C compiling - "undefined reference to"?

Question

I am making a reliable data transfer protocol and have the function prototype

void tolayer5(int, char data[]);

With the structs

struct msg {
  char data[20];
};

struct pkt {
   int seqnum;
   int acknum;
   int checksum;
   char payload[20];
};

And when I call the function in this format:

tolayer5(A, packet.payload);

Where A is an int and packet.payload is a struct pkt, I get the error "undefined reference to 'tolayer5(int, char*)'. Can you help me see what I'm missing here?

void tolayer5(int AorB, char data[])
{
  int i;
  if (TRACE>2)
  {
     printf("TOLAYER5: data received:");
     for (i=0; i<20; i++)
        printf("%c",data[i]);
     printf("\n");
  }
}

---

Thank you all for helping with the original issue! :) When trying to fix that one, however, I ran into an infinite loop that I think has something to do with me addressing characters in an array incorrectly (it's been awhile since I've done C like this. Can you help me to find where I'm creating an infinite loop?

I have updated the above code to what I'm now working with. Notice the main changes have been to my function:

void tolayer5(int AorB, char data[])

And this line inside the function: printf("%c",msgReceived.data[i]); since now it's just:

printf("%c",data[i]);

Answer 1

seems you need to link with the obj file that implements tolayer5()

Update: your function declaration doesn't match the implementation:

      void tolayer5(int AorB, struct msg msgReceived)
      void tolayer5(int, char data[])

So compiler would treat them as two different functions (you are using c++). and it cannot find the implementation for the one you called in main().

Answer 2

Make sure your declare the tolayer5 function as a prototype, or define the full function definition, earlier in the file where you use it.