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

auto

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

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.

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

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

num = 1
num = 2
num = 3

Try to remove the static keyword from the example to see the difference. Without static, myNum is created again with the value 0 every time count() is called, so the program prints num = 1 three times.

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.

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, like in the Organize Code chapter:

gcc main.c data.c -o program

The program prints shared = 50, because main.c uses the variable that is defined in data.c.

Tip: In larger programs, the extern line is usually placed in a header file (like data.h), so every file that includes it can use the variable.

Summary

This table shows how long each kind of variable lasts (its lifetime), and where it can be used: