Fork and pipe

Question 1

What is a process?

Answer 1

A running instance of a program

Question 2

What is the benefits of using multiple processes?

Answer 2

(1) Do multiple tasks at once
(2) If one process fails, others are still running
(3) Make use of existing processes

Question 3

What are the numbers corresponding to stdin, stdout, stderr?

Answer 3

stdin 0, stdout 1, stderr 2

Question 4

Which of stdin, stdout, stderr goes to the display?

Answer 4

stdout and stderr

Question 5

Where does the input of stdin come from?

Answer 5

Keyboard

Question 6

Describe the virtual memory layout of a process from top to bottom.

Answer 6

OS kernel space, stack (which grows down), empty space, heap (which grow up), bss, data, text

Question 7

What is BSS in the virtual memory layout?

Answer 7

BSS is uninitialized data.

Question 8

When a process is being created, what is this state called?

Answer 8

The process is idle

Question 9

When a process is waiting for resources, what is this state called?

Answer 9

The process is waiting.

Question 10

When a process ended and is waiting to be collect, what is this state called?

Answer 10

The process is a zombie.

Question 11

Describe the life cycle of a process.

Answer 11

Idle -> Ready -> Run -> Zombie
Run -> Wait (for resources) -> Ready ->
Run -> Ready

Question 12

What is a command to check the current running processes?

Answer 12

ps -f

Question 13

What is a system call?

Answer 13

A system call is a way for a program (in the user space) to request services from the OS (in kernel space). Some services including requesting OS managed resources like files, memories, devices.

Question 14

How does syscalls work depending on the programming language and the OS?

Answer 14

syscalls work the same in every programming language, but it is dependent on OS

Question 15

What is the difference between a normal function call and a syscall?

Answer 15

A syscall looks like a function call but it transfers control to the OS

Question 16

Describe what fork() does.

Answer 16

fork() creates a copy of the current process and starts it as a child.

Question 17

Describe what execv() and execl() does.

Answer 17

It takes an executable as the input, which completely replaces the current process (so in memory, the new code takes over). The executable starts running and the only way to terminate is through termination of the program. (Since the original process is replaced, this new process does not return to the calling program).

If execv() or execl() fails, it returns -1 and the main program continues to run.

Question 18

What does wait() and waitpid() do?

Answer 18

These system calls make a parent process wait for a child process to terminate. This allows the parent to manage child processes, retrieve their exit status, and properly cleanup resources. This also prevents creating zombie processes.

wait() waits until one child terminates.

waitpid() allows the parent to specify which child to wait for.

These two syscalls both return the terminated child’s PID.

Question 19

What does kill() do?

Answer 19

Send a signal (SIGTERM, SIGKILL, SIGINT) to another process

Question 20

Does each process have a parent?

Answer 20

Yes.

Question 21

What is a parent process’ id?

Answer 21

Ppid (Parent process identifier)

Question 22

How to get a process and its parent’s id?

Answer 22

getpid(), getppid()

Question 23

What is the reason to kill a process?

Answer 23

When a process is frozen or non-responding and it takes up too much resources.

Question 24

What does fork() return?

Answer 24

In the parent, fork() returns the process id of the newly created child.

In the child, fork() returns 0 to indicate that it is a child process.

Question 25

What happens to the parent process after calling fork()?

Answer 25

The parent continues executing concurrently.

Question 26

What does fork() do?

Answer 26

fork() creates a child process

Question 27

What does exec() do?

Answer 27

It executes a program in a process.

Question 28

What is the difference between different versions of exec()?

Answer 28

They provide different ways to pass command line arguments.

Question 29

What is argv and argv[0]?

Answer 29

argv is an array of arguments. argv[0] is the name of the program being executed

Question 30

What does SIGTERM do?

Answer 30

It asks a process to terminate.

Question 31

Can a process send a signal to any other process?

Answer 31

Yes, usually the parent sends signals to its children

Question 32

What happens when a parent process exits?

Answer 32

All of its children processes are terminated.

Question 33

What is good practice before exiting a parent process?

Answer 33

To kill and wait for the children to terminate before exiting.

Question 34

What are signals?

Answer 34

Signals are special messages sent to a process.

Question 35

What signal is it when we press ctrl-c?

Answer 35

SIGINT (terminal interrupt)

Question 36

Can SIGKILL be caught or ignored? What is one other signal like that?

Answer 36

No. SIGSTOP means stop executing and cannot be caught or ignored as well.

Question 37

What does SIGSEGV mean?

Answer 37

Invalid memory segment access

Question 38

What does SIGPIPE mean?

Answer 38

Writing on a pipe with no reader, broken pipe.

Question 39

What are the input arguments to waitpid()?

Answer 39

pid_t pid, int * status, int options

Question 40

What does waitpid() return?

Answer 40

If it successfully waits for the child to terminate, it returns the pid (pid_t type) of the child process. -1 for error. 0 If WNOHANG is specified as an option and no child process has yet terminated, waitpid() returns 0 immediately without blocking. It saves the exit status of the child in the status parameter. If options is set to 0, waitpid() will only return when a child process terminates (exits) and will not return for stopped or continued processes.

Question 41

What does pipe() do? Describe in detail.

Answer 41

It creates a one directional pipe. It sets up a pair of file descriptors that represent the two ends of the pipe: pipefd[0]: The read end of the pipe. pipefd[1]: The write end of the pipe. After creating a pipe, one process can write data to pipefd[1], and another process can read that data from pipefd[0]. This allows for inter-process communication. Usually, a parent process creates a pipe and shares it with a child. pipe() returns 0 if it is create successfully, -1 if there is an error.

Question 42

Draw out the flow of the pipe setup.

Answer 42

N/A

Question 43

Describe the blocking behaviour of pipe().

Answer 43

If a process tries to read from an empty pipe, it will block (wait) until data is written. Similarly, writing to a full pipe will block until space becomes available.

Question 44

What does dup2() do?

Answer 44

It duplicates a file descriptor, allowing redirection of stdin, stdout, stderr to a pipe or another file. int dup2(int oldfd, int newfd)

Question 45

What are the constant names for each input and output?

Answer 45

stdin (0) is STDIN_FILENO stdout (1) is STDOUT_FILENO stderr (2) is STDERR_FILENO

Question 46

What is the function to process CLI options? How to denote if we want no argument after flag k and an option value after -f?

Answer 46

getopt(argc, argv, "kf:")

Question 47

In "$ foo -s -t 10 eth.txt bigdo.txt", what are argc, argv[0], argv[1], etc?

Answer 47

argc is the number of CLI arguments argv[0] is "foo" the executable name argv[1] is -s argv[2] is -t argv[3] is 10 argv[4] is eth.txt (positional arguments) argv[5] is bigdo.txt (positional arguments)

Question 48

What to use when we have an error with getopt()?

Answer 48

optopt, a global variable used by getopt() to store the option character that caused an error