Variable declaration and definition

Question

int x;

Is this a declaration or a definition?

As I write the following code,

#include <stdio.h>

int main(void)
{
    int x;
    printf("%p",&x);
    return 0;
}

it prints some address. So as memory is allocated, int x; can't be just a declaration. So is it a definition?

This is very vague. I don't undertand the question very well. — krico
– krico, Commented Oct 25, 2010 at 7:57
Also, this code might give you a segfault depending on what platform you are running it on. — krico
– krico, Commented Oct 25, 2010 at 7:58
Use the conversion character "%p" to print pointers that you cast to (void*), otherwise casting a pointer to an integer type might lead to undefined behavior. (If you really can't avoid, cast pointers to uintptr_t, if it exists) — Jens Gustedt
– Jens Gustedt, Commented Oct 25, 2010 at 16:41

John Bode · Accepted Answer · 2010-10-25 14:15:02Z

3

From the C standard (n1256):

6.7 Declarations
...
5 A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

— for an object, causes storage to be reserved for that object;
— for a function, includes the function body;¹⁰¹⁾
— for an enumeration constant or typedef name, is the (only) declaration of the identifier.

In this case, int x; is a definition (or a defining declaration).

answered Oct 25, 2010 at 14:15

John Bode

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Comments

Armen Tsirunyan · Accepted Answer · 2010-10-25 07:57:28Z

2

int x; is a definition. extern int x; is just a declaration. extern int x = 3; is also a definition. HTH

answered Oct 25, 2010 at 7:57

Armen Tsirunyan

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8 Comments

mtvec Over a year ago

As he's just taking the address of the variable, I don't see why this is UB.

Armen Tsirunyan Over a year ago

@Job, oh, sorry, didn't see that

Parikshita Over a year ago

@Armen Tsirunyan I'm trying to show that the compiler allocates memory for int x;

Armen Tsirunyan Over a year ago

@R.. No, this isn't an unitialized pointer. This is well defined because we have an uninitialized variable and are just taking its address

Jens Gustedt Over a year ago

Use the conversion character "%p" to print pointers that you cast to (void*).

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