Using extern is only of relevance when the program you're building consists of multiple source files linked together, where some of the variables defined, for example, in source file file1.c need to be referenced in other source files, such as file2.c.

It is important to understand the difference between defining a variable and declaring a variable:

• A variable is defined when the compiler allocates the storage for the variable.
• A variable is declared when the compiler is informed that a variable exists (and this is its type); it does not allocate the storage for the variable at that point.

You may declare a variable multiple times (though once is sufficient); you may only define it once within a given scope.

Best way to declare and define global variables

Although there are other ways of doing it, the clean, reliable way to declare and define global variables is to use a header file file3.h to contain an extern declaration of the variable. The header is included by the one source file that defines the variable and by all the source files that reference the variable. For each program, one source file (and only one source file) defines the variable. Similarly, one header file (and only one header file) should declare the variable.

file3.h

extern int global_variable;  /* Declaration of the variable */

file1.c

#include "file3.h"  /* Declaration made available here */

/* Variable defined here */
int global_variable = 37;    /* Definition checked against declaration */

int increment(void) { return global_variable++; }

file2.c

#include "file3.h"
#include <stdio.h>

void use_it(void)
{
    printf("Global variable: %d\n", global_variable++);
}

That's the best way to use them.

Guidelines

Rules to be broken by experts only, and only with good reason:

• A header file only contains extern declarations of variables - never static or unqualified variable definitions.
• For any given variable, only one header file declares it (SPOT - Single Point of Truth).
• A source file never contains extern declarations of variables - source files always include the (sole) header that declares them.
• For any given variable, exactly one source file defines the variable, preferably initializing it too. (Although there is no need to initialize explicitly to zero, it does no harm and can do some good, because there can be only one initialized definition of a particular global variable in a program).
• The source file that defines the variable also includes the header to ensure that the definition and the declaration are consistent.
• A function should never need to declare a variable using extern.
• Avoid global variables whenever possible - use functions instead.

Not so good way to define global variables

With some (indeed, many) C compilers, you can get away with what's called a 'common' definition of a variable too. 'Common', here, refers to a technique used in Fortran for sharing variables between source files, using a (possibly named) COMMON block. What happens here is that each of a number of files provides a tentative definition of the variable. As long as no more than one file provides an initialized definition, then the various files end up sharing a common single definition of the variable:

file10.c

int i;   /* Do not do this in portable code */

void inc(void) { i++; }

file11.c

int i;   /* Do not do this in portable code */

void dec(void) { i--; }

file12.c

int i = 9;   /* Do not do this in portable code */

void put(void) { printf("i = %d\n", i); }

This technique does not conform to the letter of the C standard and the 'one definition rule', but the C standard lists it as a common variation on its one definition rule.Because this technique is not always supported, it is best to avoid using it, especially if your code needs to be portable. Using this technique, you can also end up with unintentional type punning. If one of the files declared i as a double instead of as an int, C's type-unsafe linkers probably would not spot the mismatch. If you're on a machine with 64-bit int and double, you'd not even get a warning; on a machine with 32-bit int and 64-bit double, you'd probably get a warning about the different sizes - the linker would use the largest size, exactly as a Fortran program would take the largest size of any common blocks.

This is mentioned in the C standard in informative Annex J as a common extension:

J.5.11 Multiple external definitions

There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).

Warning

As noted in comments here, and as stated in my answer to a similar question, using multiple definitions for a global variable leads to undefined behaviour, which is the standard's way of saying "anything could happen". One of the things that can happen is that the program behaves as you expect; and J.5.11 says, approximately, "you might be lucky more often than you deserve". But a program that relies on multiple definitions of an extern variable - with or without the explicit 'extern' keyword - is not a strictly conforming program and not guaranteed to work everywhere. Equivalently: it contains a bug which may or may not show itself.

Violating the guidelines

Note 1: if the header defines the variable without the extern keyword:

faulty_header.h

int some_var;    /* Do not do this in a header!!! */

Then each file that includes the header creates a tentative definition of the variable.

