30 Essential Interview Questions For Embedded Linux

Hello everyone, I am Deep Linux. Today I will share the interview questions for embedded Linux~ The embedded positions have been very popular in recent years, and many friends choose this direction, and the salary aspect is also quite attractive. Therefore, for those who do not want to compete in backend development, embedded systems are also a good choice.

So, are the requirements for fresh graduates in this position high? Relatively speaking, it’s a bit better than backend Java and C++. As for language requirements, embedded systems also require proficiency in C, so C/C++ candidates tend to favor embedded systems.

Today, I will take you to learn the 30 essential interview questions for embedded Linux.

Source of interview questions: https://www.nowcoder.com/interview/ (Answers are summarized by myself, please advise if there are any mistakes!)

1. What is Linux?

Linux is an open-source, free-to-use operating system kernel, initially developed by Finnish programmer Linus Torvalds in 1991. It is known for its stability, efficiency, and security, widely used in server environments, and has gradually expanded to personal computers, mobile devices, and embedded systems across multiple platforms. The Linux operating system is based on UNIX design principles, featuring good customizability and extensibility, and can be modified and distributed under a free and open license.

2. What are the differences between Unix and Linux?

Unix and Linux are both operating systems, but they have some differences. Here are a few:

Development History: Unix was initially developed by AT&T Bell Labs and became a commercial operating system. Linux, on the other hand, was developed by Linus Torvalds based on Unix.
Open Source Nature: Linux is open source, meaning anyone can view, modify, and distribute its source code. Unix primarily existed in a closed-source form in the past, and while there are some open-source versions now (like FreeBSD), it mainly still exists in commercial form.
Portability: Linux is designed to be a highly portable operating system that can run on various hardware platforms. In contrast, Unix is often bound to specific hardware architectures and is more commonly used in server and mainframe environments.
Community Support: Due to the open-source nature of Linux, there is a large global community involved that provides support, updates, and improvements. The Unix community is relatively smaller and relies more on vendor-provided technical support.
Standardization: Unix lacks a unified standard due to differences in implementations by different vendors. In contrast, Linux adheres to the POSIX (Portable Operating System Interface) standard, ensuring compatibility and interoperability.

3. What are the components of the Linux system?

Linux Kernel: The kernel is the core of the operating system, responsible for managing hardware resources, providing process scheduling, file systems, and device drivers.
Shell: The shell is the interface between the user and the operating system. It accepts user-input commands and passes them to the kernel for execution. Common Linux shells include Bash, Zsh, etc.
File System: The file system is the method of storing and organizing data. Linux supports various file systems, such as Ext4, XFS, etc.
GNU Tools: The GNU toolset is a collection of open-source software tools, including compilers, text editors, debuggers, etc., providing developers with rich functionalities and tools.
Application Libraries: Linux provides many application libraries, such as C Library (glibc), graphical interface libraries (GTK+, Qt), etc., offering convenient programming interfaces for developers.
User-space Utilities: Linux provides various utilities and applications, such as shell command interpreters, text editors, network tools, etc.

These components together constitute the Linux operating system, supporting various applications and user interactions at different levels.

4. What are the components of the Linux kernel?

Process Management: Responsible for creating, scheduling, and terminating processes, as well as managing communication and synchronization between processes.
Memory Management: Responsible for managing the system’s physical and virtual memory, including page replacement, memory allocation, and reclamation operations.
File System: Provides read/write and management functions for files on the hard disk, supporting various file system types.
Device Drivers: Used to interact with hardware devices, including initialization and data transfer operations for hardware devices.
Network Protocol Stack: Implements network communication protocols (like TCP/IP) and drivers for network devices, enabling computers to communicate over networks.
System Call Interface: Provides an interface for user-space applications to access kernel functionalities, allowing applications to request the kernel to perform specific tasks through system calls.
Interrupt Handling: Responsible for handling interrupt signals from external devices and responding to interrupt events in a predetermined manner.
Scheduler: Determines which process runs during a specific time period, employing appropriate scheduling strategies to optimize resource utilization and response time.

These components together form the Linux kernel, providing various critical functionalities at the operating system level, enabling the Linux system to efficiently manage hardware resources and provide various services.

