09 - Kernel and processes

Question 1

What is the primary role of an Operating System (OS) in the context of process management?

Answer 1

• The OS plays a central role in process management.
• It isolates applications to prevent them from affecting each other negatively.
• This isolation is fundamental to ensuring system reliability, security, privacy, and fairness.

Question 2

Define a process and a thread in the context of operating systems.

Answer 2

• A process is an instance of a running program with limited rights.
• A thread is a sequence of executed instructions within a process.
• A process can have potentially many threads.
• The address space of a process defines its accessible memory and other permissions.

Question 3

What hardware mechanisms support process protection in operating systems?

Answer 3

• Privileged instructions, available only to the kernel.
• Memory access limits to prevent user code from overwriting kernel space.
• Timers to regain control from user programs.
• Safe ways to switch between user and kernel modes.

Question 4

Explain the concept of dual-mode operation in the context of operating systems.

Answer 4

• Dual-mode operation includes Kernel mode (full hardware privileges) and User mode (limited privileges).
• Memory protection is achieved through different base and bound registers for kernel and user-level processes.
• This approach ensures safe control transfer between modes and protects kernel memory from user programs.

Question 5

What is the role of system calls in operating systems?

Answer 5

• System calls provide an interface for applications to interact with the operating system.
• They are used for various operations like process control, file management, device management, information maintenance, and communication.
• In Linux, the number of system calls has increased from around 60 in the first Unix version to over 300 in current versions.

Question 6

Explain the differences between system calls and library functions.

Answer 6

• System calls are provided by the system and are executed (mostly) in the system kernel.
  
  • They are entry points into the kernel and are therefore NOT linked into your program.
  • The source code is not portable
  • The API is portable
• Library calls include the ANSI C standard library and are therefore portable. These functions are linked into your program.

Question 7

How does the kernel handle system calls?

Answer 7

• The kernel uses a stub to locate arguments either in registers or on the user stack.
• It translates user addresses to kernel addresses and copies arguments from user memory to kernel memory.
• The kernel protects itself from malicious code and validates arguments to prevent errors.
• After executing the handler, it copies results back into user memory.

Slide 24, chapter 9

Question 8

What is User Level Process Management in UNIX?

Answer 8

• User Level Process Management allows processes to create other processes without the need for kernel recompilation.
• This capability is utilized by shell command line interpreters.
• It includes functionalities for job control, such as creation, killing, suspending, and resuming processes.
• Additionally, it provides a job coordination system for pipe-lining and sequencing of processes

Question 9

What happens when a program is compiled in UNIX?

Answer 9

• Compiling a program, like cc -c file1.c, creates a new process.
• The cc program reads the file and produces output.
• The shell waits for the completion of this process.
• The compilation process can be defined in a file, becoming a program

Question 10

Describe the kernel operations involved in process creation.

Answer 10

• The kernel allocates process data-structures.
• It allocates memory for the process.
• The program is copied from disk to the allocated memory.
• Stacks are allocated: user-level for functions and kernel-level for system-calls/interrupts.

Question 11

What occurs during process startup in UNIX?

Answer 11

• The kernel copies arguments from user memory (argc/argv).
• It copies the environment settings.
• Control is transferred to user mode using POP and IRET instructions.
• This occurs after manipulation of the kernel stack

Question 12

What steps are involved in process termination?

Answer 12

• The exit system call is invoked, typically inserted by the compiler.
• The kernel executes this call, leading to:
  
  • Freeing of stacks.
  • Freeing of process memory.
  • Freeing of kernel data-structures.
  • Notification to the “parent” process of the termination.

Question 13

What are the different policies for process creation in UNIX?

Answer 13

• Execution mode policies include whether the father and son processes execute in parallel or if the father blocks until the son terminates.
• Memory management policies determine if the son process gets new memory (data and code), new data, a shared copy of the father’s memory, or a non-shared copy.
• Resource sharing policies cover whether the father and son share all, some, or no resources (like open files, IPC objects, etc.).

Question 14

How does process creation work in Windows?

Answer 14

• The CreateProcess function is used to create and initialize the Process Control Block and the address space.
• It loads the program into the address space and copies arguments into process memory.
• The function initializes the hardware context and informs the scheduler of the new process.
• Further security configuration, like limiting privileges and changing priority, is necessary.

Question 15

What are the process creation policies in Windows?

