Program with multiple threads always giving same output

Question

I'm learning multi-threading and trying to create a program that can print two strings alternately. I have written the following code :

#include<stdio.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
pthread_mutex_t lock;
void print(char a[50]){
    pthread_mutex_lock(&lock);
    printf("%s", a);    //output console is the shared resource
    sleep(2);
    pthread_mutex_unlock(&lock);
}
void* hello(void* status){
    while(*((char*)status) != '\n'){ 
        print("Hello World\n"); 
    }
}
void* bye(void* status){
    while(*((char*)status) != '\n'){
        print("Goodbye World\n");
    }
}
int main(){
    pthread_t id1, id2;
    char status = '\0';
    int state;
    if (pthread_mutex_init(&lock, NULL) != 0) { 
        printf("\n mutex init has failed\n"); 
        exit(1); 
    }
    printf("Starting Threads (Press Enter to terminate)\n");
    state = pthread_create(&id1, NULL, hello, &status);
    if(state != 0){
        printf("Could not create thread, exiting.\n");
        exit(1);
    }
    state = pthread_create(&id2, NULL, bye, &status);
    if(state != 0){
        printf("Could not create thread, exiting.\n");
        exit(1);
    }
    scanf("%c", &status);
    printf("Out of The Threads\n");
    pthread_mutex_destroy(&lock); 
    return 0;
}

According to what I understand, the mutex should lock the print function once for the hello function and then for the bye function. But I only get this output:

Starting Threads (Press Enter to terminate)
Hello World
Hello World
Hello World
Hello World
Hello World

Why does only the hello function get allocated the print function? How do I get it do print both?

Does this answer your question? [Will a thread waiting on a mutex get the ownership, immediately after mutex\_unlock() by other thread?](https://stackoverflow.com/questions/42428049/will-a-thread-waiting-on-a-mutex-get-the-ownership-immediately-after-mutex-unlo) — jmq, Apr 29 '20 at 19:24

Hitokiri · Answer 1 · 2020-04-29T17:34:29.680

Because, you use infinitive while in each function hello and bye. When the first thread starts, it never exits the while loop unless you press enter.

But when you type enter, the loop conditions in all functions will be false, so two threads will never touch print function again.

OT, you forget to join the threads.

Try to delete the while loop in each function and use it in main function, and see what happen. For example the program below (it may be not optimized, but i just want to show why your program always print Hello World:

#include<stdio.h>
#include<pthread.h>
#include<stdlib.h>
#include<unistd.h>
pthread_mutex_t lock;
void print(char a[50]){
    pthread_mutex_lock(&lock);
    printf("%s", a);    //output console is the shared resource
    sleep(2);
    pthread_mutex_unlock(&lock);
}
void* hello(void* status){
   // while(*((char*)status) != '\n'){ 
        print("Hello World\n"); 
    //}
}
void* bye(void* status){
    //while(*((char*)status) != '\n'){
        print("Goodbye World\n");
//    }
}
int main(){
    pthread_t id1, id2;
    char status = '\0';
    int state;
    if (pthread_mutex_init(&lock, NULL) != 0) { 
        printf("\n mutex init has failed\n"); 
        exit(1); 
    }
    printf("Starting Threads (Press Enter to terminate)\n");
    while(status != '\n') {
        state = pthread_create(&id1, NULL, hello, &status);
        if(state != 0){
           printf("Could not create thread, exiting.\n");
           exit(1);
        }
        state = pthread_create(&id2, NULL, bye, &status);
        if(state != 0){
           printf("Could not create thread, exiting.\n");
           exit(1);
        }
        pthread_join(id1, NULL);
        pthread_join(id1, NULL);
        scanf("%c", &status);
    }

    printf("Out of The Threads\n");
    pthread_mutex_destroy(&lock); 
    return 0;
}

Program with multiple threads always giving same output

1 Answers1