5. What is the role of the Memory Management Unit (MMU)?

Address Translation: The MMU translates virtual addresses used by applications into corresponding physical addresses. This allows each process to believe it is using the entire memory space exclusively, without needing to share actual physical memory with other processes.
Memory Protection: The MMU protects memory isolation between different processes or between user mode and kernel mode by controlling access permission bits. For instance, only processes with specific permissions can modify certain memory areas, thus providing security and privacy protection.
Page Replacement: When the physical memory available in the system is insufficient, the MMU is responsible for swapping less-used or unused pages from physical memory to the hard disk and loading pages that need to be used into physical memory. This effectively manages the limited physical memory resources.
Cache Management: The MMU also participates in handling data transfer and consistency maintenance between the cache and main memory, playing an important role in speeding up data access.

6. What are the common operating system process scheduling strategies?

First-Come, First-Served (FCFS): Schedules processes in the order they arrive; the first to arrive is executed first.
Shortest Job Next (SJN): Selects the process estimated to have the shortest execution time to execute first, reducing the average waiting time.
Priority Scheduling: Assigns a priority to each process, determining the next process to execute based on priority. It can be static priority set at creation or dynamic priority that changes during execution.
Round Robin (RR): Divides CPU time into fixed-size time slices, allowing each process to use the CPU in turn. When the time slice expires, the currently executing process is suspended and placed at the end of the ready queue.
Multilevel Feedback Queue Scheduling: Divides the ready queue into multiple queues, each with different priorities. Newly arrived processes are placed in the high-priority queue first, and if there are no other runnable high-priority tasks, tasks in the low-priority queue get execution opportunities. Tasks in the same queue are executed in a round-robin manner.
Shortest Remaining Time Next (SRTN): Based on Shortest Job First, dynamically adjusts the execution order based on remaining execution time to further reduce waiting time.

7. What is the I/O subsystem hierarchy?

User Interface Layer: This is the top layer of user interaction, providing methods for interacting with users, such as graphical interfaces and command line interfaces. At this layer, users can initiate I/O requests in various ways.
Device Independence Layer: This layer provides abstraction and a general interface for devices, allowing upper-layer applications to perform I/O operations independently of specific hardware devices. It manages different types of device drivers and provides a unified access interface to upper-layer applications.
Device Driver Layer: This part interacts directly with hardware devices, with each specific device having a corresponding driver. Device drivers control and manage devices through low-level protocols and hardware interfaces, providing a simple and unified interface to the upper level.
I/O Controller Layer: This layer consists of physical devices, such as disk controllers and network adapters. They are responsible for transferring data between main memory and external devices or vice versa, performing related data conversion and processing operations.
Bus/Media Controller: The bus and media controller serve as a bridge connecting devices and I/O controllers. The bus is responsible for data transfer and communication, while the media controller handles specific types of media (such as disks and optical drives).

8. What are the differences between logical addresses, linear addresses, physical addresses, bus addresses, and virtual addresses?

Logical Address: The logical address is the address used in the program, specified by the programmer. It is an abstract representation relative to the program itself and is independent of actual memory or devices.
Linear Address: The linear address (also known as virtual address) is the address obtained after conversion through segmentation and paging mechanisms. The segmentation mechanism converts logical addresses into linear address space, and the paging mechanism maps the linear address to physical memory.
Physical Address: The physical address is the actual hardware physical location used when accessing memory. When the CPU issues an access request, the linear address is translated into a physical address through mapping relationships at the hardware level to find the corresponding data or instruction’s physical location.
Bus Address: The bus is the pathway for communication and data transfer between different components in a computer system. The bus has multiple signal lines, including those for addressing and data transmission. The bus address is sent on these signal lines to select specific resources or operations needed by memory or other devices.
Virtual Address: The virtual address is an abstract address provided by the operating system before the linear address is mapped to physical memory through the paging mechanism. It provides applications with a virtual view of system resources, unrestricted by the actual physical memory layout.