Note 2: if the header defines and initializes the variable, then only one source file in a given program can use the header:

broken_header.h

int some_var = 13;    /* Only one source file in a program can use this */

Note 3: if the header defines a static variable (with or without initialization), then each source file ends up with its own private version of the 'global' variable.

seldom_correct.h

static int hidden_global = 3;   /* Each source file gets its own copy  */

When the variable is actually a complex array, this can lead to extreme duplication of code. It can, very occasionally, be a sensible way to achieve some effect, but that is rather unusual.

Use the header technique I showed first. It works reliably and everywhere. Note, in particular, that the header declaring the global_variable is included in every file that uses it - including the one that defines it. This ensures that everything is self-consistent.

Similar concerns arise with declaring and defining functions - analogous rules apply. But the question was about variables specifically, so I've kept the answer to variables only.

End of Original Answer

Late Major Addition

Avoiding Code Duplication

One concern that is sometimes (and legitimately) raised about the'declarations in headers, definitions in source' mechanism describedhere is that there are two files to be kept synchronized - the headerand the source. This is usually followed up with an observation that amacro can be used so that the header serves double duty - normallydeclaring the variables, but when a specific macro is set before theheader is included, it defines the variables instead.

Another concern can be that the variables need to be defined in each ofa number of 'main programs'. This is normally a spurious concern; youcan simply introduce a C source file to define the variables and linkthe object file produced with each of the programs.

A typical scheme works like this, using the original global variableillustrated in file3.h:

file3a.h

#ifdef DEFINE_VARIABLES
#define EXTERN /* nothing */
#else
#define EXTERN extern
#endif /* DEFINE_VARIABLES */

EXTERN int global_variable;

file1a.c

#define DEFINE_VARIABLES
#include "file3a.h"  /* Variable defined - but not initialized */

int increment(void) { return global_variable++; }

file2a.c

#include "file3a.h"
#include <stdio.h>

void use_it(void)
{
    printf("Global variable: %d\n", global_variable++);
}

The problem with this scheme as shown is that it does not provide forinitialization of the global variable. With C99 and variable argumentlists for macros, you could define macros to support initialization too.You probably need two such macros, one for simple initializers with nocommas, and a second for complex initializers containing commas:

file3b.h

#ifdef DEFINE_VARIABLES
#define EXTERN                  extern
#define INITIALIZER(...)        /* nothing */
#else
#define EXTERN                  /* nothing */
#define INITIALIZER(...)        = __VA_ARGS__
#endif /* DEFINE_VARIABLES */

EXTERN int global_variable INITIALIZER(37);
EXTERN struct { int a; int b; } oddball_struct INITIALIZER({ 41, 43 });

file1b.c

#define DEFINE_VARIABLES
#include "file3b.h"  /* Variables now defined and initialized */

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

file2b.c

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

void use_them(void)
{
    printf("Global variable: %d\n", global_variable++);
    oddball_struct.a += global_variable;
    oddball_struct.b -= global_variable / 2;
}

Clearly, the code for the oddball structure is not what you'd normallywrite, but it illustrates the point. The first argument to the second invocation ofINITIALIZER is { 41 and the remaining argument (singular in thisexample) is 43 }. Without C99 or similar support for variableargument lists for macros, initializers that need to contain commas arevery problematic.

Header Guards

Any header should be protected against reinclusion, so that typedefinitions (enum, struct or union types, or typedefs generally) do notcause problems. The standard technique is to wrap the body of theheader in a header guard:

#ifndef FILE3B_H_INCLUDED
#define FILE3B_H_INCLUDED

...contents of header...

#endif /* FILE3B_H_INCLUDED */

The header might be included twice indirectly. For example, iffile4b.h includes file3b.h for a type definition that isn't shown,and file1b.c needs to use both header file4b.h and file3b.h, thenyou have some more tricky issues to resolve. Clearly, you might revisethe header list to include just file4b.h. However, you might not beaware of the internal dependencies - and the code should, ideally,continue to work.

