What happens when pthread_join is commented?

Question

#include <stdio.h>
#include "mythreads.h"
#include <stdlib.h>
#include <pthread.h>

void *
mythread(void *arg) {
    printf("%s\n", (char *) arg);
    return NULL;
}

int
main(int argc, char *argv[])
{
    if (argc != 1) {
        fprintf(stderr, "usage: main\n");
        exit(1);
    }

    pthread_t p1, p2;
    printf("main: begin\n");
    Pthread_create(&p1, NULL, mythread, "A");
    Pthread_create(&p2, NULL, mythread, "B");
    // join waits for the threads to finish
    //Pthread_join(p1, NULL); 
    //Pthread_join(p2, NULL); 
    printf("main: end\n");
    return 0;
}

This is a code from Remzi Chapter 27. Playing around, I'm curious to know why sometimes on run, I get A printed twice. I know why this is happening, because I haven't included the join statement. Why should skipping join cause this?

My output:

shubham@HP:~/OS/Code-Threads-Intro$ ./a.out 
main: begin
A
main: end
B
shubham@HP:~/OS/Code-Threads-Intro$ ./a.out 
main: begin
A
main: end
B
shubham@HP:~/OS/Code-Threads-Intro$ ./a.out 
main: begin
main: end
A
shubham@HP:~/OS/Code-Threads-Intro$ ./a.out 
main: begin
main: end
B
A
A

Answer 1

Removing the call the pthread_join() should not cause "A" to be printed twice. (Barring a bug in the implementation.)

But since your fprintf() calls share the same FILE * structure, they may not be not multithread-safe. How and why they could print "A" twice is dependent upon the implementation details of your system's fprintf() function.