9. What are the ways of memory management in operating systems, and what are their advantages and disadvantages?

Single Continuous Partition: Divides the entire physical memory into a single continuous area shared by all processes. The advantage is simplicity and ease of implementation, but the disadvantage is wasted memory space and inability to support multitasking.
Fixed Partition: Divides physical memory into several fixed-size partitions, each usable for running one process. The advantage is that multiple processes can run simultaneously, but it has internal fragmentation issues and cannot adapt to varying process size requirements.
Dynamic Partition: Dynamically divides physical memory based on process size, using bitmap or linked list data structures to manage occupied and unoccupied memory blocks. The advantage is more flexible utilization of memory space, but it may produce external fragmentation issues.
Paging: Divides logical address space and physical address space into fixed-size pages and performs address translation using page tables. The advantage is more efficient memory space utilization, but it incurs additional overhead, such as page table lookups and TLB misses.
Segmentation: Divides logical addresses into several segments based on program structure and uses segment tables for address translation. The advantage is better reflection of program structure, but it also needs to handle external fragmentation issues.
Segmented Paging: Combines segmentation and paging, first performing segmentation and then dividing each segment’s address space into pages. The advantage is a combination of the benefits of segmentation and paging, but it increases complexity and overhead.

Different memory management methods have their own advantages and disadvantages. The appropriate choice depends on system requirements and hardware platforms:

Simple continuous partitioning is suitable for simple embedded systems or environments that only need to run one process.
Fixed partitioning is suitable for processes of predictable fixed sizes and numbers, making it more appropriate when resource demands are relatively fixed.
Dynamic partitioning is suitable for multitasking systems, allowing more flexible use of memory resources.
Paging and segmented paging are suitable for virtual memory systems, better meeting multitasking and dynamic loading demands.

10. What are the communication methods between user space and the kernel?

System Calls: User space programs can request the kernel to execute privileged operations or obtain services provided by the kernel through system call interfaces. System calls are a common and widely used means of communication between user space and the kernel.
Interrupts: Hardware devices or software events can trigger interrupts, causing the processor to switch from user space to kernel mode and execute the corresponding interrupt handler. Through interrupts, user space can interact with the kernel, such as handling input/output operations.
Signals: Signals are a lightweight communication mechanism used to notify processes of certain events. User space can send signals to itself or other processes, which are handled by the kernel. Common signals include SIGINT (terminate process), SIGTERM (normal termination of process), etc.
Shared Memory: Shared memory allows multiple processes to access the same physical memory area, achieving efficient data sharing. User space needs to use specific functions to map the shared memory area and communicate with other processes through read/write operations.
Pipes: Pipes are a half-duplex, byte-stream-based communication mechanism. They can pass data between related processes. User space can create pipes and write to or read from them to communicate with other processes.
Files: User space can access special files provided by the kernel through the file system, such as device files (/dev), to communicate with the kernel. User space can open and read/write these files to perform corresponding operations.

11. What exactly does the kernel do when the API read()/write() is called?

When a user space program calls the read() function, the kernel performs the following series of operations:

Parameter Validation: The kernel validates the parameters passed to the read() function, including the file descriptor and buffer pointer. If the parameters are invalid or illegal, the kernel returns an error code.
File Lookup: Based on the file descriptor, the kernel looks up the corresponding file table entry in the kernel. This entry holds information related to the file, such as file offset, access permissions, etc.
Permission Check: The kernel checks whether the process has permission to read the file. If not enough permissions exist, the kernel returns the corresponding error code.
Buffer Allocation: The kernel allocates a temporary buffer for the read() operation and associates it with the current process’s address space.
Data Transfer: The kernel reads data from the file and copies it into the previously allocated buffer. This involves disk I/O operations and copying data between kernel space and user space.
Update Offset: After each read is completed, the kernel updates the offset in the file table entry to reflect where the next read should start from.
Return Result: Finally, the read() function returns the actual number of bytes read. If an error occurs, it returns the corresponding error code.

Similarly, when a user space program calls the write() function, the kernel performs similar operations, including parameter validation, file lookup, permission checks, buffer allocation, data transfer, etc. The difference lies in that data is written from the user space buffer into the file.

