Process Environment

Question 1

What is a process and how does the OS simulate parallel processing?

Answer 1

A process is a program in execution.

The OS runs processes concurrently by interleaving their execution.

Question 2

What are the different components of the process state?

Answer 2

• Unique ID
• Private address space
  
  • stack
  • heap
  • code and data
• Registers for context switching
• Resources
  
  • Files
  • Sockets

Question 3

How do you get the processID and what are the header files needed for this?

Answer 3

include <sys></sys>

The header files needed for this are:

You get the process ID by using the getpid() function

Question 4

What happens when a parent creates a child process using the fork function?

Answer 4

When the parent creates a child process using the fork function then the child

• Copy of the parent’s address space
• Any open files in the parent are available to the child.

Question 5

Why does fork return twice?

Answer 5

Fork returns twice, once for the parent and once for the child.

When it returns for the parent, it returns the process of the child and when it returns for the child, it returns 0.

This can be used to distinguish the parent process from the child process.

Question 6

What happens when the parent terminates before the child?

Answer 6

When the parent terminates before the child, then the child is assigned to ( or rather adopted lol ) by the init process.

Question 7

How does the parent know when a child has terminated? ( without waiting )

Answer 7

The parent is notified by the kernel through an asynchronous message known as a signal.

Question 8

What happens when you call wait inside a parent?

Answer 8

The parent blocks until a child has been terminated.

If there are no children, then wait returns immediately.

Question 9

What is a zombie process?

Answer 9

This is a process that has finished executed but it has an entry in the process table. The process has terminated but is not waited on by the parent.

Question 10

What happens when a process is a zombie process?

Answer 10

• It remains in the process table
• Its memory and resources are de-allocated
• It can be reaped, when the parent waits on it.

Question 11

Why should you be worried about zombie processes?

Answer 11

Too many zombie processes prevent the creation of new processes.

Question 12

What are the different exit functions to terminate a program?

What does atexit do and what is its prototype?

Answer 12

The different exit functions are:

• exit: Clean assassination, calls all the exit handlers
• _exit: Merciless exit, directly returns to the OS
• _Exit: same as above
• abort:
• atexit: allows you to register exit handlers that can are called during the termination of the program.
• Prototype: int atexit(void(*function)(void))

Question 13

atexit adds exit handlers to a stack of exit handlers, what does this mean for the order of registering exit handlers?

Answer 13

This means the exit handler that you want to have executed first should be registered the last, because the last one in will be executed first (LIFO) .

Question 14

What are environment variables?

Answer 14

These are key value pairs defined in the command shell.

Question 15

What is the prototype of getenv and what does it do?

Answer 15

char* getenv(char* varname):

This function gets the value of the variable with name varname

Question 16

Which component is responsible for including libraries inside a program?

Answer 16

The libraries are included inside the user code with the help of the linker file. The linker checks the set of know directories for the library.

Question 17

What is a static library?

Answer 17

A static libraries are created when the library itself is copied and inserted into the user code. This means that any changes to the libraries requrie relinking of the library.

Question 18

What are Shared or dynamic Libraries?

Answer 18

These are libraries that are not copied into the executable file but rather is refered to by executables.

If the libraries change then there is no need to relink.

Question 19

What is dynamic linking?

Answer 19

Link to the libraries during runtime.

Question 20

Describe a process’ address space.

Answer 20

Process address space is the memory that is allocated to a process by the OS

A process

• address space is Contiguous in memory
• executes in the virtual memory
• address space is known as virtual address space.

Question 21

how does the size of the address space depend on the architecture?

Are all addresses in this range accessible?

Answer 21

2^(the number of bits)

16 bit System = 2^16

32 bit System = 2^32

Not all addresses in this range are accessible

Question 22

what is kernel space and how is it different from user space?

Answer 22

Kernel space is the part of virtual address space where kernel processes run on.

User space is the part of the Virtual address space where the normal applications run.

The role of the kernel space is to manage applications running in the user space.

Question 23

how is a process divided in terms of memory?

Answer 23

A process is divided into memory segments.

Question 24

1. Each process has its own address space
2. The total of all virtual addresses is greater than the physical memory
3. Virtual address is actually disconnected from physical memory
4. Each signal is identified by a unique integer value
5. You can send signals from the terminal using the kill command.
   
