QUESTION 19 · Storage Classes

What is the default storage duration of an ordinary block-local variable?

2 marks

Answer

It has automatic storage duration unless another relevant declaration changes that rule.

auto can be written explicitly in C11, as in auto int count = 0;.
It does not provide a guaranteed zero value when an automatic object is left uninitialised.

Explain the auto storage class with an example.

5 marks

Answer in brief

In C11, auto explicitly declares an automatic block-local object. Ordinary local variables usually have the same storage duration without writing auto.

Declaration

auto int count = 0;
int other_count = 0; /* also automatic here */

Main properties

Scope is normally the enclosing block, from the declaration onwards.
The local object has no linkage.
A new block execution has its own automatic object.
An initialiser is applied when the declaration is reached. Without a suitable initialisation, an ordinary automatic object does not have a useful guaranteed starting value.

Example trace

Each call to show creates its automatic count and initialises it to zero. The increment makes it 1, so three separate calls print 1, 1 and 1. The value is not saved for the next call. A local static counter behaves differently because it refers to one lasting object.

Uses

Automatic variables are suitable for temporary calculations, counters and intermediate results that belong to one call. Recursive calls can each have their own automatic parameter and local values. This helps one call avoid overwriting another call’s ordinary locals.

Important points

Initialise before reading. Avoid returning the address of an ordinary automatic local object because its lifetime ends when its block execution ends. auto does not mean that the compiler should infer the type in the C11 examples used here; the keyword must be understood in the selected language version.

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
void show(void)
{
    auto int count = 0;
    ++count;
    printf("%d\n", count);
}
int main(void)
{
    show(); show(); show();
    return 0;
}

Sample input and output

This example uses values already written in the program.

Output

1
1
1