12. What is the purpose of system calls?

Access Hardware Devices: Through system calls, applications can request access to hardware devices, such as disks, network interface cards, printers, etc. This allows applications to avoid directly managing and controlling hardware, instead relying on the operating system as an intermediary.
File Operations: File reading and writing are essential functions for most applications. Through system calls, applications can request to open, create, read, write, and close files. The kernel is responsible for managing file access permissions, caching, disk I/O, and other low-level details.
Process Control: Through system calls, applications can create new processes, destroy processes, wait for process state changes, and perform inter-process communication (IPC). This enables multitasking and allows processes to cooperate and communicate with each other.
Memory Management: Applications need to dynamically allocate and release memory to store data. Through system calls, applications can request memory space from the operating system or release it back to the operating system for management.
Network Communication: Common socket operations in network programming, such as establishing connections, sending data, and receiving data, are all accomplished through system calls. Applications can use system calls to request network communication functionalities.
Security and Permission Control: The operating system is responsible for protecting computer resources from unauthorized access. Through system calls, applications can request authentication, permission checks, and execution of privileged operations to ensure the security of the computer system.

13. How are the Boot loader, Linux kernel, and root filesystem related?

When starting a Linux system, there is a close relationship between the Boot loader, Linux kernel, and root filesystem.

Boot loader: The Boot loader is the software located between the computer firmware and the operating system, responsible for loading and executing the operating system at startup. Common Boot loaders include GRUB, LILO, etc. The Boot loader first initializes the hardware and takes control of the computer. Then, it loads the Linux kernel into memory.
Linux Kernel: The Linux kernel is the core part of the operating system, responsible for managing computer hardware resources and providing various system services. Once loaded into memory by the Boot loader, the Linux kernel begins execution and completes a series of initialization tasks, such as setting up the process scheduler, initializing device drivers, and establishing a virtual filesystem. The kernel also detects and mounts the root filesystem.
Root Filesystem: The root filesystem is the basic directory structure of the Linux operating system, containing all other files and directories. It is usually stored on disk and mounted to associate with the operating system. After the Linux kernel starts, during the initialization process, it searches for and mounts the root filesystem as the starting point for the entire operating system. Depending on different configurations, the root filesystem can use different formats (such as ext4, XFS, etc.) to organize data.

Thus, during the Linux boot process, the Boot loader first loads the Linux kernel into memory and hands over control to it. The Linux kernel then initializes and mounts the root filesystem, allowing the operating system to access and manage files through the root filesystem. This way, the entire Linux system can start smoothly and run normally.

14. What are the two stages of the Bootloader startup process?

The Bootloader startup process can be divided into two stages: the bootloader stage and the kernel boot stage.

Bootloader Stage: At computer startup, the firmware (such as BIOS or UEFI) hands over control to the bootloader. The bootloader is located between the firmware and the operating system and is responsible for initializing hardware and loading and executing the operating system kernel. In this stage, the bootloader performs the following steps:

Initialize hardware devices: This includes setting up and detecting hardware such as the processor and memory.
Select the operating system: If multiple operating systems exist, the bootloader may provide a menu for the user to choose from.
Load kernel image: Reads the Linux kernel image file from the disk and loads it into memory.
Set kernel parameters: To correctly start the kernel, the bootloader may need to set some parameters, such as the location of the root filesystem.

Kernel Boot Stage: Once the bootloader loads the kernel into memory, it jumps to the kernel’s entry point and hands over control to the kernel. In this stage, the Linux kernel starts executing and completes the following tasks:

Initialize system resources: The Linux kernel initializes various device drivers, establishes the process scheduler, etc.
Mount the root filesystem: The Linux kernel searches for and mounts the root filesystem as the starting point for the entire operating system.
Start the init process: Once the root filesystem is mounted, the kernel starts the init process, which is the first process in user space.

Through these two stages, the Bootloader is responsible for transitioning from computer startup to loading and executing the Linux kernel, gradually handing over control to the operating system.

