EXAM STYLE QUESTIONS ON USER SPACE

USER LAND QUESTIONS

Question 1 Int pthread_create(pthread_t *thread, const

pthread_attr_t *attr, void *(*start_routine)(void *), void *arg);

The 4th argument to pthread_create Will include the return value of the thread Will include the heap of the created thread Will include the heap of the created thread if

not NULL None of the above

Question 2 Datagram sockets

Are only usable in UNIX domain sockets Should not be used for internet streaming

protocols (such as RTP) Send packets that are never broken or spliced Cannot be used on Microsoft Windows

Question 3 Socket is used to

Create communication end point Connect to a server Bind a port Only by programs that work with the internet

Question 4 Open(2) can be used

To create a new file To open existing file To open a device All of the above

Question 5 mmap(2) should never be used

In threaded application In multi process application With socket as file descriptor None of the above

Question 6 + 7 Following are functions pairs. Please specify if they

are blocking (answers – a = both , b = 1st , c = 2nd d = none) (assume default flags)

Select(2), signal/sigaction(2) b

recv(2),scanf(3) a

Question 8+9

which of the following can we wait using select (a = both b=1st c = 2nd d= none)

stream socket and dgram socket a

POSIX Cond and large matrix multiplication d

Question 10 TCP server (AF_INET, SOCK_STREAM) calls functions

in the following order Socket(2), connect(2), accept(2),recv(2) Open(2), bind(2), connect(2), send(2) Socket(2), bind(2), listen(2), accept(2) Open(2), bind(2), connect(2), recv(2)

Question 11 UNIX domain sockets are

Faster the AF_INET socket Only work on the same host One of the many forms of IPC (Inter process

communication) All of the above

Question 12 What is wrong in the following code

While (time(NULL) >time_to_start) The following code will take 100% cpu while waiting

(busy waiting) The time(2) function is deprecated The time(2) function is not thread safe There is nothing wrong with that code

Question 13 When in main we call pthread_create twice and no

thread has finished. How many threads are in the process? 1 2 3 4

Question 14 Two scenarios

Scenario A process open(2) a file. Call fork(), parent calls exit(2).Child still has some work to do.

Scenario B process open(2) a file. Call pthread_create(3), first thread finishes it’s work. Calls exit(2), 2nd thread still has work to do. BOTH scenario’s are OK ONLY A scenario is OK ONLY B scenario is OK BOTH should FAIL

Question 15

We expect to find system call man pages in man 1 man 2 man 3 man 4

Question 16 We expect to find syslog output under linux (in tau

labs) at /messages /var/log/messages /usr/log/messages ~root/log/messages

Question 17 GDB output

Program terminated with signal 11, Segmentation fault.#0 0x0000000000400492 in crash_here () at crash.c:44 *c=12; The programmer accessed a memory she has not

allocated The programmer had an endless loop/damned recursion The programmer had called a system call/function with

bad parameters The programmer divided by 0

Question 18 Process 24501 attached - interrupt to quit

select(50, [], NULL, NULL, {0, 0}) = 0 (Timeout)select(50, [], NULL, NULL, {0, 0}) = 0 (Timeout)select(50, [], NULL, NULL, {0, 0}) = 0 (Timeout)select(50, [], NULL, NULL, {0, 0}) = 0 (Timeout)select(50, [], NULL, NULL, {0, 0}) = 0 (Timeout)

What’s wrong? The programmer forgot select ruins it’s arguments There is nothing wrong with the program Select(2) FD_SET is broken Select is deprecated use pselect(2)

Question 19 In GCC

–o is the compile switch –o is the output switch –o is the optimize switch none of the above

Question 20 In UNIX

Each process has environment (char ** environ) Each process has Parent process ID Each process has current working directory All the above

Question 21 Syslogd

Is a process Only root can write to syslog Only the kernel can write to syslog None of the above

Question 22 pid_t fork(void);

fork(2) Should never be used with the standard library If we have buffers that are not flushed they may

be duplicated Should usually not be used if we also use

threads unless we call execXX(2) afterwards B+C

Question 23 Producer-Consumer

Is a situation where one task consume all the memory and does nothing useful (while the producer actually does something that is required

