C++/C 'extern' keyword explained

[SemiconductorCat]

When we built a program with multiple compilation units[ multiple source files and object files built a one single executable file] we have met situation that we need to share variables and made calls between multiple compilation units.

for a example consider the following two source files.

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#include <stdio.h>
extern void f1();

int main(int argc,char ** argv)
{
    f1();
    return 0;
}

file1.c

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#include <stdio.h>
void f1()
{
    printf ("f1() from file2\n");
    return ;
}

file2.c

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compile
#gcc -c file1.c file2.c
link
#gcc -o file file1.o file2.o
run
#./file

In "C" programming language the line 2 in file1.c is actually not necessary.because "C" allow us to use implicit declaration. But if you pass the '-Wall' compiler switch to the compiler you'll get a
warning message like.

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file1.c: In function 'main':
file1.c:5: warning: implicit declaration of function 'f1'

However in C++ , line 2 , at lest without 'extern'

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void f1();

is necessary, because C++ doesn't allow implicit declarations , because it's a strict language.
However you still can omit 'extern' prefix keyword.

so for calling functions which are on other compilation units [which are declared in other source files and will be separately compiled and will built a '.o' file], it's not necessary to use 'extern' keyword.


So How 'extern' keyword is useful? why it exists?
As you already seen , to call external function you don't need to use 'extern' prefix. But when you referencing to a variable that exists outside your file scope without redefine it,you need to use 'extern' keyword.Such as like this.

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extern int error;

which means 'error' is definied somewhere else in the code, but if you type

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int error;

it means error in declared and defined in the current file scope.

So let's take a example. Review following two source files.

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#include <stdio.h>
extern int error;

int main(int argc,char **argv)
{
    printf("error code is :%i\n",error);
    return 0;
}

main.c

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int error =3;

error.c

... There are other uses of 'extern' keyword such as 'extern "C"' to call older C library functions within C++. And to call routines that are written in assembly language.



Good Clear Coding Style.
Although you don't need to explicitly use 'extern' as a prefix when you want to call a external function, ignoring it is not a good coding practice. believe me no industry expert is ignoring that, because you're writing code to human to read , lastly for the compiler. So I recommend you to explicitly put that prefix 'extern' where appropriate.

--Happy Coding--
--sandun--

[Saman]

Very nice explanation.

I would like to add two points.

• It is good to use CPP as the extension for C++ files
• It would be better to use C++ in OOP style (that's why C++ is made for) where it removes the need of 'extern' completely.