CODEWITHSUNDEEP
c++linkage
St.Antario
St.Antario

Reputation: 27385

Why does variable declared with static, extern and without it contains differenr values

Consider the following code snippets:

#include <stdio.h>

int a=5;

int main ()
{
    extern int a;
    printf("%d\n",a); //5
}

//---------//

#include <stdio.h>

int a=5;

int main ()
{
    static int a;
    printf("%d\n",a); //0
}

//---------//

#include <stdio.h>

int a=5;

int main ()
{
    int a;
    printf("%d\n",a); //12497123435
}

I don't understand why 5 displayed when a declared with extern specifier? When we declare a with static modifier then a has linkage and a can be denoted an entity from another scope.

Upvotes: 1

Views: 56

Answers (3)

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361472

extern int a is only a declaration (i.e an announcement) that a variable with name a is defined somewhere else, which in your case refers to the global variable.

static int a defines a variable with static storage duration. Variables of such duration are zero-initialized (statically).

int a (in the function scope) defines a with automatic storage duration. It is uninitialized. Reading its value invoke undefined behavior (UB) — your third example is dangerous, as it invokes UB.

Upvotes: 0

David Schwartz
David Schwartz

Reputation: 182769

Your first example behaves as expected and is perfectly reasonable. Your second two examples are just bugs. In both of them, you create a new variable called a and read its value before setting its value.

Upvotes: 0

sepp2k
sepp2k

Reputation: 370162

When you write extern int a;, you're declaring a variable named a with extern linkage, but you do not define it. Instead a definition must exist elsewhere - usually in a different translation unit. In this case the definition of a is in the same translation unit and the declaration is completely redundant because a is already visible. That said a redundant declaration is not an error, so it behaves the same way as if the declaration did not exist at all. If the definition were in a different translation unit, you'd need the declaration, so the compiler knows that the variable exists at all. In either case the extern declaration does not introduce a new variable and a simply refers to the global variable.

static int a and int a however are definitions and introduce a new variable named a, so the value of the global variable a is no longer relevant.

The difference between static int a and int a is that they have different storage. int a has automatic storage and using an uninitialized variable with automatic storage invokes undefined behavior, which is why you get a "random" value in your case. Variables with static or extern linkage on the other hand are automatically initialized to 0, so that's why you get 0 for your static variable.

Upvotes: 6

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