   1. kill -;
   2. …
      6.

Question 25

What are the consequences or process separation?

Answer 25

1. One process can't peek into another process' data 2. it is difficult to transfer any piece of data from one process to another.

Question 26

how many cpu rings are there and what is ring 0 and ring 3 used for?

Answer 26

There are 4 cpu rings. RIng 0: Kernel space has this, this has full access including the physical memory and it can edit page tables Ring 3: ( User space has this ring ), this ring has indirect access to memory via page tables, cannot edit the page tables and also does not have permissions for some CPU instructions.

Question 27

Why does the OS set a timer interrupt before giving up control to the user processes?

Answer 27

This is because the user processes cannot be trusted to restore control back to the OS.

Question 28

What are system calls? What are some resources that are requested?

Answer 28

These are programmatic means of requesting privileged resources. Some resources are: - Communication - File management - Process control

Question 29

When are memory faults or segfaults encountered?

Answer 29

They are encountered when the process reads a memory address that is invalid or not currently mapped in the page table

Question 30

How do you make a page shareable with other processes and give an example where page sharing is used?

Answer 30

In order to make pages shareable, you need to make them read only. Read only pages can be shared among processes without any risk. They are used for shared libraries.

Question 31

What system call do you use in order to replace the current program with a new program.

Answer 31

The family of exec functions can be used for this.

Question 32

What happens to the process ID when you call the exec function, what happens ot the stack, heap and so on...?

Answer 32

The process ID is retained but the stack, heap and data are cleared Any open file descriptors are retained

Question 33

What is the prototype of the exec function and describe the usage, what does it return?

Answer 33

int execv(char\* path, char\* argsv[]) path: The name of the executable file argsv: the argument list that is passed to the main function. * The first argument should be the name of the exe * The last argument should be null Exec functions return on failure, or the just replace the current program with tne new program

Question 34

What is the copy on write feature and why is it even used?

Answer 34

Copy on write feature is basically making the parent and the child ( created after forking ) share the same address space. The pages of the parent will only be copied and given to the child if there is any attempt to write by the child. It is used because, usually the child does not change many of the pages so it is a waste copy all of these pages

Question 35

What is overcommitting?

Answer 35

When the OS promises more memory the processes than it has physically

Question 36

What are half duplex pipes?

Answer 36

Half duplex pipes are data channel that allow one way communication between a parent and a child.

Question 37

How do you create a pipe and what is the prototype?

Answer 37

Prot: int pipe ( int fd[2] ); fd[0] and fd [1] are the 2 endpoints of the pipes.

Question 38

What are signals?

Answer 38

Signals are asynchronous messages that are sent to the program to notify it that some event has occurred.

Question 39

Whar are async signal safe functions?

Answer 39

Signal safe functions are those that can be safely called from within a signal handler.

Question 40

What are the three types of signals and give examples for these.

Answer 40

The three types of signals are * Errors: * SIGSEGV * SIGINT * Notification * SIGALRM * SIGCHLD * Process Control * SIGKILL * SIGSTOP

Question 41

What are the different ways in which a process can behave when it receives a signal?

Answer 41

When a process receives a signal, it can behave one out of 4 ways * Terminate the program * Stop the program * Terminate the program and make a core dump * Ignore it.

Question 42

How do you send a signal to the same process, also mention the header file needed for this?( Prototype?)

Answer 42

You send a signal to the same process by using the raise function. int raise(int sig); The header file for this is

Question 43

How do you send a singal to a different process?

Answer 43

In order to send a signal to a different process, use the kill function. int kill(pid\_t process, int sig)

Question 44

What are some of the control commands that send the singals to foreground processes ( processes that are currently running )

Answer 44

They are: * ctri-c: * ctrl-z: * ctrl-\:

Question 45

How do you registe a signal handler?

Answer 45

you register a signal handler by using the signal function. signal(int signum, singnalhandler): NOTE: signal is deprecated and now the sigaction is used instead

Question 47

Which are the 2 signals that cannot be handled or ignored?

Answer 47

sigkill and sigstop

Question 48

What are some built in signal handlers for * ignoring a signal * default handler for a signal

Answer 48

To ignore the signal: SIG\_IGN Default handler: SIG\_DFL