements*/
		printf("Enter the elements of first matrix:\t");
		for(i=0;i<r1;i++)
		{	for(j=0;j<c1;j++)
			{
				scanf("%f",&a[i][j]);
			}
		}
		printf("Enter the elements of second matrix:\t");
		for(i=0;i<r2;i++)
		{	for(j=0;j<c2;j++)
			{
				scanf("%f",&b[i][j]);
			}
		}
		/*scanning cmpltd*/
		/*multiplication*/
		for(i=0;i<r1;i++)
		{
			for(j=0;j<c2;j++)
			{
				c[i][j]=0;
				for(k=0;k<c1;k++)
				{
					c[i][j]=c[i][j]+(a[i][k]*b[k][j]);
			
				}
				
			}
			printf("\n");
		}
	}
	else
		printf("Please Enter proper matrices");
}

This program takes two matrices as inputs and gives their product. When I have written this program without function I got the solutions. I want to use this operation in other program, that is why I need to write this as a function. But Iam getting this error.

Thnx in advance.

matr.c: In function ‘main’:
matr.c:24: error: ‘c’ undeclared (first use in this function)
matr.c:24: error: (Each undeclared identifier is reported only once
matr.c:24: error: for each function it appears in.)

Important Info from OP copied from comment below:
My sole intention to write this function is, I use it in b/w my program related to Graph theory. where I may use product(product(x,y),z) to get x*y*z.

Tags: C, Errors, Mathematics