Linux processes and signals: Exploration of normal and abnormal exit mechanisms

1. Introduction

Linux, as the world’s most popular open source operating system, provides developers with a complete and powerful set of tools to efficiently manage the life cycle of programs. Among them, the concepts of process and signal are indispensable core components. In this chapter, we will briefly explore these two concepts and lay the foundation for subsequent chapters.

1.1 Basic concepts of Linux processes

Process is the basic execution entity in Linux system. Each process has its own memory space, code, data, and system resources. This isolation mechanism ensures that errors in one process do not directly affect other processes. As Bjarne Stroustrup said in “The C++ Programming Language”: “We write and execute programs to solve problems, and processes provide an isolated environment in which to run these programs.”

The life cycle of a process spans from creation (fork or exec) to termination (exit or being killed by another process). During its life cycle, a process may interact with other processes or receive and process various signals.

#include <stdio.h>
#include <unistd.h>

int main() {
    printf("This is a simple process example\\
");
    sleep(3); // The process will terminate after 3 seconds
    return 0;
}

1.2 The role and importance of signals

Signals are a lightweight communication mechanism used for inter-process communication in UNIX and Linux systems. They are typically used to notify a process that some event has occurred, such as the termination of a child process or a keyboard interrupt request by the user. Signals provide us with a way to handle asynchronous events, allowing us to write responsive and robust programs.

Signals are often likened to human intuition or the quick response of intuition. Our brains respond quickly when we sense danger, a mechanism very similar to signaling. Signals provide a mechanism for processes to respond quickly to external events or internal errors.

For example, when we press Ctrl + C, the current foreground process will receive a SIGINT signal, informing it that the user wants to terminate the process.

#include <signal.h>
#include <stdio.h>
#include <unistd.h>

void sigint_handler(int signo) {
    printf("SIGINT signal caught!\\
");
    // Terminate the process
    exit(0);
}

int main() {
    //Set the handler for the SIGINT signal
    signal(SIGINT, sigint_handler);
    while (1) {
        printf("Waiting for SIGINT signal...\\
");
        sleep(1);
    }
    return 0;
}

In this code, we set up a SIGINT signal handler. When the user presses Ctrl + C, the program will capture this signal and execute the corresponding handler.

2. Normal exit methods (Normal Exit Mechanisms)

2.1 Return from main function (Return from main)

In C++, a program exits gracefully when its execution reaches the end of the main function or returns explicitly using a return statement. This is the most natural and direct way to end the program life cycle.

For example:

#include <iostream>

int main() {<!-- -->
    std::cout << "Hello, World!" << std::endl;
    return 0; //The program exits normally
}

As Bjarne Stroustrup said in “The C++ Programming Language”: “Successful execution of a program usually returns 0, and failure returns a non-zero value.”

However, human thinking and existence are not always linear. Our experiences and emotions can come with complex twists and turns, but that’s how we grow and learn. Likewise, a program may end at any point in its life cycle for various reasons. But in most cases, reaching the end of the main function is the most common way to end.

2.2 Using the exit() function to end the process (Using the exit () Function)

In addition to returning from the main function through the return statement, the program can also be terminated by calling the exit() function. The exit() function is part of the C standard library and is used to end the process and return a status code to the operating system.

When exit() is called, all static object destructors will be executed, all open file descriptors will be closed, and all memory used by the process will be released.

#include <iostream>
#include <cstdlib>

int main() {<!-- -->
    std::cout << "Calling exit() function" << std::endl;
    exit(0); //The program exits normally
}

Here exit(0) and return 0 have the same effect in the main function. But exit() can be called anywhere in the program, not just in the main function.

Just like human decision-making when faced with difficult situations, a program may choose to terminate early because it encounters an unsolvable problem or to prevent further damage.

2.3 Normal Exit of Threads: pthread_exit() (Normal Exit of Threads: pthread_exit())

In a multi-threaded environment, a thread can terminate its execution gracefully by calling pthread_exit() without affecting other threads. This is similar to the exit() function in a process, but only applies to the current thread.