Is a situation where one task consume some resource (memory/IO/CPU) damaging more important producer tasks

Is a situation where tasks that produce more output also consume more resources

Is a situation where we have two tasks one producing work packet and the other handling (=consume) them

Question 24 We can use socket messages

To deliver information between process To make a cond like mechanism between process

(only after receiving the messages you can work) When we want to do I/O multiplexing and use select

to read from stdin and another process (all in a single process)

All the above

Question 25 Deadlock can occur

Between threads & processes Between sockets & threads Between environment variable & sockets Between terminals & sockets

Question 26 Comparing mmap(2) and read(2)/write(2) Apis

Read/write should be used When we want to maximize performance When we have read(2)/write(2) permissions When we want to maximize portability B+C

Question 27 // lines from C program

// … if ((src = mmap (0, statbuf.st_size, PROT_READ,

MAP_SHARED, fdin, 0)) == (caddr_t) -1) // error handling removedif ((dst = mmap (0, statbuf.st_size, PROT_READ |

PROT_WRITE, MAP_SHARED, fdout, 0)) == (caddr_t) -1)// error handling removedmemcpy (dst, src, statbuf.st_size); } /* main */

Question 27 The files will not be copied because we didn’t call

msync(2)

The files will not be copied because we didn’t call munmap(2)

The files will be copied after we fall from main (assuming the computer was still on)

A+B

IMPORTANT NOTE REGARDING Q 27

Falling from main does not call munmap

Mmap marks buffer as dirty (as we’ll learn later)

After a while the kernel will put the dirty buffers to disk

If we call munmap we can force write. If we write again we can save writes

But under no circumstances exit checks for memory buffers and calls munmap

Question 28+29 Virtual memory

Works in Linux Works in UNIX Works in Windows All of the above 

Virtual memory Reside in memory until we call rmmod Is a different kind of memory Allows running programs that require more memory then we

currently have Slows the PC down (due to page faults) and therefore is not used in

modern OS

Question 30 Which of the below is not a scheduling criterion

Max CPU utilization Max throughput Min response time None of the above

Question 31 The simplest way to transfer multiple arguments to

thread is Put args in a file. Transfer filename Put args in hash table. Transfer key to each thread Put args in struct. Transfer struct * There is no simple way

Question 31+32+33

Following are functions pairs. Please specify if they are blocking (answers – a = both , b = 1st , c = 2nd d = none) (assume default flags)

Toupper(3) and Lseek(2)

munmap(2) and Log(3) (linking with –lm)

Getch(3) and getopt(3)

Question 34+35

which of the following can we wait using select (a = both b=1st c = 2nd d= none)

UDS-Stream Mmap file

Standard error (stderr) FILE *

Question 36 Virtual memory algorithms (LRU, 2nd fault etc.)

Minimize page faults Minimize their own processing time Minimize process wait time on ready queue Minimize virtual memory usage

Question 37 – which of the two scenarios is legal

Scenario A – process calloc(3) memory on the heap. Calls fork(2) father and child process use fcntl(2) for locking and communicate via the calloced memory as shared memory

Scenario B – process callc memory on the heap. Calls pthread_create(3) both threads use fcntl(2) for locking and communicate via the calloced memory as shared memory

Question 38 In UNIX zombie is

A type of daemon that does no I/O The saved returned value of a child process that was

not yet wait(2)ed for by father A process that is doing busy waiting for a long time MS windows

Question 39 Which statement is not true

Threads are simpler to create then processes We can only use Producer-Consumer in Threads

using posix cond Threads share the entire memory. Further more they

also share the process environment (FD etc.) POSIX threads require compiling with –lpthread on

Linux

Question 40 In UDS-Stream

We implement all TCP algorithms (window size and checksum) and gain performance because we limit them to local host

