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

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

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

• A variable is defined when the compiler allocates the storage forthe variable.

You may declare a variable multiple times (though once is sufficient);you may only define it once within a given scope.A variable definition is also a declaration, but not all variabledeclarations are definitions.

Best way to declare and define global variables

The clean, reliable way to declare and define global variables is to usea header file to contain an extern declaration of the variable.

The header is included by the one source file that defines the variableand by all the source files that reference the variable.For each program, one source file (and only one source file) defines thevariable.Similarly, one header file (and only one header file) should declare thevariable.The header file is crucial; it enables cross-checking betweenindependent TUs (translation units — think source files) and ensuresconsistency.

Although there are other ways of doing it, this method is simple andreliable.It is demonstrated by file3.h, file1.c and file2.c:

file3.h

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

file1.c

#include "file3.h"  /* Declaration made available here */#include "prog1.h"  /* Function declarations *//* Variable defined here */int global_variable = 37;    /* Definition checked against declaration */int increment(void) { return global_variable++; }

file2.c

#include "file3.h"#include "prog1.h"#include <stdio.h>void use_it(void){    printf("Global variable: %d\n", global_variable++);}

That's the best way to declare and define global variables.

The next two files complete the source for prog1:

The complete programs shown use functions, so function declarations havecrept in.Both C99 and C11 require functions to be declared or defined before theyare used (whereas C90 did not, for good reasons).I use the keyword extern in front of function declarations in headersfor consistency — to match the extern in front of variabledeclarations in headers.Many people prefer not to use extern in front of functiondeclarations; the compiler doesn't care — and ultimately, neither do Ias long as you're consistent, at least within a source file.

prog1.h

extern void use_it(void);extern int increment(void);

prog1.c

#include "file3.h"#include "prog1.h"#include <stdio.h>int main(void){    use_it();    global_variable += 19;    use_it();    printf("Increment: %d\n", increment());    return 0;}

• prog1 uses prog1.c, file1.c, file2.c, file3.h and prog1.h.

The file prog1.mk is a makefile for prog1 only.It will work with most versions of make produced since about the turnof the millennium.It is not tied specifically to GNU Make.

prog1.mk

# Minimal makefile for prog1PROGRAM = prog1FILES.c = prog1.c file1.c file2.cFILES.h = prog1.h file3.hFILES.o = ${FILES.c:.c=.o}CC      = gccSFLAGS  = -std=c11GFLAGS  = -gOFLAGS  = -O3WFLAG1  = -WallWFLAG2  = -WextraWFLAG3  = -WerrorWFLAG4  = -Wstrict-prototypesWFLAG5  = -Wmissing-prototypesWFLAGS  = ${WFLAG1} ${WFLAG2} ${WFLAG3} ${WFLAG4} ${WFLAG5}UFLAGS  = # Set on command line onlyCFLAGS  = ${SFLAGS} ${GFLAGS} ${OFLAGS} ${WFLAGS} ${UFLAGS}LDFLAGS =LDLIBS  =all:    ${PROGRAM}${PROGRAM}: ${FILES.o}    ${CC} -o $@ ${CFLAGS} ${FILES.o} ${LDFLAGS} ${LDLIBS}prog1.o: ${FILES.h}file1.o: ${FILES.h}file2.o: ${FILES.h}# If it exists, prog1.dSYM is a directory on macOSDEBRIS = a.out core *~ *.dSYMRM_FR  = rm -frclean:    ${RM_FR} ${FILES.o} ${PROGRAM} ${DEBRIS}

Guidelines

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

• A header file only contains extern declarations of variables — neverstatic 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 toinitialize explicitly to zero, it does no harm and can do some good,because there can be only one initialized definition of a particularglobal variable in a program).

• The source file that defines the variable also includes the header toensure 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.

