Thread: Why I don't have to include code.c in this example?

In code.c

Code:

int accum = 0;                                                                                                                                                               
int sum (int x, int y)                                                                                                                                                       
{                                                                                                                                                                            
    int t = x + y;                                                                                                                                                           
    accum += t;                                                                                                                                                              
    return t;                                                                                                                                                                
}

In test.c

Code:

//#include "code.c"                                                                                                                                                          
                                                                                                                                                                             int main()                                                                                                                                                                   
{                                                                                                                                                                            
  return sum(1,3);                                                                                                                                                           
}

I use

gcc -O2 -c code.c

and get code.o

gcc -O2 -o prog code.o test.c

It's interesting to see I don't have to include code.c in my code, why is that?