C Storage Classes

In this page:

Auto Storage Class
Register Storage Class
Static Storage Class
Extern Storage Class
Selecting Storage Classes

Auto Storage Class

auto is the default storage class for any local variable you declare without specifying one explicitly -- it allocates temporary memory for the current function call and is destroyed automatically once that call ends.

Example: Auto Storage Class

#include <stdio.h>
int main() {
	auto int x = 5;
	printf("%d", x);
	return 0;
}

Register Storage Class

register asks the compiler to store a variable directly in a CPU register instead of regular memory (RAM), aiming for faster access -- though modern compilers usually make this optimization decision on their own regardless of the hint.

Example: Register Storage Class

#include <stdio.h>
int main() {
	register int i;
	for (i = 0; i < 3; i++) {
		printf("%d ", i);
	}
	return 0;
}

Static Storage Class

static changes a local variable's lifetime so it retains its value between separate calls to the same function, instead of resetting each time -- useful for things like a counter that needs to persist across multiple invocations.

Example: Static Storage Class

#include <stdio.h>
void counter() {
	static int count = 0;
	count++;
	printf("%d ", count);
}
int main() {
	counter();
	counter();
	counter();
	return 0;
}

Extern Storage Class

extern declares that a variable is defined in a different source file, letting multiple .c files share and modify the same global variable without each one needing its own separate copy.

Example: Extern Storage Class

#include <stdio.h>
int sharedValue = 42;
extern int sharedValue;
int main() {
	printf("%d", sharedValue);
	return 0;
}

Selecting Storage Classes

Choosing the right storage class is about matching intent to behavior: static for a value that must persist across calls, auto for ordinary short-lived locals, and register only as a rarely-needed performance hint on tight loop counters.

Example: Selecting Storage Classes

#include <stdio.h>
void counter() {
	static int persistent = 0;
	int temporary = 0;
	persistent++;
	temporary++;
	printf("%d %d ", persistent, temporary);
}
int main() {
	counter();
	counter();
	return 0;
}

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