Using return codes - Mini Project: Build & Apply

Using return codes

📖 Scenario: You are writing a small program that simulates a simple login system. The program checks if a username and password are correct and returns a code to indicate success or failure.

🎯 Goal: Build a C program that uses return codes from a function to indicate if login was successful or not.

📋 What You'll Learn

Create a function that checks username and password and returns 0 for success and 1 for failure

Use variables to store the correct username and password

Call the function with test username and password

Print the return code to show if login succeeded or failed

💡 Why This Matters

🌍 Real World

Return codes are used in many programs to signal if an operation succeeded or failed, like login systems, file operations, or network requests.

💼 Career

Understanding return codes helps in debugging and writing clear, maintainable code that communicates status between functions and modules.

1

Create variables for correct username and password

Create two char arrays called correct_username and correct_password with values "admin" and "1234" respectively.

C

// Your code here

Hint

Use char arrays to store text strings in C.

2

Create a function to check login and return codes

Write a function called check_login that takes two char* parameters named username and password. The function should return 0 if both match correct_username and correct_password, otherwise return 1.

C

char correct_username[] = "admin";
char correct_password[] = "1234";

// Write the check_login function below

Hint

Use strcmp from string.h to compare strings. Return 0 for success, 1 for failure.

3

Call the function with test values

Create two char* variables called test_user and test_pass with values "admin" and "1234". Then call check_login(test_user, test_pass) and store the result in an int variable called result.

C

char correct_username[] = "admin";
char correct_password[] = "1234";

int check_login(char* username, char* password) {
    if (strcmp(username, correct_username) == 0 && strcmp(password, correct_password) == 0) {
        return 0;
    } else {
        return 1;
    }
}

// Create test_user and test_pass and call check_login

Hint

Use string pointers for test username and password. Store the function return in result.

4

Print the return code result

Write a printf statement to print "Login result: %d\n" with the variable result.

C

char correct_username[] = "admin";
char correct_password[] = "1234";

int check_login(char* username, char* password) {
    if (strcmp(username, correct_username) == 0 && strcmp(password, correct_password) == 0) {
        return 0;
    } else {
        return 1;
    }
}

char* test_user = "admin";
char* test_pass = "1234";
int result = check_login(test_user, test_pass);

// Print the result

Hint

Use printf to show the login result number.

Practice

1. What does a return code of 0 usually mean in C functions?

easy

A. The function completed successfully

B. The function encountered an error

C. The function did not run

D. The function returned a pointer

Solution

1. Step 1: Understand return codes meaning

   In C, a return code of 0 typically means success or no error.

2. Step 2: Interpret the code 0

   Since 0 means success, the function completed its task without problems.

3. Final Answer:

   The function completed successfully -> Option A

4. Quick Check:

   Return code 0 = success [OK]

Hint: Remember: 0 means success, non-zero means error [OK]

Common Mistakes:

• Thinking 0 means error
• Confusing return code with function output
• Assuming non-zero means success

2. Which of the following is the correct way to return an error code from a C function?

easy

A. return 0;

B. return 'error';

C. return -1;

D. return true;

Solution

1. Step 1: Identify valid return types for error codes

   Return codes are integers, so returning a number like -1 is valid for error.

2. Step 2: Check each option

   return 0; returns 0 (success), B returns a multi-char constant (invalid), C returns true (boolean, invalid in C), D returns -1 (valid error code).

3. Final Answer:

   return -1; -> Option C

4. Quick Check:

   Use integer codes for errors [OK]

Hint: Return integer codes; negative often means error [OK]

Common Mistakes:

• Returning strings instead of integers
• Using boolean values in C return codes
• Returning 0 for errors

3. What will be the output of this C program?

#include <stdio.h>
int check(int x) {
    if (x > 0) return 0;
    else return 1;
}
int main() {
    int result = check(-5);
    printf("Return code: %d\n", result);
    return 0;
}

medium

A. Return code: 1

B. Return code: -5

C. Return code: 0

D. No output

Solution

1. Step 1: Analyze the check function behavior

   If x > 0, it returns 0; otherwise, it returns 1.

2. Step 2: Apply input value -5

   Since -5 is not greater than 0, the function returns 1.

3. Final Answer:

   Return code: 1 -> Option A

4. Quick Check:

   Input -5 <= 0, so return 1 [OK]

Hint: Check conditions carefully to find returned code [OK]

Common Mistakes:

• Assuming negative input returns 0
• Confusing return value with input
• Ignoring else branch

4. Identify the error in this C function that uses return codes:

int process(int val) {
    if (val == 0)
        return 0;
    else
        return;
}

medium

A. Using 0 as success code

B. No error, code is correct

C. Function should return void

D. Missing return value in else branch

Solution

1. Step 1: Check function return type and return statements

   The function returns int, so every return must have an int value.

2. Step 2: Identify missing return value

   The else branch has 'return;' without a value, which is invalid for int return type.

3. Final Answer:

   Missing return value in else branch -> Option D

4. Quick Check:

   int functions must return int always [OK]

Hint: Every return in int function must return int value [OK]

Common Mistakes:

• Returning nothing in non-void function
• Confusing void and int return types
• Ignoring compiler warnings

5. You want to write a function int saveData() that returns 0 on success, 1 if file not found, and 2 if write error. Which is the best way to handle these return codes in main()?

hard

A. Ignore the return code and always print "Success"

B. Check if return code is 0, else print error message based on code

C. Return the code from main without checking

D. Use return codes only for debugging, not in main

Solution

1. Step 1: Understand return codes purpose

   Return codes help main know if saveData succeeded or failed and why.

2. Step 2: Choose proper handling in main

   Main should check if return code is 0 (success), else print specific error messages for 1 or 2.

3. Final Answer:

   Check if return code is 0, else print error message based on code -> Option B

4. Quick Check:

   Use return codes to handle errors clearly [OK]

Hint: Check return codes to handle errors clearly [OK]

Common Mistakes:

• Ignoring return codes in main
• Printing success without checking
• Using return codes only for debugging