C: undefined reference to a variable when using extern

Question

I try to declare a global variable config:

//general.h

struct config_t {
    int num;
};

extern struct config_t config;  //The global variable

Then I define config variable in general.c:

//general.c
#include "general.h"

struct config_t config = {
    num = 5;
};

But, when I try to use the global variable 'config' in my main function, I get the error:

undefined reference to `config':

Main program:

//main.c
#include "general.h"

main() {
    config.num = 10;
}

Why is it?

To avoid linker errors, _always_ use _header guards_ `#ifndef MY_HEADER_H #define MY_HEADER_H /* contents */ #endif`. You _must_ have this in every header file you ever make. — Lundin, Apr 29 '14 at 14:27
And like sergey answered (which is the pertinent point in this case), don't forget to link all your code together. — Deduplicator, Apr 29 '14 at 14:28
@Lundin A header guard won't help you outside a compilation unit. So "undefined reference" and "redefinition of" from `ld` are mostly unaffected by it. — Błażej Michalik, Nov 01 '16 at 19:28

score 10 · Accepted Answer · answered Apr 29 '14 at 14:22

This looks like a linker error. You need to make sure you link your executable properly:

cc -c general.c
cc -c main.c
cc general.o main.o
./a.out

The -c flag instructs your compiler not to link yet. In order to link the object file containing config needs to be available at that moment.

1 Answers1