It is possible to declare and define body functions in header files, there is no restrictions to create your all code inside a header file.

SourceCode.c:
#include "SourceCode.h"

SourceCode.h:
#include <stdio.h>
#include <conio.h>

int Calc(int p_iNum1, int p_iNum2);

int main()
{
     printf("Hi there! 1 + 1 = %d",Calc(1,1));

     getch();

     return 0;
}

int Calc(int p_iNum1, int p_iNum2)
{
     return p_iNum1 + p_iNum2;
}