We can send information to remote host if we use NFS mounted files as path

We can use mmap(2) to speed up receiving time None of the above

Question 40 - GRID GRID book-keeping

Is used to decide how much to charge user for resource consumption

Is one of the methods used for GRID user management

Is only done for I/O of over 150MB Is done by each computer to monitor how much of

it's resources the GRID consumes

Question 41 Socket(2) system call

Is non blocking for UDS Is required for all forms of IPC Requires –lsocket compile flag A+B

Question 42 Which of the following can be close(2)d

Thread mmap(2)ed file Process Cond

Question 43 When we implement Daemons

We setsid() to change our running user to root so that we run with admin privileges

We close stdin to avoid memory thrashing after loosing controlling terminal

We chdir(“/”) so that we can use syslog in /var/log/messages instead of <cwd>/var/log/messages

We fork(2) twice or more to decrease our priority compared to user tasks

None of the above

Question 44+45+46 Regarding each pair write true or false

Virtual memory managed by MMU Include real memory Includes shared libraries Includes disk files Includes memory mapped files Includes remote memory on GRID environment Includes swapped pages

Question 47 Signal handler function

Gets an int for argument the pid of the process receiving the signal

Gets an int for argument the severity of the signal Gets an int for argument the signal called Gets an int for argument – enum for the suggested

action

Question 48 SDL

Is a cross platform library used to decode videos Can be used on Macintosh to simulate a VM Can be used on Linux to display video and audio Only works with Bruce Springsteen songs A+C

Question 49 Two scenarios

Scenario A – Process accept(2) TCP connection. Calls fork(2). Two things happen simultaneously. Child handles connection. Parent close(2) the socket.

Scenario B – Process accept(2) TCP connection Calls Pthread_create(3). Two things happen simultaneously. New thread handles the connection. Main thread close(2) the connection.

Question 50 Two scenarios

Two process work in producer consumer environment. They have a connection using UDS. When producer is done producing he sends a byte to consumer. Consumer is waiting on select(2)-when FD_ISSET Consumer will recv(2) 1 byte.

Two Threads work in producer consumer environment. They have a connection using UDS. When producer is done producing he sends a byte to consumer. Consumer is waiting on select(2)-when FD_ISSET Consumer will recv(2) 1 byte.

Question 51+52 51 - TWO scenarios

Process forks(3) both parent and child calls strtok(3) We call pthread_create both threads call strtok_r(3)

52 – TWO scenarios (what is LEGAL!) We don’t use threads. We compile with –lpthread We use threads. We compile with –lpthread.

Question 53 Which of the following signals cannot be caught

SIGKILL SIGFPE SIGSUSP None of the above A+C

Question 54 TWO scenarios

Process A wants to communicate with process B using UDS-Stream. Process A and Process B runs on different hosts. We need max throughput.

Thread A wants to communicate with thread B using UDP . Thread A and Thread B runs on different hosts. We cannot afford to lose data

Question 55 TWO Scenarios

We want to bind(2) TCP and UDP sockets. We want both sockets to be given the same name. (bind(2)ed to the same port number)

We want to bind(2) UDS-S and UDS-D sockets. We want both sockets to be given the same name. (bind(2)ed to the same path)

Question 56 Using factory I can

Call constructors that require arguments that I can’t call otherwise

Create singletons in procedural languages such as C and Assembler

Add constructors to object that use the factory thus adding functionality

Decouple implementation and representation

Question 57 Codec is

Compressor/decompressor such as winzip File format – such as mp3 Encoder/decoder such as XviD CoDEC (Digital equipment cooperation) a follower of

digital products

Question 58 Av_register_all()

Register all the codecs in the OS Register all the codecs in the ffmpeg factory Initialize all the codecs in the OS Initialize all the codecs used in the stream

Question 59 OOP programming in C

Is impossible. Is possible if we use C99 standard that add class as a

reserved word in C Is actually C++ for example – we add class and

constructors but we don’t use STL so it’s still C. Is often done in UNIX. Sometimes conceptually and

sometimes mechanically as the language doesn’t have some mechanics

Question 60 All Factories and the Process signal table both hold

Event handlers Pointers to functions SIGCHLD Ints as keys