C Storage Classes

C Storage Classes

Storage classes define the lifetime, visibility, and memory location of variables.

There are four main storage class specifiers in C:

• auto
• static
• register
• extern

Note: Difference Between Scope and Storage Classes Scope defines where a variable can be used, and storage classes define how long it lasts and where it's stored. This chapter continues from the C Scope chapter.

auto

The auto keyword is used for local variables. It is default for variables declared inside functions, so it's rarely used explicitly.

Example

int main() {
  auto int x = 50;  // Same as just: int x = 50;
  printf("%d\n", x);
  return 0;
}

static

The static keyword changes how a variable or function behaves in terms of lifetime and visibility:

• Static local variables keep their value between function calls.
• Static global variables/functions are not visible outside their file.

Example

void count() {
  static int myNum = 0; // Keeps its value between calls
  myNum++;
  printf("num = %d\n", myNum);
}

int main() {
  count();
  count();
  count();
  return 0;
}

Try to remove the static keyword from the example to see the difference.

register

The register keyword suggests that the variable should be stored in a CPU register (for faster access).

You cannot take the address of a register variable using &.

Note: The register keyword is mostly obsolete - modern compilers automatically choose the best variables to keep in registers, so you usually don't need to use it.

Example

int main() {
  register int counter = 0;
  printf("Counter: %d\n", counter);
  return 0;
}

extern

The extern keyword tells the compiler that a variable or function is defined in another file.

It is commonly used when working with multiple source files.

#include <stdio.h>

extern int shared;  // Declared here, defined in another file

int main() {
  printf("shared = %d\n", shared);
  return 0;
}

int shared = 50;  // Definition of the variable

Compile both files together:

gcc main.c data.c -o program