15. What are the common Linux commands?

File and Directory Operations:

ls: List directory contents
cd: Change working directory
pwd: Display current working directory
mkdir: Create directory
touch: Create a file or update the file timestamp
cp: Copy files or directories
mv: Move files or rename files/directories
rm: Delete files or directories

File Viewing and Editing:

cat: View file content
less/more: View file content page by page
head/tail: View the beginning/end part of a file
nano/vi/vim: Text editors

File Permission Management:

chmod: Modify file permissions
chown/chgrp: Change file owner/group

System Information Queries:

uname: Display system information
top/htop: Monitor system resource usage in real-time
df/du: View disk space usage

Process Management:

ps/top: View process list and resource usage
kill: Terminate a process

Network Related:

ifconfig/ip addr: Display network interface information
ping/traceroute: Test network connectivity and routing paths
curl/wget: Download network resources – ssh/scp/rsync: Remote login, copy, and synchronize data

16. What is a Shell script?

A Shell script is a script language used for writing batch commands and automating tasks. In Linux and Unix systems, the Shell is the interface between the user and the operating system kernel, interpreting and executing user-input commands. A Shell script combines a series of Shell commands in a specific syntax and order to achieve some complex functionality.

Using Shell scripts, various tasks such as file operations, process management, and system configuration can be performed. By writing scripts, commonly used operations and processes can be encapsulated, simplifying repetitive tasks and improving work efficiency. Additionally, Shell scripts can implement flexible logical processing through control flow statements such as conditional statements and loops.

Common Shell script languages include Bash (Bourne Again SHell), Csh (C SHell), Ksh (Korn SHell), etc., among which Bash is the most commonly used and widely supported. Writing a simple Shell script only requires using a text editor to create a file with a .sh suffix, specifying which type of Shell interpreter to use at the beginning of the file to start writing code.

17. What do GCC, GDB, and makefile do?

GCC stands for the GNU Compiler Collection, which is a widely used set of programming language compiler tools. GCC supports multiple programming languages, such as C, C++, Objective-C, Fortran, etc., and can be used on multiple operating systems. It converts source code into machine code or intermediate code to generate executable files or libraries.

GDB stands for the GNU Debugger, which is a powerful debugging tool that helps developers locate and resolve errors in programs. GDB allows you to run programs and provides various debugging functionalities, such as setting breakpoints, viewing variable values, and tracing function call stacks. By working with the compiler, GDB can pinpoint the line of code where an error occurred and help analyze the problem.

A Makefile is a text file that describes the dependencies between source code files and the compilation rules. Makefiles are typically used with the make command to automate the building of software projects. By defining targets, dependencies, and corresponding commands, when the make command is executed, it checks which files have been modified or need to be recompiled and automatically builds the entire project according to the rules defined in the Makefile. Makefiles can also include other configuration options and parameters, making the build process more flexible and customizable.

18. What is a Makefile?

A Makefile is a text file containing a series of rules and instructions that describe the dependencies between various source code files in a software project and how to compile and build these source code files. Makefiles are typically used with the make command to automate the building of projects.

In a Makefile, you can define targets, dependencies, and corresponding build commands. A target is a file to be generated or a task to be executed; dependencies indicate other files or tasks that a target depends on; and the build commands describe how to generate the target based on its dependencies. When the make command is executed, it checks which files have been modified or need to be recompiled and performs the corresponding build operations according to the rules defined in the Makefile.

By using Makefiles, we can easily manage complex software projects, avoid recompiling unchanged parts, and achieve automated and customizable build processes.

19. What are the methods of inter-process communication?

Pipes: A pipe is a half-duplex communication method that can pass data between parent and child processes or between processes with a common ancestor.
Named Pipes: Similar to pipes, but can be used for communication between unrelated processes.
Semaphores: Semaphores are used to implement synchronization and mutual exclusion access to shared resources, counting among multiple processes and controlling access.
Shared Memory: By mapping a certain area of memory into the address space of multiple processes, shared memory allows different processes to share access to that memory area.
Message Queues: Message queues provide a message buffer, allowing different processes to communicate by sending and receiving messages to/from the message queue.
Signals: Signals are an asynchronous notification mechanism used to send signals to target processes when specific events occur.
Sockets: Sockets are a commonly used IPC mechanism in network programming, allowing processes on different hosts to communicate.