Further, it starts to get tricky because you might include file4b.hbefore including file3b.h to generate the definitions, but the normalheader guards on file3b.h would prevent the header being reincluded.

So, you need to include the body of file3b.h at most once fordeclarations, and at most once for definitions, but you might need bothin a single translation unit (TU - a combination of a source file andthe headers it uses).

Multiple inclusion with variable definitions

However, it can be done subject to a not too unreasonable constraint. Let'sintroduce a new set of file names:

• external.h for the EXTERN macro definitions, etc.
• file1c.h to define types (notably, struct oddball, the type of oddball_struct).
• file2c.h to define or declare the global variables.
• file3c.c which defines the global variables.
• file4c.c which simply uses the global variables.
• file5c.c which shows that you can declare and then define the global variables.
• file6c.c which shows that you can define and then (attempt to) declare the global variables.

In these examples, file5c.c and file6c.c directly include the headerfile2c.h several times, but that is the simplest way to show that themechanism works. It means that if the header was indirectly included twice, it would also be safe.

The restrictions for this to work are:

1. The header defining or declaring the global variables may not itself define any types.
2. Immediately before you include a header that should define variables, you define the macro DEFINE_VARIABLES.
3. The header defining or declaring the variables has stylized contents.

external.h

/*
** This header must not contain header guards (like <assert.h> must not).
** Each time it is invoked, it redefines the macros EXTERN and INITIALIZER
** based on whether macro DEFINE_VARIABLES is currently defined.
*/
#undef EXTERN
#undef INITIALIZER

#ifdef DEFINE_VARIABLES
#define EXTERN                  extern
#define INITIALIZER(...)        /* nothing */
#else
#define EXTERN                  /* nothing */
#define INITIALIZER(...)        = __VA_ARGS__
#endif /* DEFINE_VARIABLES */

file1c.h

#ifndef FILE1C_H_INCLUDED
#define FILE1C_H_INCLUDED

struct oddball
{
    int a;
    int b;
};

extern void use_them(void);
extern int increment(void);
extern int oddball_value(void);

#endif /* FILE1C_H_INCLUDED */

file2c.h

/* Standard prologue */
#if defined(DEFINE_VARIABLES) && !defined(FILE2C_H_DEFINITIONS)
#undef FILE2C_H_INCLUDED
#endif

#ifndef FILE2C_H_INCLUDED
#define FILE2C_H_INCLUDED

#include "external.h"   /* Support macros EXTERN and INITIALIZER */
#include "file1c.h"     /* Type definition for struct oddball */

/* Global variable declarations / definitions */
EXTERN int global_variable INITIALIZER(37);
EXTERN struct oddball oddball_struct INITIALIZER({ 41, 43 });

/* Standard epilogue */
#ifdef DEFINE_VARIABLES
#define FILE2C_H_DEFINITIONS
#undef DEFINE_VARIABLES     /* Safety first */
#endif /* DEFINE_VARIABLES */

#endif /* FILE2C_H_INCLUDED */

file3c.c

#define DEFINE_VARIABLES
#include "file2c.h"  /* Variables now defined and initialized */

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

file4c.c

#include "file2c.h"
#include <stdio.h>

void use_them(void)
{
    printf("Global variable: %d\n", global_variable++);
    oddball_struct.a += global_variable;
    oddball_struct.b -= global_variable / 2;
}

file5c.c

#include "file2c.h"     /* Declare variables */

#define DEFINE_VARIABLES
#include "file2c.h"  /* Variables now defined and initialized */

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

file6c.c

#define DEFINE_VARIABLES
#include "file2c.h"     /* Variables now defined and initialized */

#include "file2c.h"     /* Declare variables */

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

This scheme avoids most problems. You only run into a problem if aheader that defines variables (such as file2c.h) is included byanother header (say file7c.h) that defines variables. There isn't aneasy way around that other than "don't do it".

