Problem with extern variable

[hzmonte]

<header.h>

#ifndef HEADER_H
#define HEADER_H
extern int x;
#endif

<main.c>

#include "header.h"

int main()
{
  int x = 5;
  printf("x=%d\n", x);
  func(x);
  printf("x=%d\n", x);
  return 0;
}

<func.c>

#include "header.h"

void func(int y)
{
  y=9;
  x = 5;
}

% gcc main.c func.c
Undefined first referenced
symbol in file
x /var/tmp//ccQGVCcI.o
ld: fatal: Symbol referencing errors. No output written to a.out

Why? func.c #includes header.h which declares x! Thanks.

[Prelude]

>Why? func.c #includes header.h which declares x!
Nice try, but it doesn't work that way. extern doesn't magically give other functions special access rights to a local variable of another function. You basically need to define a global x in one of your source files (like main.c), and extern tells another source file (like func.c) that x actually exists elsewhere so that the code will compile.

[hzmonte]

OK, I have amended my programs:

<header.h>

#ifndef HEADER_H
#define HEADER_H
extern int x;
#endif

<main.c>

#include "header.h"
int x = 5;
int main()
{
  printf("x=%d\n", x);
  func();
  printf("x=%d\n", x);
  return 0;
}

<func.c>

#include "header.h"

void func(void)
{
  x = 15;
}

% gcc main.c func.c
% a.out
x=5
x=5

Why hasn't the second printf prints out x=15? Isn't x now a global variable that everyone can change?

[bithub]

First of all, that code won't compile as it's shown since main.c doesn't see any declaration of func(). Second of all, x would print out 15 the second time, so there must be some other problem (that isn't shown) in your code.

[hzmonte]

thx. it is working now. I seemed to forget to remove an obsolete func.o.