20. What are the synchronization mechanisms between threads?

Mutex: A mutex is used to protect shared resources, allowing only one thread to access the resource at a time; other threads must wait.
Reader-Writer Lock: A reader-writer lock can support multiple read operations or a single write operation simultaneously, improving concurrency performance.
Condition Variable: Condition variables are used to implement waiting and notification mechanisms between threads, allowing execution to continue only when certain conditions are met.
Semaphore: A semaphore can limit the number of threads accessing a certain resource simultaneously and provide mutual exclusion access to the resource.
Barrier: A barrier ensures that multiple threads are blocked until all threads reach a certain point before continuing execution.
Atomic Operation: An atomic operation is an indivisible operation that can achieve data synchronization and access control in a concurrent environment.

21. What is the difference between threads and processes?

Resource Occupation: Each process has its own independent address space, file descriptors, and other system resources, while threads share the resources of the same process. This makes the overhead of switching between threads smaller than that between processes.
Execution Unit: Each process has its own execution environment and code, acting as an independent entity. In contrast, threads are attached to specific processes and share the same code and data segments.
Communication Mechanism: Communication between processes requires using mechanisms provided by the operating system, such as pipes, message queues, shared memory, etc. In contrast, threads can communicate directly by reading and writing shared variables since they share the same process’s address space.
Scheduling: Scheduling refers to the operating system allocating CPU time slices for processes or threads to execute. For multiple processes, the operating system is responsible for scheduling decisions; for multiple threads, since they belong to the same process, context switching and scheduling can be more efficient.
Creation and Destruction Overhead: Creating or destroying a new thread incurs much less overhead than creating or destroying a new process.

22. What is a deadlock? What are the causes of deadlock?

A deadlock is a state in concurrent computing where two or more processes (or threads) cannot continue executing because they are waiting for each other to release resources. The main causes of deadlock are as follows:

Mutual Exclusion Condition: Resources are exclusively allocated, meaning only one process (or thread) can use a resource at a time.
Hold and Wait Condition: A process (or thread) holds certain resources while requesting additional resources.
No Preemption Condition: Resources allocated to a process (or thread) cannot be forcibly reclaimed and can only be released voluntarily by the holder.
Circular Wait Condition: A set of processes (or threads) exists where each process is waiting for a resource owned by the next process in the cycle.

When all four conditions are met simultaneously, deadlock may occur. When deadlock occurs, these processes (or threads) will be in an infinite waiting state, and the system cannot resolve this state on its own. To prevent and resolve deadlock issues, the following strategies can be adopted:

Break the Mutual Exclusion Condition: Allow multiple processes to share certain resources, such as shared files.
Break the Hold and Wait Condition: Require processes to release all resources they hold before requesting additional resources, rather than requesting all resources at the beginning.
Break the No Preemption Condition: Allow the operating system to reclaim resources held by processes when necessary.
Break the Circular Wait Condition: Establish a linear order for all resources and require processes to request resources in that order, avoiding circular dependencies. Resource preallocation strategies can also be adopted.

The goal of the above methods is to break any of the conditions for deadlock occurrence, thus preventing or resolving deadlock states. However, when designing concurrent systems, it is essential to plan resource usage and request methods reasonably to minimize the likelihood of deadlock.

23. What are the four necessary conditions for deadlock?

Mutual Exclusion Condition: At least one resource is exclusively allocated to a process or thread, meaning only one process or thread can use it at a time. Other processes or threads must wait for that resource to be released.
Hold and Wait Condition: A process or thread holds certain resources while requesting additional resources that are occupied by other processes or threads. When multiple processes hold some resources and are waiting for others, deadlock may occur.
No Preemption Condition: Resources already allocated to a process or thread cannot be forcibly reclaimed; they can only be released voluntarily by the holder. In other words, any resource that has been acquired cannot be forcibly taken away.
Circular Wait Condition: There exists a set of processes or threads where each process is waiting for a resource held by the next process, forming a circular dependency. For example, P1 waits for resources held by P2, P2 waits for resources held by P3, and P3 waits for resources held by P1.

If all four conditions are satisfied simultaneously and no external intervention breaks any of them, a deadlock state will occur. Therefore, when designing concurrent systems, it is necessary to avoid deadlock or adopt corresponding strategies to resolve deadlock states.