You can partially work around the problem by revising file2c.h to:

file2c.h - revised

/* Standard prologue */
#if defined(DEFINE_VARIABLES) && !defined(FILE2C_H_DEFINITIONS)
#undef FILE2C_H_INCLUDED
#endif

#ifndef FILE2C_H_INCLUDED
#define FILE2C_H_INCLUDED

#include "external.h"   /* Support macros EXTERN and INITIALIZER */
#include "file1c.h"     /* Type definition for struct oddball */

#if !defined(DEFINE_VARIABLES) || !defined(FILE2C_H_DEFINITIONS)

/* Global variable declarations / definitions */
EXTERN int global_variable           INITIALIZER(37);
EXTERN struct oddball oddball_struct INITIALIZER({ 41, 43 });

#endif /* !DEFINE_VARIABLES || !FILE2C_H_DEFINITIONS */

/* Standard epilogue */
#ifdef DEFINE_VARIABLES
#define FILE2C_H_DEFINITIONS
#undef DEFINE_VARIABLES         /* See discussion below */
#endif /* DEFINE_VARIABLES */

#endif /* FILE2C_H_INCLUDED */

The issue becomes 'should the header include #undef DEFINE_VARIABLES?'If you omit that from the header and wrap any defining invocation with#define and #undef:

#define DEFINE_VARIABLES
#include "file2c.h"
#undef DEFINE_VARIABLES

in the source code (so the headers never alter the value ofDEFINE_VARIABLES, then you should be clean. It is just a nuisance tohave to remember to write the the extra line. An alternative might be:

#define HEADER_DEFINING_VARIABLES "file2c.h"
#include "externdef.h"

where externdef.h contains:

#if defined(HEADER_DEFINING_VARIABLES)
#define DEFINE_VARIABLES
#include HEADER_DEFINING_VARIABLES
#undef DEFINE_VARIABLES
#undef HEADER_DEFINING_VARIABLES
#endif /* HEADER_DEFINING_VARIABLES */

This is getting a tad convoluted, but seems to be secure (using therevised version of file2c.h, and with no #undef DEFINE_VARIABLES inthe file2c.h).

file7c.c

/* Declare variables */
#include "file2c.h"

/* Define variables */
#define HEADER_DEFINING_VARIABLES "file2c.h"
#include "externdef.h"

/* Declare variables - again */
#include "file2c.h"

/* Define variables - again */
#define HEADER_DEFINING_VARIABLES "file2c.h"
#include "externdef.h"

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

file8c.h

/* Standard prologue */
#if defined(DEFINE_VARIABLES) && !defined(FILE8C_H_DEFINITIONS)
#undef FILE8C_H_INCLUDED
#endif

#ifndef FILE8C_H_INCLUDED
#define FILE8C_H_INCLUDED

#include "external.h"   /* Support macros EXTERN and INITIALIZER */
#include "file2c.h"     /* struct oddball */

#if !defined(DEFINE_VARIABLES) || !defined(FILE8C_H_DEFINITIONS)

/* Global variable declarations / definitions */
EXTERN struct oddball another INITIALIZER({ 14, 34 });

#endif /* !DEFINE_VARIABLES || !FILE8C_H_DEFINITIONS */

/* Standard epilogue */
#ifdef DEFINE_VARIABLES
#define FILE8C_H_DEFINITIONS
#endif /* DEFINE_VARIABLES */

#endif /* FILE8C_H_INCLUDED */

file8c.c

/* Define variables */
#define HEADER_DEFINING_VARIABLES "file2c.h"
#include "externdef.h"

/* Define variables */
#define HEADER_DEFINING_VARIABLES "file8c.h"
#include "externdef.h"

int increment(void) { return global_variable++; }
int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

However, the problems are relatively unlikely to occur in practice,especially if you take the standard advice to

Avoid global variables

Does this exposition miss anything?

Confession: The 'avoiding duplicated code' scheme outlined here wasdeveloped because the issue affects some code I work on (but don't own),and is a niggling concern with the scheme outlined in the first part ofthe answer. However, the original scheme leaves you with just twoplaces to modify to keep variable definitions and declarationssynchronized, which is a big step forward over having exernal variabledeclarations scattered throughout the code base (which really matterswhen there are thousands of files in total). However, the code in thefiles with the names fileNc.[ch] (plus external.h and externdef.h)shows that it can be made to work. Clearly, it would not be hard tocreate a header generator script to give you the standardized templatefor a variable defining and declaring header file.

