Declarations

Properties of variables

• Every variable in a C program has three properties:

Storage duration

• Automatic storage duration: is allocated when the surrounding block is executed; storage is deallocated when the block terminates, causing the variable to lose its value (no default init value).
• Static storage duration: variable can stay at the same storage location as long as the program is running, allowing it to retain its value indefinitely.

Scope

• Block scope: the variable is visible from its point of declaration to the end of the enclosing block
• File scope: the variable is visible from its point of declaration to the end of the enclosing file.

Linkage

• External linkage: the variable may be shared by several files in a program.
• Internal linkage: the variable is restricted to a single file, but may be shared by the functions in that file.
• No linkage: the variable belongs to a single function and can’t be shared at all.

Storage classes

auto storage class

• The auto storage class is legal only for variables that belong to a block. An auto variable has automatic storage duration (not surprisingly), block scope, and no linkage
• Default for variables declared inside a block.

static storage class

static int i;
// static - file scope - internal package

void f(void)
{
    static int j;
    // static - block scope - no linkage
}

extern storage class

• The extern storage class enables several source files to share the same variable.

extern int i;
// static - file scope - ? linkage
void f(void)
{
    extern int j;
    // static - block scope - ? linkage
}
/*
If the variable was declared static earlier in the file (outside of any function definition), then it has
internal linkage. Otherwise (the normal case), the variable has external linkage
*/

register storage class

Function storage class

Example

int a;
extern int b;
static int c;

void f(int d, register int e)
{
    auto int g;
    int h;
    static int i;
    extern int j;
    register int k;
}

Name	Storage duration	Scope	Linkage
a	static	file	external
b	static	file	x
c	static	file	internal
d	automatic	block	none
e	automatic	block	none
g	automatic	block	none
h	automatic	block	none
i	static	block	none
j	static	block	external
k	automatic	block	x

Declarators

• Deciphering complex declarations:

• Using type definitions to simplify declarations