#include <iostream>
#include <pthread.h>

void* threadFunc(void* arg) {<!-- -->
    std::cout << "Thread is exiting" << std::endl;
    pthread_exit(nullptr);
}

int main() {<!-- -->
    pthread_t thread;
    pthread_create( & amp;thread, nullptr, threadFunc, nullptr);
    pthread_join(thread, nullptr);
    std::cout << "Main thread continues" << std::endl;
    return 0;
}

In the above code, the sub-thread exits through pthread_exit(), but the main thread continues to execute and “Main thread continues” is displayed.

Just like us as individuals in life, each thread has its own tasks and goals. When one thread completes its task and “exits the life cycle”, other threads can still continue their lives.

Conclusion

Normal exit is part of the program life cycle, whether it is returning from the main function, using the exit() function, or using pthread_exit() in a multi-threaded environment. These mechanisms allow a program to end in an orderly and expected manner, ensure that resources are properly released, and return information about the program’s state to the caller or the operating system. Just as people return home after completing their journey, programs end normally after completing their tasks.

3. Exceptional Exit Mechanisms

As a robust operating system, Linux is designed with a complex signal mechanism to handle various abnormal situations. These exceptions, such as a program trying to access illegal memory or execute an illegal instruction, usually cause the process to receive a signal. Next, we’ll dive into common causes of abnormal exits and their associated signals.

3.1 Common causes of exceptions

3.1.1 Segmentation Fault – SIGSEGV

A segfault occurs when a program attempts to access an area of memory that it does not have permission to access. For example, dereferencing a null pointer or accessing memory outside the bounds of an array.

int *ptr = nullptr;
*ptr = 10; // This will raise SIGSEGV

This code attempts to assign an integer value to a null pointer, causing a segfault. As Bjarne Stroustrup said in “The C++ Programming Language”: “Direct access to raw pointers is generally dangerous and should be avoided.”

3.1.2 Floating Point Exception (SIGFPE)

When performing illegal arithmetic operations, such as dividing by zero, the program will receive the SIGFPE signal.

int x = 0;
int y = 10/x; // this will raise SIGFPE

In this case, trying to divide by zero results in a floating point exception. Human thinking is often thought of as linear, but in computing many things are nonlinear and require us to think differently.

3.1.3 Illegal Instruction (Illegal Instruction – SIGILL)

This signal is triggered when the CPU attempts to execute an illegal or undefined instruction. This could be because the program’s binary code is corrupted or because of some other error.

3.2 Default behaviors and handling of signals

When a process receives one of the above signals, its default behavior is usually to terminate the process if no specific handler is set for the signal. But processes can choose to catch these signals and execute custom handlers.

For example, you can use the signal() or sigaction() function to set a handler for a specific signal.

#include <signal.h>
#include <stdio.h>

void sig_handler(int signo) {
    if (signo == SIGSEGV) {
        printf("Received SIGSEGV, handling...\\
");
    }
}

int main() {
    signal(SIGSEGV, sig_handler); // Set the SIGSEGV handler
    int *ptr = nullptr;
    *ptr = 10; // Will call the above handler instead of terminating the process
    return 0;
}

In the above code, we set up a simple handler for SIGSEGV. When a process tries to access illegal memory, it does not terminate immediately, but calls the handler we defined.

When delving into the behavior of these signals, we can refer to the Linux kernel source code. For example, the handling of SIGSEGV is usually defined in the kernel’s mm/fault.c file, which describes how the system responds when a memory violation occurs.

To sum up, Linux provides developers with a powerful tool for handling abnormal situations through its signaling mechanism. By understanding and utilizing these tools, we can write more robust and reliable programs. Just as we do when we face challenges in life, programs need mechanisms to handle and recover from exceptions to ensure their continued, stable operation.

4. Termination by External Signals

In Linux systems, interaction and communication between processes often rely on signal mechanisms. External signals, as a special kind of signal, are often used to interrupt, stop or end the process.

