C Undefined reference

Question

I have the following code:

main.c

#include "checksum.h"

void main()
{
    char *Buf ="GPGGA204502.005106.9813N11402.2921W1090.91065.02M-16.27M";
    checksum(Buf);
}

checksum.c

#include <stdio.h>
#include <string.h>

checksum(char *Buff)
{
    int i;
    unsigned char XOR;
    unsigned long iLen = strlen(Buff);
    printf("Calculating checksum...\n");
    for (XOR = 0, i = 0; i < iLen; i++)
        XOR ^= (unsigned char)Buff[i];
    printf("%X \n",XOR);
}

checksum.h

#ifndef CHECKSUM_H_INCLUDED
#define CHECKSUM_H_INCLUDED

void checksum(char *Buff);

#endif

When compiling I get the following error:

/tmp/ccFQS7Ih.o: In function `main':
main.c:(.text+0x18): undefined reference to `checksum'
collect2: ld returned 1 exit status

I can't figure out what the problem is?

Can you post the compiler command? You need to link the checksum.o when building main. — hmjd
– hmjd, Commented Mar 20, 2012 at 10:34
You have gotten a warning also when compiling (and if not, your compiler settings are bad). Your function declaration (that's what in the .h file) and its defintion (that what is in the .c) do not match. By not writing the void before checksum, the function is re-declared as a function returning an int. — Patrick Schlüter
– Patrick Schlüter, Commented Mar 20, 2012 at 12:09

Shahbaz · Accepted Answer · 2012-03-20 16:23:25Z

3

You are compiling only one file not both. More precisely, you are not linking the files together.

I don't know your compiler, but with gcc, it would be something like this:

gcc -c main.c          <-- compile only
gcc -c checksum.c      <-- compile only
gcc main.o checksum.o  <-- link the two

Edit: To automate this process, take a look at the make program which reads Makefiles.

edited Mar 20, 2012 at 16:23

answered Mar 20, 2012 at 10:34

Shahbaz

47.8k19 gold badges120 silver badges187 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

StianL Over a year ago

Hmm, that did the trick! Is there a way to automate this when using gcc? I't is for AVR, but I'm testing some code in linux terminal...

asaelr Over a year ago

You can also do gcc main.c checksum.c

Shahbaz Over a year ago

@StianL, I tried explaining makes and Makefiles twice now, but my computer kept crashing for no apparent reason and I lost all I had written. Your bad luck. Anyway, follow the links I wrote in my edit and you should be able to conjure up a simple Makefile quite quickly.

Dharma · Accepted Answer · 2012-03-20 13:10:35Z

2

You could also try gcc -o program.out main.c checksum.c which will compile and link both files together

answered Mar 20, 2012 at 13:10

Dharma

695 bronze badges

Comments

Anthony · Accepted Answer · 2012-07-18 22:34:14Z

1

I think: in checksum.c, you should include checksum.h.

edited Jul 18, 2012 at 22:34

Anthony

12.5k10 gold badges72 silver badges107 bronze badges

answered Mar 20, 2012 at 10:46

anhldbk

4,5975 gold badges33 silver badges37 bronze badges

1 Comment

user529758 Over a year ago

-1. That's a linker error, it doesn't occur during the compile stage, but during linkage.

Collectives™ on Stack Overflow

C Undefined reference

3 Answers 3

3 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related