Global variable in header file without extern

Question

Using global variable in header file we can change value/use variable in another file(same value can access/modify in multiple files), then why extern can be used ? What is difference between global and extern ?

Below is the example that i have tried It doesnt make difference between extern and global variable in c.

Program compile and run successfully without any error.

t.h

int i;

t1.c

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

int main()
{
        i=10;
        printf("%s i = %d\n",__func__, i);
        t2();
        printf("%s i = %d\n",__func__, i);
        i=200;
        printf("%s i = %d\n",__func__, i);
        t3();
        printf("%s i = %d\n",__func__, i);

return 0;
}

t2.c

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

void t2()
{
        printf("%s i=%d\n",__func__, i);
        i = 100;
        printf("%s i=%d\n",__func__, i);
}

t3.c

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

void t3()
{
        printf("%s i=%d\n",__func__, i);
        i = 300;
        printf("%s i=%d\n",__func__, i);
}

Output :- gcc t1.c t2.c t3.c

main i = 10
t2 i=10
t2 i=100
main i = 100
main i = 200
t3 i=200
t3 i=300
main i = 300

Have you read [this answer](https://stackoverflow.com/a/1433387/694733)? — user694733, Aug 02 '19 at 09:27
And maybe this:https://stackoverflow.com/questions/53008247/what-is-the-most-elegant-way-to-work-with-global-variables — Mike, Aug 02 '19 at 09:40
@user694733 yes i already read but dont know why to use extern if without extern program can run — Pankaj Suryawanshi, Aug 02 '19 at 09:40
@Mike i read that question but question is different, why to use extern if without extern it is working. — Pankaj Suryawanshi, Aug 02 '19 at 09:46
have a look here:https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files. — Mike, Aug 02 '19 at 09:50
@AnttiHaapala I understood but " if the definitions disagree" what does it mean ? i am not getting error, in which situation definition disagree ? Can you answer it in detail. — Pankaj Suryawanshi, Aug 03 '19 at 13:56
Question that actually has a correct answer on this topic, https://stackoverflow.com/questions/1490693/tentative-definitions-in-c-and-linking or https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files/1433387#1433387 — M.M, Feb 07 '20 at 05:25
@user694733 lol, 10 A4 pages explaination for one single keyword. now i know why c/c++ is hard — TomSawyer, Apr 26 '20 at 18:28

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

See C11 6.9.2p2

A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition.

So that's what you have: a tentative definition.

Try different variations in file t.h

int i;
int i = 42;
static int i;
static int i = 42;
extern int i;

The description of the various options at gcc documentation may be of help

-fno-common

In C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the extern keyword, which do not allocate storage.

Unix C compilers have traditionally allocated storage for uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references.

The -fno-common option specifies that the compiler should instead place uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is defined in more than one compilation unit. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way.

Yes I agree tentative definition but if its working scenario then why to use extern. — Pankaj Suryawanshi, Aug 02 '19 at 09:28
Because you really should **define** the variable **in one .c file**. — pmg, Aug 02 '19 at 09:31
now i am defining it in multiple files right ? But if i will define it in only in t1.c and use it in t2.c then also it will work without extern. — Pankaj Suryawanshi, Aug 02 '19 at 09:34
without `extern`, using `gcc -fno-common`, I get a "multiple definition of `i'" at the linker stage~ — pmg, Aug 02 '19 at 09:41
yes but it will give error only when i declared same variable again in another file. — Pankaj Suryawanshi, Aug 02 '19 at 09:43
using gcc -fno-common also give you error if you declare extern — Pankaj Suryawanshi, Aug 02 '19 at 09:50
I routinely use [these command-line options](https://stackoverflow.com/a/1667114/25324) for problematic code. — pmg, Aug 02 '19 at 09:52

Global variable in header file without extern

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