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jitenagarwal19
jitenagarwal19

Reputation: 308

understanding the extern keyword in c

#include <stdio.h>

int main()
{
    extern int a;
    extern int a;
    int a = 10;
    return 0;
}

what is the problem with this code? Since multiple declaration is allowed in c what is the problem with this code

Upvotes: 0

Views: 663

Answers (2)

A.s. Bhullar
A.s. Bhullar

Reputation: 2788

From your question I observe that you are visualizing your program something like this

#include <stdio.h>

int main()
{
    extern int a;  //declaration
    extern int a;  //declaration
    int a = 10;    //declaration + definiton
    return 0;
}

With above understanding of extern keyword. Your question is obvious.

Let us understand use extern of keyword thoroughly.

Extern variable declaration is a promise to the compiler that there would be a definition of a global variable some place else. Read This. In other words extern keyword tell the compiler that forget about this variable at the moment and left it to linker to link it with its definition. That is extern variables are actually linked to its definition by linker. Moreover Local variables have no linkage at all. So while searching for its definition compiler found a definition without linkage. Thats the error.

As a rule of thumb just remember when you declare any variable as extern inside any function then you can only define it outside of that function.(However there is no use of it).

Upvotes: 0

Mahonri Moriancumer
Mahonri Moriancumer

Reputation: 6013

The problem with the code is that the compiler is first informed that a is a global variable (due to the extern keyword); and then a is defined as a local 'automatic' variable. Hence there is a conflict in the defined scope of a

As an alternative to automatic variables, it is possible to define variables that are external to all functions, that is, variables that can be accessed by name by any function. (This mechanism is rather like Fortran COMMON or Pascal variables declared in the outermost block.) Because external variables are globally accessible, they can be used instead of argument lists to communicate data between functions. Furthermore, because external variables remain in existence permanently, rather than appearing and disappearing as functions are called and exited, they retain their values even after the functions that set them have returned. —The C Programming Language

An external variable must be defined, exactly once, outside of any function; this sets aside storage for it. The variable must also be declared in each function that wants to access it; this states the type of the variable. The declaration may be an explicit extern statement or may be implicit from context. ... You should note that we are using the words definition and declaration carefully when we refer to external variables in this section. Definition refers to the place where the variable is created or assigned storage; declaration refers to places where the nature of the variable is stated but no storage is allocated. —The C Programming Language

Upvotes: 1

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