C - usage of extern qualifier

Question

I have following three files. In the header file, I declare a global variable and tried to access it other files using extern. But I got the linker errors.

Header1.h

  #ifndef HEADER1_H
  #define HEADER1_H

  #include<stdio.h>

  int g = 10;
  void test();

  #endif

test.c

  #include "header1.h"

  extern int g;

  void test()
  {
       printf("from print function g=%d\n", g);
  }

main.c

  #include "header1.h"

  extern int g;

  int main()
  {
     printf("Hello World g=%d\n", g);
     test();
     getchar();
     return 0;
  }

Linker error:

  LNK2005   "int g" (?g@@3HA) already defined in main.obj   
  LNK1169   one or more multiply defined symbols found

My understanding about extern is that a variable can be defined only once but can be declared multiple times. I think I follow it this way - I defined the global variable g in the header file and tried to access it in the .c files.

Could you please correct my understanding? What actually causes the linker error here? I did not define g multiple times.

"a variable can be defined only once" is true, yet as a global int g; (not used here) is a tentative definition. Research tentative definition for details. — chux
– chux, Commented Apr 19, 2018 at 22:01

dbush · Accepted Answer · 2018-04-19 20:46:21Z

1

You get a multiple definition error because you put the definition in the header file. Because both source files include the header file, that results in g being defined in both places, hence the error.

You want to put the declaration in the header file and the definition in one source file:

In header1.h:

extern int g;

In test.c:

int g = 10;

And nothing in main.c.

answered Apr 19, 2018 at 20:46

dbush

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1 Comment

James K Over a year ago

It is worth reminding that a #include statement, for most intents and purposes, is identically equivalent to copy/pasting that file's content into the file doing the including.

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