Question

C program:

1:

• Write a program that uses 5 threads. Initialize a shared variable with a value of 100.
• Each thread must add its Thread ID (tid) to the shared variable.
• Once a thread has done the addition, print the ID of the thread. [Note: thread does the printing]
• Output the value of the shared variable once all threads have finished incrementing it. [Note: main code does the printing]
• Provide a text file with your comments on the results of activity 1 - show what was printed and make any comments relevant to your learnings on processes and multi-threading.

2:

• Repeat the program but make a modification.
• Add Mutex to provide control on modifying the variable. It is important to make use of mutexes so that only one thread is incrementing the shared variable at a time.
• Add comments to your text file and provide results and observations for activity 2.

Answer 1

1. C code for adding thread IDs without mutex -

#include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 #define SHRD_VALUE 100 static int sumValue; void *AddThreadIds(void *threadid) {    long tid = (long)threadid; //lineX    sumValue = sumValue + tid; //lineY    printf("Thread ID - %d\n", tid);    pthread_exit(NULL); } int main () {         pthread_t threads[NUM_THREADS];    int i;    for(i = 0; i < NUM_THREADS; i++ )       pthread_create(&threads[i], NULL, AddThreadIds, (void *)i);    for (int i = 0; i < NUM_THREADS; i++)         pthread_join(threads[i], NULL);        printf("Sum of all thread IDs - %d\n", sumValue+SHRD_VALUE);    pthread_exit(NULL); }

Output -

Thread ID - 0

Thread ID - 2

Thread ID - 3

Thread ID - 1

Thread ID - 4

Sum of all thread IDs - 110

Comments - The global variable sumValue is prone to get erroneous value at the end as multiple threads active at any point of time may change the value incorrectly. Mutex Ensures that such thinng never happens

2. C code for adding thread IDs with mutex -

#include <iostream> #include <cstdlib> #include <pthread.h> using namespace std; #define NUM_THREADS 5 #define SHRD_VALUE 100 static int sumValue; pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER; void *AddThreadIds(void *threadid) { pthread_mutex_lock(&m); long tid = (long)threadid; sumValue = sumValue + tid; printf("Thread ID - %d\n", tid); pthread_mutex_unlock(&m); pthread_exit(NULL); } int main () { pthread_t threads[NUM_THREADS]; int i; for(i = 0; i < NUM_THREADS; i++ ) pthread_create(&threads[i], NULL, AddThreadIds, (void *)i); for (int i = 0; i < NUM_THREADS; i++) pthread_join(threads[i], NULL); printf("Sum of all thread IDs - %d\n", sumValue+SHRD_VALUE); pthread_exit(NULL); }

Output -

Thread ID - 2

Thread ID - 1

Thread ID - 0

Thread ID - 4

Thread ID - 3

Sum of all thread IDs - 110

I have added Mutex Lock & unlock codes at respective critical section place to ensure that only single thread is present in critical section at a given point of time.