24. What are the methods for handling deadlock?

Deadlock Prevention: Prevents deadlock by breaking one of the necessary conditions for its occurrence. Common methods include resource allocation strategies, safe sequence algorithms, and avoiding cycles.
Deadlock Avoidance: Dynamically detects whether operations will lead to deadlock based on the system state and resource request situation, avoiding operations that could lead to deadlock. This can be achieved through the Banker’s algorithm, resource allocation graphs, etc.
Deadlock Detection and Recovery: Periodically detects deadlocks in the system and takes measures to resolve the deadlock state once detected. Common methods include resource deprivation, process termination, and recovery.
Deadlock Ignorance: In specific cases, if deadlock occurs but the impact is minimal or the cost of handling it is too high, it may be ignored or tolerated. For instance, in some embedded systems, to simplify design and reduce costs, not all potential deadlock situations are handled.

Each method has its applicable scenarios and limitations, and choosing the appropriate handling method requires comprehensive consideration of system needs, resource utilization, performance, and reliability.

25. How to prevent deadlock from occurring?

Break the Mutual Exclusion Condition: Ensure that different processes or threads do not hold the same lock simultaneously when accessing resources. For example, use shared resources instead of exclusive resources to avoid mutual exclusion.
Break the Hold and Wait Condition: Require processes to release all resources they have acquired before requesting additional resources, thus avoiding situations where a process holds some resources while waiting for others.
Break the No Preemption Condition: Allow the system to forcibly reclaim certain resources held by processes to meet the demands of other processes. This means that an executing process may be interrupted and paused.
Break the Circular Wait Condition: To avoid deadlock, all resources that may be used in the system should be globally numbered, and each process should request resources in the order of these numbers. In other words, a process should only request resources in a certain order, rather than arbitrarily.

The above are common methods for preventing deadlock. It is important to note that preventing deadlock may lead to decreased system performance or other issues, so it is necessary to comprehensively consider the system’s needs and constraints when choosing suitable prevention strategies.

26. What basic knowledge does networking cover?

Network Concepts and Architecture: Understanding the concepts, components, and different network architectures of computer networks, such as client-server models and peer-to-peer networks.
Protocols and Communication: Studying different network protocols, including the TCP/IP protocol suite, HTTP, FTP, DNS, etc., and understanding the data transmission process in networks.
IP Addressing and Subnetting: Learning the representation of IP addresses, including IPv4 and IPv6, and how to perform subnetting and routing configuration.
Network Devices: Mastering the functions and working principles of various network devices, such as switches, routers, firewalls, etc., and understanding their roles in the network.
Network Security: Understanding common network security threats and attack methods, learning to protect network security, including authentication, firewall settings, encryption technologies, etc.
Network Management and Monitoring: Learning how to manage and monitor network performance, including configuration and troubleshooting, using tools for performance monitoring and optimization.
Wireless Network Technologies: Understanding the principles of wireless local area networks (WLAN) and common standards, such as Wi-Fi and Bluetooth, and learning about wireless network configuration and security.
Cloud Computing and Network Virtualization: Mastering the basic concepts and service models of cloud computing, understanding network virtualization technologies such as SDN and NFV.

27. What is TCP programming?

TCP programming refers to the programming techniques used for network communication based on the Transmission Control Protocol (TCP). TCP is a connection-oriented, reliable transport protocol widely used in computer networks.

Using TCP programming, a reliable, bidirectional communication connection can be established between two computers on the network. Through a TCP connection, reliable data transmission can be performed, ensuring that data arrives in order and automatically handling loss, duplication, and errors. This enables developers to build various network applications, such as chat programs, file transfer tools, and remote login.

In TCP programming, it is typically necessary to create a server and one or more clients. The server listens on a specified port and waits for client connection requests. The client actively initiates connection requests and establishes connections with the server. Once a connection is established, both parties can perform data reading and writing operations.

TCP programming usually involves the following steps:

Create a socket: The server and client create their respective sockets.
Bind address and port: The server binds the socket to a specified IP address and port.
Listen for connections: The server begins listening for connection requests from clients.
Accept connections: The server accepts client connection requests and establishes a connection with them.
Data transmission: Data is transmitted through the established connection, including sending and receiving data.
Close the connection: After communication is complete, the server and client can close the connection.

