Implementing Callback Functions in C

Introduction

In this project, you will learn how to implement a callback function in C language. Callback functions are a powerful technique in C programming, especially in the context of Linux system development, where they are commonly used.

👀 Preview

$ gcc test_callback.c callback.c -o test_callback
$ ./test_callback
Alarm1:0
Alarm2:1
Alarm3:2

🎯 Tasks

In this project, you will learn:

• How to define and use callback functions in C
• How to register and trigger alarms using a callback function
• How to handle edge cases and errors in the callback function implementation

🏆 Achievements

After completing this project, you will be able to:

• Understand the concept of callback functions and their usage in C programming
• Implement a callback-based alarm system, including registering and triggering alarms
• Write robust and defensive code to handle potential issues in the callback function
• Apply your knowledge of callback functions to other areas of C programming, such as event-driven systems or asynchronous operations

Skills Graph

Understand the Callback Function

In this step, you will learn about the concept of callback functions in C language and how they are used in Linux C program development.

Callback functions are a way to pass a function as an argument to another function, which can then call the passed function at a later time. This is a powerful technique that allows for more flexible and modular code.

In the code provided in the callback.c file, the callback function is used to implement an alarm system. The alarm struct represents an alarm, and the register_alarm function is used to register an alarm. The hit_alarm function is then used to trigger the registered alarms.

Identify and Fix the Defects

Now, let's take a look at the provided code in the callback.c file and fix the defects.

1. Segmentation Fault: This is likely because the alarm_list is not being correctly populated or accessed, leading to illegal memory access.
2. Improper Index Handling in register_alarm: The index variable is local to register_alarm and is reinitialized to 0 every time the function is called. This causes all alarms to be registered at index 1, overwriting each other.

Update the callback.c file as follows:

#include "callback.h"

alarm alarm_list[MAX_ALARMS];
int current_index = 0; // Global index tracker

void register_alarm(alarm a) {
    if (current_index < MAX_ALARMS) {
        alarm_list[current_index] = a;
        current_index++;
    }
    // If the number of registered alarms exceeds MAX_ALARMS, do nothing (don't report an error)
}

int hit_alarm(int index) {
    if (index < 0 || index >= MAX_ALARMS || !alarm_list[index])
        return 1;
    (*alarm_list[index])(index);
    return 0;
}

The key changes are:

1. Added a global current_index variable to track the current index for registering alarms.
2. In register_alarm, we check if current_index is less than MAX_ALARMS before registering the alarm. If the number of registered alarms exceeds MAX_ALARMS, we don't report an error and simply continue.
3. In hit_alarm, we added additional checks to ensure that the index is within the valid range and that an alarm is actually registered at that index. If not, we return 1 to indicate that no alarm was triggered.

Test the Callback Function

Now that we have fixed the defects in the callback.c file, let's test the callback function by running the provided test_callback.c program.

Compile the program:

gcc test_callback.c callback.c -o test_callback

Run the program:

./test_callback

Output:

Alarm1:0
Alarm2:1
Alarm3:2

The output should now match the expected output, and the program should run without any segmentation faults or other errors.

Congratulations! You have successfully debugged and fixed the callback function implementation.

Summary

Congratulations! You have completed this project. You can practice more labs in LabEx to improve your skills.