An extern variable is a declaration (thanks to sbi for the correction) of a variable which is defined in another translation unit. So that means that the variable is defined in another file...

Say you have two .c-files test1.c and test2.c. If you define a global variable int test1_var; in test1.c and you'd like to access this variable in test2.c you have to use extern int test1_var; in test2.c.

Complete sample:

$ cat test1.c 
int test1_var = 5;
$ cat test2.c
#include <stdio.h>

extern int test1_var;

int main(void) {
    printf("test1_var = %d\n", test1_var);
    return 0;
}
$ gcc test1.c test2.c -o test
$ ./test
test1_var = 5

Extern is the keyword you use to declare that the variable itself resides in another translation unit.

So you can decide to use a variable in a translation unit and then access it from another one, then in the second one you declare it as extern and the symbol will be resolved by the linker.

If you don't declare it as extern you'll get 2 variables named the same but not related at all, and an error of multiple definitions of the variable.

extern tells the compiler to trust you that the memory for this variable is declared elsewhere, so it doesnt try to allocate/check memory.

Therefore, you can compile a file that has reference to an extern, but you can not link if that memory is not declared somewhere.

Useful for global variables and libraries, but dangerous because the linker does not type check.

I like to think of an extern variable as a promise that you make to the compiler.

When encountering an extern, the compiler can only find out its type, not where it "lives", so it can't resolve the reference.

You are telling it, "Trust me. At link time this reference will be resolvable."

Adding an extern turns a variable definition into a variable declaration. See this thread as to what's the difference between a declaration and a definition.

The correct interpretation of extern is that you tell something to the compiler. You tell the compiler that, despite not being present right now, the variable declared will somehow be found by the linker (typically in another object (file)). The linker will then be the lucky guy to find everything and put it together, whether you had some extern declarations or not.

extern keyword is used with the variable for its identification as a global variable.

It also represents that you can use the variable declared using extern keyword in any file though it is declared/defined in other file.

In C a variable inside a file say example.c is given local scope. The compiler expects that the variable would have its definition inside the same file example.c and when it does not find the same , it would throw an error.A function on the other hand has by default global scope . Thus you do not have to explicitly mention to the compiler "look dude...you might find the definition of this function here". For a function including the file which contains its declaration is enough.(The file which you actually call a header file). For example consider the following 2 files :
example.c

#include<stdio.h>
extern int a;
main(){
       printf("The value of a is <%d>\n",a);
}

example1.c

int a = 5;

Now when you compile the two files together, using the following commands :

step 1)cc -o ex example.c example1.cstep 2)./ex

You get the following output : The value of a is <5>

externAllows one module of your program to access a global variable or function declared in another module of your program.You usually have extern variables declared in header files.If you don't want a program to access your variables or functions, you use static which tells the compiler that this variable or function cannot be used outside of this module.

first of extern keyword is not used for defineing variable rather it is used for declaring variable. I can say extern is storage class not datatype.

extern is used to let other c file / externalComponent know know this variable is already defined somewhere, Example if you building a library no need define global variable mandatarily some where in library itself. library will be compiled directly but while linking the file it checks for the definition.

A global variable means that once you define it in any source file, you can access it in another source file without having to define it all over again. To do so, you define and initialize it BEFORE main() in your original function instead of after main, as is normally done.eg:

char codename=gets();

main()
{...
...
your code...
...
...
}

Now, whenever you want to use the global variable codename in another program, you simply type it in as you would normally have defined it, like your other variables, EXCEPT THAT YOU PUT AN extern BEFORE THE VARIABLE NAME.

eg: if I wanted to use extern in another program, I'd type something like this:

main()
{
extern char codename;

printf("%s",codename);

..
...
..

}