Automatic storage

Learn automatic storage duration (local variables).

Automatic storage

Variables declared inside a function have automatic storage. They live on the stack and disappear when the function returns.

void show(void) {
    int x = 42;  /* automatic */
    printf("%d\n", x);
}

Here x is recreated at each call; its memory is given back when the call ends. This needs no free; C cleans it for you. Automatic storage is the simplest approach for most local variables.

Limits of automatic storage

Automatic storage has a limit: when the function returns, the variable is gone. So you cannot return the address of a local array.

int *bad(void) {
    int x = 5;
    return &x;  /* dangerous */
}

Here x is deleted when the function returns; the returned address is no longer valid. For large or long-lived memory you should use malloc. For small temporary data, automatic storage is always the best choice.

Exercise Print with the show function

Define void show(void) with int x = 42 inside and print x with printf. In main call show.

Show example solution

void show(void) {
    int x = 42;
    printf("%d\n", x);
}
show();

Test yourself

Match storage class to property

automatic

Gone when function ends
stack

Local variables
no free needed

Automatic cleanup
each call

Recreated
Which is true about automatic storage?
- It is memory allocated with malloc
- It is the storage of local variables
- It lives for the whole program
- It is released with free
Show answer

It is the storage of local variables — Automatic storage is for local variables.

Fill in the blank: function definition

____ show(void) {
    int x = 42;
}

Show answer

void show(void) {
    int x = 42;
}

Next lesson: malloc

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