C Scope and Storage Classes

Scope tells where a variable can be used. Storage class tells how long it lives and where it is stored.

Local and global variables

Type	Declared	Can be used	Default value
Local	Inside a function or block	Only in that block	Garbage value
Global	Outside all functions	Everywhere after the declaration	0

#include <stdio.h>

int total = 100;      // global

void show(void) {
    int local = 5;    // local to show()
    printf("%d %d\n", total, local);
}

int main(void) {
    show();
    total = 200;
    show();
    return 0;
}

Output:

If a local variable has the same name as a global one, the local variable is used inside the function.

Storage classes

Class	Meaning
auto	Default for local variables. Created when the block starts and removed when it ends.
static	Keeps its value between function calls. Initialized only one time.
extern	Tells that the variable is defined in another file or later.
register	A hint to keep the variable in a CPU register for speed. The compiler may ignore it.

static example

#include <stdio.h>

void counter(void) {
    static int count = 0;
    int normal = 0;

    count++;
    normal++;
    printf("static=%d normal=%d\n", count, normal);
}

int main(void) {
    counter();
    counter();
    counter();
    return 0;
}

Output:

Next: C Arrays.

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