Question

Consider the following C++ code: 7 string animals[5] = {"dog", "cat", "turkey", "shark", "lion"); B 9 void* printword(void* arg) 10 { 11 int id = *((int*) arg); 12 13 cout << "Current word: « animals[id] << endl; 15 pthread_exit(NULL); 16 } 17 18 int main() 19 { 20 pthread_t thread_id[5]; 21 22 for(int num = 0; num < 5; num++) 23 { 24 1/ pass the current value of id as a parameter to the new thread 25 pthread_create(&thread_id(num), NULL, printword, (void*) &num); 26 } 27 28 1/ wait until all threads finish executing 29 for (int num = @; num < 5; num++) { 30 pthread_join(thread_id[num), NULL); 31 } 32 33 cout << "Process done!\n"; 34 35 return 0; 36 } Below are a few sample outputs obtained when this program is executed: Sample Output 1: Current word: Current word: Current word: Current word: Current word: catturkeysharklion Process done!

c++

Answer 1

CODE :

#include<iostream>
#include<pthread.h>
#include<string>
using namespace std;


string animals[5] = {"dog", "cat","turkey","shark","lion"};

void *printWord(void *arg ){

        int id = *((int*)arg);
        cout<<"Current word : "<<animals[id] << endl;
        pthread_exit(NULL);

}

int main(){

        pthread_t thread_id[5];
        for(int num = 0; num < 5; num++){

                pthread_create(&thread_id[num],NULL,printWord,(void*)&num);
                pthread_join(thread_id[num],NULL);

        }
        
        cout<<"Process done!\n";
        return 0;

}

Output :

F:\CODES\VS\C\t.exe Current word : dog Current word : cat Current word : turkey Current word : shark Current word : lion Process done! Process exited after 0.1193 seconds with return value o Press any key to continue..

Explanation :

• In the question, code first creates all the threads and then tries to join them all to the program thread at the end.
• pthread_join(thread_id[num], NULL): This function will suspend current thread execution until the thread specified (thread_id[num]) execution terminates and then prints the return value from the thread and resumes the current thread.If we want nothing to print we pass NULL.
• But if we create all the threads at the beginning just like first for loop in the question and try to join all the threads at the end, the current thread is suspended more than one time and is not executed properly. It may also end due to its own join call. This will happen to all the threads since all the threads are created and start executing at all.
• So the output that is provided is the result due to this logical error.
• If we try to create a single thread and join it after its creation, the problem could be solved.
• So, in the single for-loop I have created the thread and then join it to the parent immediately after its creation.
• This will help to suspend the execution of the other threads if so created after the first one and wait until its execution.
• Similarly, we can run threads without suspending their execution and waiting until their termination to execute the next thread.