4.1 kill command and its application (The kill Command and Its Usage)

When we mention external signals, the kill command is often the first tool that comes to mind. Although called “kill,” this command does much more than just end a process. In fact, it can send almost any type of signal to a specified process.

# Send SIGTERM (default signal) to process PID
kill PID

# Send SIGKILL to process PID
kill -9 PID

Here -9 represents the digital representation of the SIGKILL signal. Each signal has a corresponding number.

Signals and Human Thought (Signals and Human Thought)

The role of signals in the process is a bit like intuition or sudden ideas in the human mind. As Bjarne Stroustrup said in “The C++ Programming Language”: “Intuition is often unpredictable, but it can always influence our behavior.” When a process receives a signal, it must decide how to respond-just like we decide what to do with a sudden thought or feeling.

4.2 Sources and Effects of External Signals

In addition to using the kill command, a process may receive external signals for other reasons. For example, when the user presses Ctrl + C in a process running in the foreground, the process receives the SIGINT signal.

The impact of a signal depends on how the process handles it. Some signals (such as SIGKILL) cannot be caught, which means that the process cannot prevent its default behavior – in this case, terminating the process.

Signals as Interruptions in Life (Signals as Interruptions in Life)

We can compare signals in progress to sudden interruptions in life. Just as in life we sometimes have to decide whether to answer a phone call or respond to a sudden task change, processes must also decide how to respond to signals.

This chapter provides you with an in-depth analysis of how external signals work in Linux and how processes respond to these signals. Hopefully this helps you better understand the interaction between Linux processes and signals, and how they impact our applications.

5. Notification of Child Process Status Changes

5.1 Role and Use of the SIGCHLD Signal (Role and Use of the SIGCHLD Signal)

In a Linux system, when the state of a child process changes, the parent process will receive the SIGCHLD signal (SIGCHLD Signal). This state change can be the termination, stop, or continuation of the child process. Why is such a mechanism needed?

As Bjarne Stroustrup said in “The C++ Programming Language”: “In complex systems, communication and synchronization between components are indispensable.” In a multi-process environment, the parent process and the child process are like two independent components and require some mechanism to synchronize their states. The SIGCHLD signal provides such a bridge so that the parent process can know when the child process ends and take appropriate action.

For example, the parent process may need to reclaim resources used by the child process, or start a new child process after the child process terminates. Without the SIGCHLD signal, the parent process may be in a “dead zone” where the status of the child process cannot be determined.

5.2 Using wait() and waitpid() to obtain the child process status (Using wait() and waitpid() to Retrieve Child Process Status)

When a child process terminates, it does not disappear from the system immediately. It still exists in a state known as a “zombie process” (zombie process) until the parent process explicitly asks for its exit status. This is what the wait() and waitpid() functions are for.

#include <sys/wait.h>
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);

These functions allow the parent process to obtain the termination status of the child process and perform further operations accordingly. For example, the parent process can check whether the child process terminated due to receiving a certain signal and make a decision accordingly.

Consider the following code example:

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>

int main() {<!-- -->
    pid_t pid = fork();

    if (pid == 0) {<!-- --> // child process
        printf("Child process is running...\\
");
        sleep(5); // Simulate some work
        printf("Child process is exiting...\\
");
        exit(0);
    } else if (pid > 0) {<!-- --> // parent process
        int status;
        wait( & amp;status); // Wait for the child process to terminate
        if (WIFEXITED(status)) {<!-- -->
            printf("Child exited with status %d\\
", WEXITSTATUS(status));
        }
    } else {<!-- -->
        perror("fork failed");
        exit(1);
    }

    return 0;
}

In the above code, the parent process uses the wait() function to wait for the child process to terminate, and prints its exit status after the child process terminates.

From the perspective of kernel source code, the wait() and waitpid() functions are implemented in the kernel/exit.c file. They are tightly integrated with the kernel’s process management subsystem to ensure that resources are properly reclaimed and provide feedback to the parent process about the status of the child process.

