C Undefined reference

Question

I got some problem with the following codes particularly in header.c where i can't access the extern int x variable in header.h... Why? Does extern variable in .h not global? How can i use this on other files?

===header.h===

#ifndef HDR_H
#define HDR_H

extern int x;
void function();

#endif

===header.c===

#include <stdio.h>
#include "header.h"

void function()
{
    printf("%d", x); //****undefined reference to x, why?****
}

===sample.c===

int main()
{
    int x = 1;
    function();
    printf("\n%d", x);
    return 0;
}

possibly just remove the `int` before `x` in your main function. this would prevent a new local variable being created in the main function with the same name as the global variable — bph, Aug 08 '12 at 09:08
See also more info about the extern int in [http://stackoverflow.com/questions/7610321/difference-between-extern-int-a-extern-int-a-42][1] [1]: http://stackoverflow.com/questions/7610321/difference-between-extern-int-a-extern-int-a-42 — Michel Keijzers, Aug 08 '12 at 09:15

score 9 · Answer 1 · answered Aug 08 '12 at 09:09

9

The declaration

extern int x;

tells the compiler that in some source file there will be a global variable named x. However, in the main function you declare a local variable x. Move that declaration outside of main to make it global.

answered Aug 08 '12 at 09:09

Some programmer dude

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score 3 · Answer 2 · answered Aug 08 '12 at 09:08

The extern keyword say the variable exists but does not create it. The compiler expects that another module will have a global variable with that name, and the linker will do the right thing to join them up.

You need to change sample.c like this:

/* x is a global exported from sample.c */
int x = 1;

int main()
{
    function();
    printf("\n%d", x);
    return 0;
}

score 1 · Answer 3 · answered Aug 08 '12 at 09:06

extern declares a variable, but does not define it. It basically tells the compiler there is a definition for x somewhere else. To fix add the following to header.c (or some other .c file but only one .c file):

int x;

Note that in main() the local variable x will hide the global variable x.

log0 · Answer 4 · 2012-08-08T09:12:18.970

1

Indeed extern int x; means x will be defined in another place/translation unit.

The compiler expects to find a definition of x in the global scope somewherelse.

edited Aug 08 '12 at 09:12

answered Aug 08 '12 at 09:06

log0

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bph · Answer 5 · 2012-08-08T21:02:50.990

0

I would reorganise/modify your code like this and get rid of header.c

===sample.h===

#ifndef SAMPLE_H
#define SAMPLE_H

extern int x;
void function();

#endif

===sample.c===

#include <stdio.h>
#include "sample.h"

int x;

void function()
{
    printf("%d", x);
}

int main()
{
    x = 1;
    function();
    printf("\n%d", x);
    return 0;
}

edited Aug 08 '12 at 21:02

answered Aug 08 '12 at 09:32

bph

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C Undefined reference

5 Answers5