^{The source code and text of this answer are available in mySOQ (Stack Overflow Questions)repository on GitHub in thesrc/so-0143-3204sub-directory.}

^{If you're not an experienced C programmer, you could (and perhapsshould) stop reading here.}

Not so good way to define global variables

With some (indeed, many) C compilers, you can get away with what'scalled a 'common' definition of a variable too.'Common', here, refers to a technique used in Fortran for sharingvariables between source files, using a (possibly named) COMMON block.What happens here is that each of a number of files provides a tentativedefinition 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 thevariable:

file10.c

#include "prog2.h"long l;   /* Do not do this in portable code */void inc(void) { l++; }

file11.c

#include "prog2.h"long l;   /* Do not do this in portable code */void dec(void) { l--; }

file12.c

#include "prog2.h"#include <stdio.h>long l = 9;   /* Do not do this in portable code */void put(void) { printf("l = %ld\n", l); }

This technique does not conform to the letter of the C standard and the'one definition rule' — it is officially undefined behaviour:

J.2 Undefined behavior

An identifier with external linkage is used, but in the program theredoes not exist exactly one external definition for the identifier, orthe identifier is not used and there exist multiple externaldefinitions for the identifier (6.9).

§6.9 External definitions ¶5

An external definition is an external declaration that is also adefinition of a function (other than an inline definition) or anobject.If an identifier declared with external linkage is used in anexpression (other than as part of the operand of a sizeof or_Alignof operator whose result is an integer constant), somewhere inthe entire program there shall be exactly one external definition forthe identifier; otherwise, there shall be no more thanone.¹⁶¹⁾

¹⁶¹⁾ Thus, if an identifier declared with external linkageis not used in an expression, there need be no external definition forit.

However, the C standard also lists it in informative Annex J as one ofthe Common extensions.

J.5.11 Multiple external definitions

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

Because this technique is not always supported, it is best to avoidusing it, especially if your code needs to be portable.Using this technique, you can also end up with unintentional typepunning.

If one of the files above declared l as a double instead of as along, C's type-unsafe linkers probably would not spot the mismatch.If you're on a machine with 64-bit long and double, you'd not evenget a warning; on a machine with 32-bit long and 64-bit double,you'd probably get a warning about the different sizes — the linkerwould use the largest size, exactly as a Fortran program would take thelargest size of any common blocks.

Note that GCC 10.1.0, which was released on 2020-05-07, changes thedefault compilation options to use-fno-common, which meansthat by default, the code above no longer links unless you override thedefault with -fcommon (or use attributes, etc — see the link).

The next two files complete the source for prog2:

prog2.h

extern void dec(void);extern void put(void);extern void inc(void);

prog2.c

#include "prog2.h"#include <stdio.h>int main(void){    inc();    put();    dec();    put();    dec();    put();}

• prog2 uses prog2.c, file10.c, file11.c, file12.c, prog2.h.

Warning

As noted in comments here, and as stated in my answer to a similarquestion, using multipledefinitions for a global variable leads to undefined behaviour (J.2;§6.9), which is the standard's way of saying "anything could happen".One of the things that can happen is that the program behaves as youexpect; and J.5.11 says, approximately, "you might be lucky more oftenthan you deserve".But a program that relies on multiple definitions of an extern variable— with or without the explicit 'extern' keyword — is not a strictlyconforming program and not guaranteed to work everywhere.Equivalently: it contains a bug which may or may not show itself.

Violating the guidelines

There are, of course, many ways in which these guidelines can be broken.Occasionally, there may be a good reason to break the guidelines, butsuch occasions are extremely unusual.

faulty_header.h

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

Note 1: if the header defines the variable without the extern keyword,then each file that includes the header creates a tentative definitionof the variable.As noted previously, this will often work, but the C standard does notguarantee that it will work.

broken_header.h

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

Note 2: if the header defines and initializes the variable, then onlyone source file in a given program can use the header.Since headers are primarily for sharing information, it is a bit sillyto create one that can only be used once.

seldom_correct.h

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

Note 3: if the header defines a static variable (with or withoutinitialization), then each source file ends up with its own privateversion of the 'global' variable.