Answer 15

• In Windows, the execution mode policy is that the father and son execute in parallel.
• For memory management, the son gets new memory (data and code).
• Regarding resource sharing, the father and son do not share most of the resources.

Question 16

What are the key steps in UNIX process management?

Answer 16

• UNIX process management involves two steps: copying the current process and executing a different program.
• Key system calls include fork for creating a copy of the current process, exec for changing the program run by the current process, wait for waiting for a process to finish, signal for sending notifications to other processes.

Question 17

What are the key policies in UNIX process creation?

Answer 17

• Execution mode: Parent and child processes can execute in parallel.
• Memory management: The child process can receive a non-shared or shared copy of the parent’s memory.
• Resource sharing: Parent and child processes can share all, some, or no resources (e.g., files, IPC objects).

Question 18

How does the UNIX fork system call function?

Answer 18

• The fork call creates a new process, duplicating the current process.
• It returns twice: once in the parent process (returns child PID) and once in the child process (returns 0).
• After fork, changes in variables are local to each process.

Question 19

Describe the execve system call in UNIX.

Answer 19

• execve() executes a program specified by a filename.
• It replaces the current process image with a new one, loading the program into the current address space.
• Arguments and environment variables are passed to the new program, and previous process state is lost.

Question 20

What is the purpose of the ‘system’ function in UNIX?

Answer 20

• The system function is used to execute a shell command in a new process.
• It combines fork and execve: the child process executes the command, and the parent waits for its completion.
• The function returns the exit code of the child process.

Question 21

How does UNIX handle the return codes of processes?

Answer 21

• The parent process can retrieve the child’s return code using wait or waitpid functions.
• A process is considered a zombie until its return code is collected by its parent.
• The operating system maintains information about the terminated process and its return code until the parent processes it.

Question 22

What is the function of posix_spawn in process creation?

Answer 22

• posix_spawn() creates a new child process that executes a specified file.
• It is used on systems that lack support for the fork(2) system call, typically on small embedded systems without MMU support.
• This function replicates shell behavior for process creation.

Question 23

What are zombies in UNIX and how are they handled?

Answer 23

• A zombie is a process that has completed execution but still has an entry in the process table.
• It remains until its parent calls wait to clean up and collect the exit status.
• If a parent process doesn’t call wait, the child remains a zombie, which can lead to resource depletion.

Question 24

How are processes identified in UNIX?

Answer 24

• Each process in UNIX is identified by a Process ID (PID), which is a unique number.
• Processes can find out their own PID using the getpid system call.
• The maximum value for PID is usually 32768.

Question 25

What are signals in UNIX and how do they function?

Answer 25

* Signals are a mechanism for handling asynchronous events in UNIX. * They inform a process that a particular event has occurred. * Examples include illegal memory access, process abortion (Ctrl-C), or system call-generated events (like a son process termination). * Processes interact with the kernel using system calls, while the kernel uses signals to interact with processes.

Question 26

What are the methods for handling signals?

Answer 26

* Polling: Processes check periodically for events, but it's inefficient and not always correct. * Upcalls: Kernel notifies processes of events, allowing for efficient and direct handling. * Signals can be handled by default (kernel action), ignored, or managed with a user-level handler.

Question 27

How are signals handled in UNIX?

Answer 27

* Default handling: The kernel handles the signal, usually terminating the process. * Ignoring the signal: The process does not acknowledge the signal. * Handling with a user-level handler: Custom code in the process executes in response to the signal. * Some signals like SIGKILL and SIGSTOP cannot be ignored or handled and must be executed by default.

Question 28

What are some examples of UNIX signals and their default actions?

Answer 28

SIGHUP (1): Terminates the process due to terminal line hangup. SIGINT (2): Terminates the process due to an interrupt program (Ctrl-C). SIGQUIT (3): Terminates the process and produces core dumps. SIGKILL (9): Kills the process. SIGALRM (14): Indicates a timer has expired. SIGCHLD (20): Ignored, indicates child status has changed. SIGSTOP (17): Stops the process. SIGCONT (19): Continues if stopped.

Question 29

How do you register a signal handler in UNIX?

Answer 29

* Use the signal() function from . * The function takes the signal code (SIGXXX) and a pointer to the signal handler function. * It can also take SIG_IGN for ignoring the signal or SIG_DFL for default handling. * The function returns the previous signal handler.