CODEWITHSUNDEEP
cvariablesglobal-variableslocal-variablesvariable-declaration
Soumya Pawar
Soumya Pawar

Reputation: 37

Variable declarations and datatypes

In the following statements first three are definitions and the last one is the declaration:

auto int i;

static int j;

register int k;

extern int l;

What's the reason for the same?

Upvotes: 0

Views: 50

Answers (2)

user2371524
user2371524

Reputation:

I assume the question is about the terms declaration and definition in C.

  • A declaration tells the compiler name and type of "something".

  • A definition is a declaration, but additionally "creates" the "something" that is declared. E.g. for a variable, this would introduce some storage space for this variable.

In your first three examples, the variables are actually created. The storage classes auto, static and register all just specify a storage duration. In contrast, the storage class extern tells the compiler that this variable is known, but it might exist in a different translation unit.

Maybe an example comparing the declaration and definition of functions will make the concept easier to understand:

// function declaration:
int foo(int x);
// (now we know a function foo should be "somewhere", but it doesn't exist yet)

// function definition:
int foo(int x) {
    return x+1;
}

Upvotes: 1

pankaj birat
pankaj birat

Reputation: 76

In first three(int i, static int j, register int k) is a definition. It denotes the space for the integer to be in this translation unit and advices the linker to link all references to i against this entity. If you have more or less than exactly one of these definitions, the linker will complain.

But in last extern int l, is a declaration, since it just introduces/specifies l, no new memory address/space is allocated. You can have as many extern int l in each compilation unit as you want. A declaration introduces names into a translation unit or redeclares names introduced by previous declarations.

Upvotes: 1

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