Through TCP programming, reliable, connection-oriented network communication can be achieved, suitable for applications that require data integrity and order.

28. What is the difference between kernel mode and user mode?

Kernel mode and user mode are two different execution modes in operating systems used to distinguish code running at different privilege levels.

Kernel Mode:

In kernel mode, programs can execute all privileged instructions and access system resources.
Code running in kernel mode has the highest privileges and can directly access low-level hardware devices and core functionalities of the operating system.
Kernel mode is mainly used for internal operating system tasks, such as managing processes, file systems, and device drivers.
If errors or exceptions occur while running in kernel mode, the operating system can capture and handle them promptly.

User Mode:

In user mode, programs can only execute a restricted set of instructions and cannot directly access low-level hardware devices and operating system resources.
User programs run in user mode, considered external applications relative to the kernel.
User programs cannot directly manipulate system resources or call privileged instructions; they must request the operating system to perform these operations.
If a user program attempts to execute privileged instructions or access restricted resources, it triggers an exception, which is handled by the operating system.

29. What is the relationship between processes and threads?

Processes (Process) and threads (Thread) are the basic units of concurrent execution in operating systems, and they have a certain relationship.

A process can be seen as an execution instance of a program, including code, data, and resources. Each process has its own independent memory space, running in different address spaces, and cannot directly access each other’s memory.

A thread is an execution path within a process. A process can contain multiple threads, and these threads share the same memory space. In other words, multiple threads within the same process can access and modify the same variables and resources.

The following are the relationships between processes and threads:

Resource Ownership: Each process has its own independent address space, file descriptors, open files, and other resources, while threads share these resources of the process they belong to.
Execution Unit: Each process has at least one main thread (Main Thread) and can create multiple child threads. All threads share the code segment but have their own independent stack space.
Context Switching: In a multitasking environment, the operating system performs context switching to schedule different processes or threads for execution. When switching between multiple processes, the entire context information must be saved and restored; whereas switching between multiple threads only requires saving and restoring the thread’s context, which incurs less overhead.
Synchronization and Communication: Sharing data and communication between processes requires specific mechanisms (such as pipes, message queues, shared memory, etc.), while threads can communicate through shared memory and directly read and write variables.

30. What are the communication methods between user space and kernel?

System Calls: User space programs can request services or execute privileged operations from the kernel through system calls. System calls are a mechanism that switches control from user space to kernel space and passes parameters for processing. For example, operations like opening files and reading/writing files can be achieved through system calls.
Inter-Process Communication (IPC): IPC is a mechanism for exchanging data and communication between different processes, and communication can also occur between user space and kernel space. Common IPC methods include pipes, message queues, shared memory, semaphores, etc.
Memory Mapping: Memory mapping technology allows mapping a file or device into the virtual address space of user space, enabling user programs to directly read and write to that memory area. This achieves data transfer between user space and kernel buffers.
Reading/Writing Special Device Files: In some cases, user programs need to interact directly with hardware devices and can communicate with the kernel by reading or writing special device files. For example, opening device files like /dev/ttyS0 for serial communication.

It is important to note that when communicating between user space and kernel space, the correctness and safety of data must be ensured, and the relevant interfaces and specifications provided by the operating system must be followed. Different operating systems may have different communication methods and mechanisms, and the methods listed above are some common general methods.

Recommended Reading:

Understanding C++ Memory Management: Pointers, References, and Memory Allocation
Breaking the Norm: The New Data Structure Maple Tree in Linux Kernel
Diving Deep into Linux Kernel Source Code: Unveiling Its Amazing Architecture and Design
Efficient Network Communication Technology Revealed: Principles and Practices of Socket Programming
Decoding the Linux Kernel Magic: The Secret Effects and Applications of Memory Barriers
Linux Kernel Filesystem: A Storage Magic More Powerful Than Any God!
Memory Allocation No Longer a Mystery: In-Depth Analysis of malloc Function Implementation Principles and Mechanisms
Exploring Core Technologies of Network Communication: Handwriting a TCP/IP User-Space Protocol Stack to Boost Performance