week 4 c programming

Question 1

what is a socket

Answer 1

socket behaves similar to a file (can read/write) once connected
A client connects to a server

Question 2

what is the preprocessor for the socket

Answer 2

include <sys/socket.h>

Question 3

what is syntax for the socket

Answer 3

int sockfd = socket(AF_INET6,
SOCK_STREAM, 0);

Question 4

describe sockets in servers

Answer 4

Server binds to port, listens on the socket, and
accepts each client connection

Question 5

syntax of sockets in a server

Answer 5

bind(sockfd, &srv_struct, len)
listen(sockfd, backlog)
newsockfd = accept(sockfd,
&cl_struct, len);

Question 6

describe sockets in client

Answer 6

Client connects to IP/port, and then read() and
write() on socket
Now can read() and write() on newsockfd and
close() later

Question 7

syntax of sockets in client

Answer 7

connect(sockfd, &addr_struct, len)
write(sockfd, buffer, len)
…
close(sockfd)
Same as server regarding read/write/close

Question 8

what are the solutions to Applications are demanding more resources

Answer 8

Alternative solution:
* Put many processing cores on the microprocessor chip.
* The number of cores doubles with each generation.

Question 9

what does the CPU go from to

Answer 9

single core to multi core

Question 10

what is coherence

Answer 10

Coherence’ is the quality of being logical and consistent.
During program execution, data must remain coherent

Question 11

describe coherence in a multicore system

Answer 11

On a multicore system, a data variable may reside in multiple
caches and might get updated ‘locally’ by its local core
→ this causes coherence problem, and protocols are needed

Question 12

what are the two options to program multicore systems

Answer 12

Option 1: Program directly targeting processor cores
* Programmer takes care of synchronization
* Painful and error-prone
* Option 2: Use a concurrency platform
* It abstracts processor cores, handles synchronization and
communication protocols, and performs load balancing.
* Hence offers much easier multicore programming environment
* Examples:
* Pthreads and WinAPI threads
* OpenMP

Question 13

what is a concurrent server

Answer 13

all servers running at the same time
All clients get served by the server.
Even if one client causes blocking, the other clients do not have to wait

Question 14

describe parallel tasks in concurrent servers

Answer 14

Task2 is parallel to Task1 and Task3
* Parallel tasks are always concurrent.
* Concurrent tasks may not be parallel (Task1 and Task3)
* So, ‘concurrency’ is a more general term

Question 15

how do we describe programs in a serial system

Answer 15

sequence of
instructions executed one-by-
one

Question 16

what is a thread

Answer 16

A thread of execution is the smallest sequence of programmed
instructions that can be managed independently by a scheduler,
which is typically a part of the operating system

Question 17

syntax to include pthreads

Answer 17

include <pthread.h></pthread.h>

• There are many functions related to pthreads.
• On a UNIX-based system, a list of these functions can
  typically be obtained with man -k pthread
• To know about a specific function see the man page.
• When linking, you must link to the pthread library using

Question 18

create a thread using pthread

Answer 18

int pthread_create(
pthread_t *thread_id, // ID number for thread
const pthread_attr_t *attr, // controls thread attributes
void (function)(void *), // function to be executed
void *arg // argument of function
)
int pthread_create(
pthread_t *thread_id, // ID number for thread
NULL, // we set it to default thread attributes
void (function)(void *), // function to be executed
void *arg // argument of function
);

returns -> 0 if thread is successful
returns non zero value to indicate error

Question 19

give me functions that can be passed through pthreads

Answer 19

void *foo1();
void foo2(int *);
int *foo3(int *);

Question 20

what functions can not be pointed through pthreads

Answer 20

int *foo4(int , int);

Question 21

T *foo (T *, T *); function with multiple data types
how can we create a pthread that accepts this function

Answer 21

Solution: Pack arguments into a struct and create a wrapper,
which takes the compound argument and unpacks before
passing the unpacked arguments to foo()
typedef struct Compound {
T *a, *b;
} Compound_t;
T * foo_wrapper(Compound_t *c) {
// This can be passed to pthread_create
T *d;
d = foo(c->a, c->b);
}

Question 22

describe Shared data objects in a concurrent system

Answer 22

Cooperation between concurrent threads leads to the sharing of
Global data objects, Heap objects Files, etc.
Lack of synchronization leads to chaos and wrong calculations

Question 23

what are the problems with :
Shared data objects in a concurrent system
2 threads
3 scenarios

Answer 23

Scenario 1:
Main thread computes r1+r2 before thread1 and thread2 produces the
results. Both r1 and r2 will be wrong
Scenario 2:
Main thread computes r1+r2 after thread1 but before thread2 produces.
So, r1 will have correct but r2 will have wrong values.
Thus r will be wrong.
Scenario 3:
Luckily, main thread computes r1+r2 after thread1 and thread2 produce.
Luckily, r will be correct.

