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
– Lundin, Commented Apr 29, 2014 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
– Deduplicator, Commented Apr 29, 2014 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
– Błażej Michalik, Commented Nov 1, 2016 at 19:28

Sergey L. · Accepted Answer · 2014-04-29 14:22:20Z

11

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.

answered Apr 29, 2014 at 14:22

Sergey L.

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