If the variable is actually a complex array, for example, this can leadto extreme duplication of code. It can, very occasionally, be asensible way to achieve some effect, but that is very unusual.

Summary

Use the header technique I showed first.It works reliably and everywhere.Note, in particular, that the header declaring the global_variable isincluded 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 theanswer to variables only.

End of Original Answer

^{If you're not an experienced C programmer, you probably should stop reading here.}

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 */#include "prog3.h"int increment(void) { return global_variable++; }

file2a.c

#include "file3a.h"#include "prog3.h"#include <stdio.h>void use_it(void){    printf("Global variable: %d\n", global_variable++);}

The next two files complete the source for prog3:

prog3.h

extern void use_it(void);extern int increment(void);

prog3.c

#include "file3a.h"#include "prog3.h"#include <stdio.h>int main(void){    use_it();    global_variable += 19;    use_it();    printf("Increment: %d\n", increment());    return 0;}

• prog3 uses prog3.c, file1a.c, file2a.c, file3a.h, prog3.h.

Variable initialization

The problem with this scheme as shown is that it does not provide forinitialization of the global variable. With C99 or C11 and variable argumentlists for macros, you could define a macro to support initialization too.(With C89 and no support for variable argument lists in macros, there is noeasy way to handle arbitrarily long initializers.)

file3b.h

#ifdef DEFINE_VARIABLES#define EXTERN                  /* nothing */#define INITIALIZER(...)        = __VA_ARGS__#else#define EXTERN                  extern#define INITIALIZER(...)        /* nothing */#endif /* DEFINE_VARIABLES */EXTERN int global_variable INITIALIZER(37);EXTERN struct { int a; int b; } oddball_struct INITIALIZER({ 41, 43 });

^{Reverse contents of #if and #else blocks, fixing bug identified byDenis Kniazhev}

file1b.c

#define DEFINE_VARIABLES#include "file3b.h"  /* Variables now defined and initialized */#include "prog4.h"int increment(void) { return global_variable++; }int oddball_value(void) { return oddball_struct.a + oddball_struct.b; }

file2b.c

#include "file3b.h"#include "prog4.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 secondinvocation of INITIALIZER is { 41 and the remaining argument(singular in this example) is 43 }. Without C99 or similar supportfor variable argument lists for macros, initializers that need tocontain commas are very problematic.

^{Correct header file3b.h included (instead of fileba.h) perDenis Kniazhev}

The next two files complete the source for prog4:

prog4.h

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

prog4.c

#include "file3b.h"#include "prog4.h"#include <stdio.h>int main(void){    use_them();    global_variable += 19;    use_them();    printf("Increment: %d\n", increment());    printf("Oddball:   %d\n", oddball_value());    return 0;}

• prog4 uses prog4.c, file1b.c, file2b.c, prog4.h, file3b.h.

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 such as:

#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's introduce 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 includedtwice, it would also be safe.

The restrictions for this to work are:

1. The header defining or declaring the global variables may not itselfdefine 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, INITIALIZE** based on whether macro DEFINE_VARIABLES is currently defined.*/#undef EXTERN#undef INITIALIZE#ifdef DEFINE_VARIABLES#define EXTERN              /* nothing */#define INITIALIZE(...)     = __VA_ARGS__#else#define EXTERN              extern#define INITIALIZE(...)     /* nothing */#endif /* DEFINE_VARIABLES */

file1c.h

#ifndef FILE1C_H_INCLUDED#define FILE1C_H_INCLUDEDstruct 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, INITIALIZE */#include "file1c.h"     /* Type definition for struct oddball */#if !defined(DEFINE_VARIABLES) || !defined(FILE2C_H_DEFINITIONS)/* Global variable declarations / definitions */EXTERN int global_variable INITIALIZE(37);EXTERN struct oddball oddball_struct INITIALIZE({ 41, 43 });#endif /* !DEFINE_VARIABLES || !FILE2C_H_DEFINITIONS *//* Standard epilogue */#ifdef DEFINE_VARIABLES#define FILE2C_H_DEFINITIONS#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; }

The next source file completes the source (provides a main program) for prog5, prog6 and prog7:

prog5.c

#include "file2c.h"#include <stdio.h>int main(void){    use_them();    global_variable += 19;    use_them();    printf("Increment: %d\n", increment());    printf("Oddball:   %d\n", oddball_value());    return 0;}

• prog5 uses prog5.c, file3c.c, file4c.c, file1c.h, file2c.h, external.h.

