Static storage class - Mini Project: Build & Apply

Understanding Static Storage Class in C

📖 Scenario: Imagine you are writing a small program to count how many times a function is called. You want to keep this count inside the function itself, so it remembers the count between calls but is not visible outside the function.

🎯 Goal: You will create a function that uses a static variable to count how many times it has been called, then print the count each time the function runs.

📋 What You'll Learn

Create a function called call_counter.

Inside call_counter, declare a static int variable named count initialized to 0.

Each time call_counter is called, increase count by 1.

Print the current value of count inside call_counter.

Call call_counter three times from main.

💡 Why This Matters

🌍 Real World

Static variables are useful when you want to keep track of information inside a function without exposing it outside. For example, counting events, caching results, or managing state in embedded systems.

💼 Career

Understanding static storage class is important for C programmers working on embedded systems, system programming, or performance-critical applications where controlling variable lifetime and visibility is crucial.

1

Create the call_counter function with a static variable

Write a function called call_counter that declares a static int count variable initialized to 0.

C

// Define the call_counter function
// Your code here

Hint

Remember, static variables inside functions keep their value between calls.

2

Increase the static variable count by 1 inside call_counter

Inside the call_counter function, add code to increase the count variable by 1.

C

void call_counter() {
    static int count = 0;
    // Increase count by 1
    // Your code here
}

Hint

Use count++ to add one to the count.

3

Print the current value of count inside call_counter

Add a printf statement inside call_counter to print the message: "Function called %d times\n" using the count variable.

C

void call_counter() {
    static int count = 0;
    count++;
    // Print the count
    // Your code here
}

Hint

Use printf to show the count value with the message.

4

Call call_counter three times from main and print output

Write the main function that calls call_counter three times.

C

#include 

void call_counter() {
    static int count = 0;
    count++;
    printf("Function called %d times\n", count);
}

int main() {
    // Call call_counter three times
    // Your code here
    return 0;
}

Hint

Call the function three times in main to see the count increase.

Practice

1. What does the static keyword do when used with a variable inside a function in C?

easy

A. Keeps the variable value between function calls

B. Makes the variable global

C. Deletes the variable after the function ends

D. Allows the variable to be accessed from other files

Solution

1. Step 1: Understand static inside a function

   A static variable inside a function keeps its value even after the function finishes.

2. Step 2: Compare with other options

   Static does not make variables global or accessible outside the file; it preserves value only inside the function.

3. Final Answer:

   Keeps the variable value between function calls -> Option A

4. Quick Check:

   Static inside function = value preserved [OK]

Hint: Static inside function means value stays between calls [OK]

Common Mistakes:

• Thinking static makes variable global
• Assuming static deletes variable after function ends
• Confusing static with extern keyword

2. Which of the following is the correct way to declare a static variable inside a function in C?

easy

A. static count int = 0;

B. static int count = 0;

C. int static count = 0;

D. count static int = 0;

Solution

1. Step 1: Recall correct static syntax

   The correct syntax places static before the type: static int count = 0;.

2. Step 2: Check other options

   Other options have wrong order of keywords, which is invalid in C.

3. Final Answer:

   static int count = 0; -> Option B

4. Quick Check:

   Static keyword comes before type [OK]

Hint: Static always comes before the type in declaration [OK]

Common Mistakes:

• Placing static after the variable name
• Mixing order of keywords
• Using invalid syntax like 'static count int'

3. What will be the output of the following C code?

void func() {
    static int x = 0;
    x++;
    printf("%d ", x);
}

int main() {
    for(int i = 0; i < 3; i++) {
        func();
    }
    return 0;
}

medium

A. 1 1 1

B. 0 1 2

C. 1 2 3

D. 3 3 3

Solution

1. Step 1: Understand static variable behavior in func()

   The static variable x is initialized once to 0 and keeps its value between calls.

2. Step 2: Trace the loop calls

   Each call increments x by 1 and prints it. So outputs are 1, then 2, then 3.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Static variable increments across calls [OK]

Hint: Static variable keeps incrementing each call [OK]

Common Mistakes:

• Assuming static resets each call
• Confusing initial value with printed value
• Expecting output to be zeros

4. Identify the error in the following code snippet:

void func() {
    static int x;
}

int main() {
    x = 5;
    func();
    return 0;
}

medium

A. No error, code runs fine

B. Missing initialization of static variable x

C. Static variables cannot be declared inside functions

D. Static variable x cannot be assigned outside its function

Solution

1. Step 1: Check variable scope

   The static variable x is declared inside func(), so it is local to that function.

2. Step 2: Analyze assignment in main()

   Trying to assign x = 5; in main() causes an error because x is not visible outside func().

3. Final Answer:

   Static variable x cannot be assigned outside its function -> Option D

4. Quick Check:

   Static inside function = local scope [OK]

Hint: Static inside function is local, can't access outside [OK]

Common Mistakes:

• Thinking static variables are global
• Ignoring variable scope rules
• Assuming static variables must be initialized

5. Consider two C files: file1.c and file2.c.

In file1.c:

static int count = 0;

void increment() {
    count++;
}

int getCount() {
    return count;
}

In file2.c:

extern int count;

int main() {
    increment();
    printf("%d", getCount());
    return 0;
}

What will happen when you compile and run these files together?

hard

A. Compilation error due to undefined reference to count

B. Prints 1

C. Prints 0

D. Runtime error due to static variable

Solution

1. Step 1: Understand static at file scope

   The static int count has internal linkage, visible only in file1.c.

2. Step 2: Function access

   increment() and getCount() are external functions that can access the static count since they are in the same file.

3. Step 3: Analyze extern usage

   The declaration extern int count; in file2.c expects a global variable count defined elsewhere, but since count is static in file1.c, it is not visible externally, causing a linker error.

4. Final Answer:

   Compilation error due to undefined reference to count -> Option A

5. Quick Check:

   Static variables have internal linkage, extern cannot access them [OK]

Hint: Static variables limited to file scope, extern can't access [OK]

Common Mistakes:

• Assuming static variables are global across files
• Expecting extern to find static variables
• Ignoring linker errors