Question 24

what is synchronisation in threads

Answer 24

Synchronization in threads programming to make sure that
some events happen in order

Question 25

Synchronization mechanism in Pthreads 3 methods

Answer 25

* Joins * Mutual exclusions (“Mutex”) * Condition variables

Question 26

what type of function is pthread_join()

Answer 26

pthread_join() is a blocking function

Question 27

syntax with pthread Null one using Null and onw without using Null

Answer 27

Without NULL int pthread_join( pthread_t thread_id, // ID of thread to "join" void **value_pntr // address of function’s return value ); with NULL int pthread_join( pthread_t thread_id, // ID of thread to "join" NULL );

Question 28

write code for synchronising threads

Answer 28

int r1 = 0, r2 = 0; int main(void){ pthread_t thread1, thread2; pthread_create(&thread1, NULL, (void *) do_one_thing, (void *) &r1); pthread_create(&thread2, NULL, (void *) do_another_thing, (void *) &r2); pthread_join(thread1, NULL); pthread_join(thread2, NULL); do_wrap_up(r1, r2); return 0; }

Question 29

what is a race condition in threads

Answer 29

A race condition occurs when two or more threads perform operations on the same data, but the results depend on the order in which these operations are performed

Question 30

how can we solve race condition in C

Answer 30

we can enforce mutual exclusion → threads get exclusive access to the shared resource in turn

Question 31

what is mutual exclusion

Answer 31

threads get exclusive access to the shared resource in turn

Question 32

what does pthread offer to implement mutual exclusion

Answer 32

‘mutex’ to enforce exclusive access by a thread to a variable or a set of variables

Question 33

Syntax for mutex in pthreads

Answer 33

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

Question 34

are mutex declared as static or global variables

Answer 34

global

Question 35

syntax for mutex for serializing access to a shared resource

Answer 35

... pthread_mutex_lock(&mutex1); counter++; pthread_mutex_unlock(&mutex1); ... Threads access ‘counter’ serially one after another

Question 36

code that uses 2 threads and pthread mutex

Answer 36

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER; int counter = 0; int main() { int rc1, rc2; pthread_t thread1, thread2; // Two threads execute functionC() concurrently pthread_create(&thread1, NULL, &functionC, NULL); pthread_create(&thread2, NULL, &functionC, NULL); pthread_join(thread1, NULL); pthread_join(thread2, NULL); return 0; } void *functionC() { pthread_mutex_lock(&mutex1); counter++; printf("Counter value: %d\n", counter); pthread_mutex_unlock(&mutex1); }

Question 37

syntax forhow can we allow exclusive access to set of shared objects

Answer 37

serialize access to a set of shared objects

Question 38

code for serialize access to a set of shared objects

Answer 38

pthread_mutex_lock(&mutex1); // Access shared object1; // Access shared object2; pthread_mutex_unlock(&mutex1);

Question 39

what is the critical section in mutex lock/unlock in C when serializing access to a set of shared objects

Answer 39

pthread_mutex_lock(&mutex1); //crticial region // Access shared object1; // Access shared object2; //end of critical region pthread_mutex_unlock(&mutex1); shared objects in between the unlock lock are the critical region The code segment that resides between mutex_lock() and mutex_unlock() is called ‘critical section’ or ‘critical region’. Critical section is executed serially by the threads

Question 40

is critical section executed serially or concurrently

Answer 40

serially

Question 41

syntax for pthread try lock

Answer 41

int pthread_mutex_trylock(pthread_mutex_t *mutex); * Tries to lock a mutex object. * If the mutex object is available, then it is locked and 0 is returned. * Otherwise, the function call returns nonzero. It will not

Question 42

what is pthread_mutex_trylock

Answer 42

Tries to lock a mutex object. * If the mutex object is available, then it is locked and 0 is returned. * Otherwise, the function call returns nonzero. It will not wait for the object to be free

Question 43

/ Thread1 void *do_one_thing() { ... pthread_mutex_lock(&mutex1); pthread_mutex_lock(&mutex2); ... pthread_mutex_lock(&mutex2); pthread_mutex_lock(&mutex1); ... } / Thread2 void *do_another_thing() { ... pthread_mutex_lock(&mutex2); pthread_mutex_lock(&mutex1); ... pthread_mutex_lock(&mutex1); pthread_mutex_lock(&mutex2); ... } what leads to a pitfall betweens these two threads

Answer 43

Thread1 obtains mutex1 Thread1 waits to obtain mutex2 Thread2 obtains mutex2 Thread2 waits to obtain mutex1 Both threads stalled indefinitely → Deadlock

Question 44

what is a conditional variable

Answer 44

A condition variable is used to synchronize threads based on the value of data (i.e., a condition). * One thread waits until data reaches a particular value or until a certain event occurs. * Then, another thread sends a signal when the event occurs. * Receiving the signal, the waiting thread wakes up

Question 45

synatx for pthread conditional variable

Answer 45

A condition variable is a variable of type pthread_cond_t * A thread goes to waiting state based on ‘condition to happen’ by: Function takes two arguments: the condition variable and a mutex variable associated with the condition variable. * Waking thread based on condition pthread_cond_t condition_cond = PTHREAD_COND_INITIALIZER; pthread_cond_wait(&condition_cond, &condition_mutex); pthread_cond_signal(&condition_cond)