• prog6 uses prog5.c, file5c.c, file4c.c, file1c.h, file2c.h, external.h.

• prog7 uses prog5.c, file6c.c, file4c.c, file1c.h, file2c.h, external.h.

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 intofile2d.h:

file2d.h

/* Standard prologue */#if defined(DEFINE_VARIABLES) && !defined(FILE2D_H_DEFINITIONS)#undef FILE2D_H_INCLUDED#endif#ifndef FILE2D_H_INCLUDED#define FILE2D_H_INCLUDED#include "external.h"   /* Support macros EXTERN, INITIALIZE */#include "file1c.h"     /* Type definition for struct oddball */#if !defined(DEFINE_VARIABLES) || !defined(FILE2D_H_DEFINITIONS)/* Global variable declarations / definitions */EXTERN int global_variable INITIALIZE(37);EXTERN struct oddball oddball_struct INITIALIZE({ 41, 43 });#endif /* !DEFINE_VARIABLES || !FILE2D_H_DEFINITIONS *//* Standard epilogue */#ifdef DEFINE_VARIABLES#define FILE2D_H_DEFINITIONS#undef DEFINE_VARIABLES#endif /* DEFINE_VARIABLES */#endif /* FILE2D_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"

externdef.h

/*** This header must not contain header guards (like <assert.h> must not).** Each time it is included, the macro HEADER_DEFINING_VARIABLES should** be defined with the name (in quotes - or possibly angle brackets) of** the header to be included that defines variables when the macro** DEFINE_VARIABLES is defined.  See also: external.h (which uses** DEFINE_VARIABLES and defines macros EXTERN and INITIALIZE** appropriately).**** #define HEADER_DEFINING_VARIABLES "file2c.h"** #include "externdef.h"*/#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 thefile2d.h, with no #undef DEFINE_VARIABLES in the file2d.h).

file7c.c

/* Declare variables */#include "file2d.h"/* Define variables */#define HEADER_DEFINING_VARIABLES "file2d.h"#include "externdef.h"/* Declare variables - again */#include "file2d.h"/* Define variables - again */#define HEADER_DEFINING_VARIABLES "file2d.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, INITIALIZE */#include "file2d.h"     /* struct oddball */#if !defined(DEFINE_VARIABLES) || !defined(FILE8C_H_DEFINITIONS)/* Global variable declarations / definitions */EXTERN struct oddball another INITIALIZE({ 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 "file2d.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; }

The next two files complete the source for prog8 and prog9:

prog8.c

#include "file2d.h"#include <stdio.h>int main(void){    use_them();    global_variable += 19;    use_them();    printf("Increment: %d\n", increment());    printf("Oddball:   %d\n", oddball_value());    return 0;}

file9c.c

#include "file2d.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;}

• prog8 uses prog8.c, file7c.c, file9c.c.

• prog9 uses prog8.c, file8c.c, file9c.c.

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?

NB These are toy programs with just barely enough code to make themmarginally interesting. There is repetition within the examples thatcould be removed, but isn't to simplify the pedagogical explanation.(For example: the difference between prog5.c and prog8.c is the nameof one of the headers that are included. It would be possible toreorganize the code so that the main() function was not repeated, butit would conceal more than it revealed.)

An extern variable is a declaration (thanks to sbi for the correction) of a variable which is defined in another translation unit. That means the storage for the variable is allocated 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$ ./testtest1_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.