kernel_programming

NOTE: You -> Userspace (application/Software) --> Shell --> Kernel --> Hardware(Pheriperal)

Reference: https://www.codingame.com/playgrounds/84444/running-u-boot-linux-kernel-in-qemu/pre-requisites

1. Process Management (CPU)

• Purpose: The kernel manages the execution of processes, including multitasking, process creation, termination, and context switching between processes.
• Details: It allocates CPU time to processes and handles process scheduling, ensuring that multiple processes can run simultaneously without interfering with each other.

2. Memory Management (RAM, HDD)

• Purpose: The kernel controls the system's memory, managing how memory is allocated to processes and ensuring efficient use of RAM.
• Details: It manages virtual memory, page swapping, and memory protection, making sure each process has its own memory space and that no process can access another process's memory.

3. Device Management (hard drives, printers, network interface)

• Purpose: The kernel manages communication between the system and its hardware devices, such as hard drives, printers, and network interfaces.
• Details: It includes drivers that provide the necessary instructions for controlling hardware devices and handles I/O operations, ensuring that data is correctly transferred between devices and the system.

4. File System Management

• Purpose: The kernel organizes and manages data storage on disk drives and handles file operations like reading, writing, and deleting files.
• Details: It ensures that files are stored and retrieved efficiently, and it provides file security and permissions to control access. NOTE: need a mind map

5. System Calls and Security

• Purpose: The kernel provides an interface for user applications to request services from the OS, such as file operations or network communication.
• Details: It also enforces security policies, ensuring that only authorized users and processes can access certain resources or perform specific actions.

Here’s a more detailed, technical look at each function of the kernel, including common command-line utilities and system call signatures relevant to each section:

1. Process Management

• System Call Example:
  • fork() - Creates a new process by duplicating the calling process.
  • exec() - Replaces the current process image with a new process image.
  • wait() - Makes the parent process wait until its child process terminates.
• Command-Line Utilities:
  • ps - Displays information about currently running processes.
  • top - Shows real-time information about system processes and resource usage.
  • kill - Sends a signal to a process, usually to terminate it.
  • nice / renice - Adjusts the priority of a process.
• Code Snippet (C):

  pid_t pid = fork();
  if (pid == 0) {
      // Child process
      execl("/bin/ls", "ls", NULL);
  } else if (pid > 0) {
      // Parent process
      wait(NULL);
  }

2. Memory Management

• System Call Example:
  • mmap() - Maps files or devices into memory.
  • brk() - Changes the end of the data segment, effectively allocating more memory.
  • munmap() - Unmaps a previously mapped memory region.
• Command-Line Utilities:
  • free - Displays the amount of free and used memory in the system.
  • vmstat - Reports information about processes, memory, paging, block IO, traps, and CPU activity.
  • pmap - Reports memory map of a process.
• Code Snippet (C):

  void *ptr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  if (ptr == MAP_FAILED) {
      perror("mmap failed");
  }

3. Device Management

• System Call Example:
  • ioctl() - Device-specific input/output operations.
  • read() - Reads data from a file descriptor.
  • write() - Writes data to a file descriptor.
  • open() - Opens a file descriptor for a device.
  • close() - Closes a file descriptor.
• Command-Line Utilities:
  • lsblk - Lists information about block devices.
  • dmesg - Prints or controls the kernel ring buffer (usually logs related to device activity).
  • hdparm - Gets and sets hard disk parameters.
  • lspci - Lists all PCI devices.
• Code Snippet (C):

  int fd = open("/dev/sda", O_RDONLY);
  if (fd < 0) {
      perror("Failed to open device");
  }
  // Read data from the device
  read(fd, buffer, sizeof(buffer));
  close(fd);

4. File System Management

• System Call Example:
  • open() - Opens a file and returns a file descriptor.
  • read() - Reads data from a file descriptor.
  • write() - Writes data to a file descriptor.
  • close() - Closes a file descriptor.
  • stat() - Retrieves file status information.
  • unlink() - Deletes a file.
• Command-Line Utilities:
  • ls - Lists directory contents.
  • df - Reports file system disk space usage.
  • du - Estimates file space usage.
  • mount / umount - Mounts and unmounts filesystems.
• Code Snippet (C):

  int fd = open("file.txt", O_RDONLY);
  if (fd < 0) {
      perror("Failed to open file");
  }
  struct stat file_stat;
  if (fstat(fd, &file_stat) < 0) {
      perror("Failed to get file status");
  }
  close(fd);

5. System Calls and Security

• System Call Example:
  • getuid() - Returns the real user ID of the calling process.
  • setuid() - Sets the user ID of the calling process.
  • chmod() - Changes the file mode bits (permissions) of a file.
  • chown() - Changes the owner and group of a file.
  • capget() / capset() - Manipulates capabilities of the calling process (Linux-specific).
• Command-Line Utilities:
  • chmod - Changes the file permissions.
  • chown - Changes the file owner and group.
  • sudo - Executes a command as another user, typically root.
  • iptables - Configures the Linux kernel firewall.
• Code Snippet (C):

  uid_t uid = getuid();
  printf("Current User ID: %d\n", uid);
  
  if (setuid(0) < 0) {
      perror("Failed to change user ID");
  }

These commands, system calls, and code snippets illustrate the kernel's role in managing processes, memory, devices, file systems, and security.