why am i getting warnings errors due to assignment of a variable's address to a pointer in the-CodePudding

I am just getting started with pointers and this following program is being flagged by compiler for some reason I am not able comprehend. the code is as follows:

#include <stdio.h>

int dec = 0;
int *d;
d = &dec;

int main() {
    return 0;
}

there is no error when I am stuffing these declarations in to main's body. the version of gcc I am using is gcc version 12.2.0(downloaded using MSYS2) and code editor MS visual code.can anybody post an explanation for this?

as i have stated above i have randomly started typing a program to get familiar with pointers, i expected there to be no variation in the treatment of pointers regardless of where they are being declared and intialised.

CodePudding user response：

You're attempting to perform an assignment outside of a function, which is not allowed. What you can do is initialize:

int *d = &dec;

CodePudding user response：

You may use only declarations in file scopes.

In the provided program you are using an assignment statement

d = &dec;

in the file scope. So the compiler issues an error.

Instead you could write for example

#include <stdio.h>

int dec = 0;
int *d = &dec;

int main( void ) {
    return 0;
}

As the variable dec has static storage duration then the expression &dec is an address constant and my be used as an initializer for the variable d that also has static storage duration.

From the C Standard (6.7.9 Initialization)

4 All the expressions in an initializer for an object that has static or thread storage duration shall be constant expressions or string literals.

and (6.6 Constant expressions)

7 More latitude is permitted for constant expressions in initializers. Such a constant expression shall be, or evaluate to, one of the following:

— an arithmetic constant expression,

— a null pointer constant,

— an address constant, or

— an address constant for a complete object type plus or minus an integer constant expression.

and

9 An address constant is a null pointer, a pointer to an lvalue designating an object of static storage duration, or a pointer to a function designator; it shall be created explicitly using the unary & operator or an integer constant cast to pointer type, or implicitly by the use of an expression of array or function type. The array-subscript [] and member-access . and -> operators, the address & and indirection * unary operators, and pointer casts may be used in the creation of an address constant, but the value of an object shall not be accessed by use of these operators.