5.2.1 A Deeper Dive into the System Calls (A Deeper Dive into the System Calls)

Digging into the Linux kernel source code, we can see that wait() is actually a simplified version of waitpid(), which internally calls wait4(). These functions obtain the status information of the child process by interacting with the kernel’s process management subsystem, so that the parent process can make appropriate decisions.

We won’t go into every line of kernel code here, but, for interested readers, I recommend looking directly at the Linux kernel source code, especially the parts related to process management, to get deeper insights.

5.3 Resonance Between Humans and Technology

When considering these technical details, we can’t help but ask: Why is it designed this way?

As the old adage goes: “Man’s outer world is a reflection of his inner world.” In technology, the systems we create often reflect the way we think and understand the world. The SIGCHLD signal and the wait() system call are like the communication bridge between parents and children. When a child process (or child) leaves or enters a new state, the parent process (or parent) needs to know and respond accordingly.

This connection between father and son processes is actually a manifestation of the eternal themes of mankind: inheritance, communication and care. When we delve deeper into technology, we often see this theme that is deeply connected to humanity.

6. Conclusion

During our deep dive into Linux processes and signals, we encountered a number of key concepts and mechanisms. These concepts are not only fundamental to understanding how the operating system works, but are crucial for every developer who wants to program effectively in a Linux environment.

6.1 Review of key points of processes and signals

First, we understand that a process is the basic execution entity in the operating system. It has its own address space, resources and execution path. At the same time, signals are the basic mechanism for inter-process communication in Linux and UNIX systems. As Bjarne Stroustrup said in “The C++ Programming Language”: “The structure and execution process of the program should be clear and unambiguous”, and the process and signal mechanism in Linux indeed provide such clarity.

We also saw that a process can exit through normal means, such as return or exit(), or terminate due to abnormal behavior, such as receiving SIGSEGV code> signal. These exit methods are deeply embedded in the foundation of human thinking: we can either choose to end a task normally, or we can be forced to abort due to some external interference or internal error.

6.2 How to effectively manage and control the life cycle of the process

Managing and controlling the lifecycle of a process is a complex task, but with the right tools and strategies, we can make it simpler and more controllable. For example, using the wait() and waitpid() system calls, a parent process can obtain status information about its child processes. These interfaces are implemented in detail in the Linux kernel source code, especially in the kernel/exit.c file.

Additionally, handling signals efficiently is key to managing process lifecycle. By setting up appropriate signal handlers, we can ensure that the process takes appropriate action when it receives certain signals, rather than simply terminating. The implementation of this method can be found in arch/x86/kernel/signal.c, which shows how signal handling interacts with the kernel.

6.3 In-depth thinking

While delving into the technical details, we should also think about deeper questions: Why is the operating system designed like this? How do they reflect our thinking and decision-making processes? The concepts of processes and signals, like so many other things in our lives, are the result of finding a balance between trade-offs.

Overall, Linux processes and signals provide us with a powerful, flexible and clear framework to help us better understand and control program execution. By deeply understanding these concepts, we can not only write more efficient and stable code, but also better understand the digital world in which we live.

Conclusion

In our programming learning journey, understanding is an important step for us to move to a higher level. However, mastering new skills and ideas always requires time and persistence. From a psychological point of view, learning is often accompanied by constant trial and error and adjustment, which is like our brain gradually optimizing its “algorithm” for solving problems.

This is why when we encounter mistakes, we should view them as opportunities to learn and improve, rather than More than just an obsession. By understanding and solving these problems, we can not only fix the current code, but also improve our programming skills and prevent making the same mistakes in future projects.

I encourage everyone to actively participate and continuously improve their programming skills. Whether you are a beginner or an experienced developer, I hope my blog will be helpful on your learning journey. If you find this article useful, you may wish to click to bookmark it, or leave your comments to share your insights and experiences. You are also welcome to make suggestions and questions about the content of my blog. Every like, comment, share and attention is the greatest support for me and the motivation for me to continue sharing and creating.

Read my CSDN homepage and unlock more exciting content: Bubble’s CSDN homepage