UNIX and C Processes

Question 1

What is POSIX?

Answer 1

Portable Operating System Interface; IEEE Computer Society standard from 1988 that enables developers to write portable applications across UNIX variants and other operating systems.

Question 2

What does POSIX define?

Answer 2

POSIX defines an application programming interface (API), command line shells, utility interfaces, and describes fundamental services needed for applications.

Question 3

What is the Single UNIX Specification?

Answer 3

A standard created by The Open Group in 1995 to ensure compatibility across UNIX platforms; technically conforms to POSIX.

Question 4

Which Linux distributions are officially certified UNIX systems?

Answer 4

Only Inspur K-UX and EulerOS are officially certified UNIX systems.

Question 5

What are examples of fully certified UNIX systems?

Answer 5

Oracle Solaris, HP-UX, IBM AIX, Apple MacOS X (version 10.5 onwards), Inspur K-UX, and Huawei EulerOS.

Question 6

What are the two fundamental elements of UNIX?

Answer 6

Everything in UNIX is either a file or a process.

Question 7

What is a PID?

Answer 7

Process IDentifier; a unique identifier assigned to every process in a UNIX system.

Question 8

What are the parent-child relationships in UNIX processes?

Answer 8

Every process has exactly one parent (except the system swapper with PID 0), and can have zero or more children.

Question 9

What is the system swapper/init process?

Answer 9

The system swapper has PID 0 and is the ancestor of every process; init or systemd (PID 1 on Linux) is part of the Linux kernel.

Question 10

How is a child process created in UNIX?

Answer 10

A child process is created (“spawned”) when the system fork command is called.

Question 11

What does the ps command do?

Answer 11

Shows a snapshot of processes.

Question 12

What does the top command do?

Answer 12

Shows a real-time list of processes.

Question 13

What does & do when placed after a command?

Answer 13

Runs the process in the background.

Question 14

What does the jobs command show?

Answer 14

Lists the background child processes of the current process.

Question 15

What does the bg command do?

Answer 15

Puts a paused job into the background.

Question 16

What does the fg command do?

Answer 16

Brings a background job into the foreground.

Question 17

What does the kill command do?

Answer 17

Terminates a process.

Question 18

What does the nohup command do?

Answer 18

Keeps a child process running even when the parent process finishes.

Question 19

What does the nice command do?

Answer 19

Lowers the priority of a child process as you spawn it.

Question 20

What does the renice command do?

Answer 20

Lowers the priority of a current running process.

Question 21

How do you show all processes from the same terminal?

Answer 21

ps

Question 22

How do you show full details of your processes?

Answer 22

ps -f

Question 23

How do you show all processes whose command line includes “python” (ignoring case)?

Answer 23

ps -ef | grep -i python

Question 24

How do you show the parent/child hierarchy of all processes?

Answer 24

ps -efH | less

Question 25

What's the difference between foreground and background processes?

Answer 25

Foreground: shell waits until process finishes. Background: user can continue interacting with the shell while process runs.

Question 26

How do you run a program in the background?

Answer 26

Add & after the command (e.g., tempsleep &)

Question 27

How do you put a foreground process into the background?

Answer 27

Press CTRL-Z to stop it, then use bg %[job number]

Question 28

What happens to background processes when the parent process finishes?

Answer 28

They are killed unless protected with nohup

Question 29

How do you kill a process using its job number?

Answer 29

kill %[job number]

Question 30

How do you kill a process using its PID?

Answer 30

kill [PID]

Question 31

How do you start a process with lower priority?

Answer 31

nice [command]

Question 32

How do you change the priority of a running process?

Answer 32

renice -n [value] -p [PID]

Question 33

How do you enable a child process to continue running after logout?

Answer 33

nohup [command] &

Question 34

What does the fork() function do in C?

Answer 34

Creates a duplicate process (child) of the calling process (parent).

Question 35

What value does fork() return in the parent process?

Answer 35

The PID of the child process.

Question 36

What value does fork() return in the child process?

Answer 36

0

Question 37

What value does fork() return if it fails?

Answer 37

A negative value

Question 38

What elements do parent and child processes share after fork()?

Answer 38

They share variable values from before the fork and any open file descriptors (e.g., stdout)

Question 39

How does a parent wait for a specific child process to finish?

Answer 39

Using waitpid(pid, &status, 0)

Question 40

How does a parent wait for any child process to finish?

Answer 40

Using waitpid(0, &status, 0)

Question 41

What flag makes waitpid() return immediately if no child has exited?

Answer 41

WNOHANG

Question 42

What header file is required for waitpid() function?

Answer 42

#include

Question 43

What do the exec family of functions do?

Answer 43

Replace the current process with a new program.

Question 44

Does code after a successful exec call ever execute?

Answer 44

No, the process is completely replaced. Code after exec only executes if exec fails

Question 45

What is the prototype of the execvp function?

Answer 45

int execvp(const char *file, char *const argv[]);

Question 46

What is a pipe in Linux/C?

Answer 46

A unidirectional flow of data between two processes.

Question 47

When must a pipe be opened relative to the fork?

Answer 47

Before the fork, so that parent and child share it.

Question 48

How is a pipe created in C?

Answer 48

Using the pipe(fd) function, where fd is an array of two file descriptors.

Question 49

What are the file descriptors in a pipe used for?

Answer 49

fd[0] is the reading end (front) of the pipe, and fd[1] is the writing end (rear) of the pipe.

Question 50

Are pipes buffered?

Answer 50

No, pipes are unbuffered.

Question 51

What is a signal in UNIX/Linux?

Answer 51

A software interrupt delivered to a process to notify it of an event.

Question 52

What does the kill command actually do?

Answer 52

It sends a signal to a process (by default, SIGTERM).

Question 53

What signal does CTRL-C send to a process?

Answer 53

SIGINT (interrupt signal)

Question 54

Which signal cannot be caught by a program?

Answer 54

SIGKILL (causes immediate termination by the kernel)

Question 55

How can a program set up a handler for a signal?

Answer 55

Using the signal() function to register a handler function for a specific signal

Question 56

How do you define a signal handler in C?

Answer 56

Create a function like void sig_handler(int signo) and register it with signal(SIGNAL, sig_handler)

Question 57

How do you send a USR1 signal to a process?

Answer 57

kill -USR1 [PID] or kill -USR1 %[job number]

Question 58

What signals can programmers use for their own purposes?

Answer 58

SIGUSR1 and SIGUSR2

Question 59

Can a program catch a SIGSEGV signal?

Answer 59

Yes, though handling segmentation faults properly is complex.

Question 60

What are three ways processes can communicate?

Answer 60

Signals, files, and pipes.

Question 61

Which communication method requires knowing the PID of the target process?

Answer 61

Signals

Question 62

Which communication method allows data transfer but requires processes to be parent/child or siblings?

Answer 62

Pipes

Question 63

Which communication method allows data transfer between any processes?

Answer 63

Files (though file locking must be considered)

Question 64

Which communication method is asynchronous (receiving process doesn't need to keep checking)?

Answer 64

Signals