I am just starting on how threading really works on linux (Ubuntu) using pthreads library. And I am stuck on first program. See the code first :

#include <pthread.h>

#include <iostream>
using namespace std;

void* th_func(void* arg) {
    cout << "Indside Thread Start Routine !!n";
    return NULL;
}

void foo() { cout << "Inside Foo !!n"; }

int main(void) {
    int ret;
    foo();
    pthread_t thread;
    if (!(ret = pthread_create(&thread, NULL, th_func, NULL))) {
        cout << "ret = " << ret << endl;
    }
    return 0;
}

Simple compilation and test run of the program here 🙁 I intentionally use 0,1,… before $ just for the sake of indexing and trying to explain more efficiently )

0$ g++ t0.cpp
1$ ./a.out
Inside Foo !!
ret = 0
Indside Thread Start Routine !!
2$ ./a.out
Inside Foo !!
ret = 0
Indside Thread Start Routine !!
3$ ./a.out
Inside Foo !!
ret = 0
Indside Thread Start Routine !!
Indside Thread Start Routine !!
4$ ./a.out
Inside Foo !!
ret = 0
Indside Thread Start Routine !!
5$ ./a.out
Inside Foo !!
ret = 0

I am happy with all the test run except the run indexed with 3$. First let me share my understanding on the other run.

• At run 1$, 2$ and 4$, the main thread ended after the created thread ended resulting the start routine (function in this case th_func) to execute before program ends.
• At run 5$ however the main thread ended before the created thread which causes created thread to terminate before code given to it processes. (th_func not printing cout statement)

Let me come to the 3$ one which grabs all my attention or probably stuck in my head. My questions are :

1. Why the created thread not terminated after printing whatever on cout statement and returning NULL ? (As DESCRIPTION section point no.2 suggests from man page of pthread_create:

DESCRIPTION top
The pthread_create() function starts a new thread in the calling
process. The new thread starts execution by invoking
start_routine(); arg is passed as the sole argument of
start_routine().

The new thread terminates in one of the following ways:

It calls pthread_exit(3), specifying an exit status value that
is available to another thread in the same process that calls
pthread_join(3).

It returns from start_routine(). This is equivalent to
calling pthread_exit(3) with the value supplied in the return
statement.

It is canceled (see pthread_cancel(3)).

Any of the threads in the process calls exit(3), or the main
thread performs a return from main(). This causes the
termination of all threads in the process.

2. If it returns the why print the statement of cout (cout << "Indside Thread Start Routine !!n";) twice ? Is thread created twice ?

I put up my head on the internet for several hours but honestly could not find answer. Perhaps my code is not thread safe or idk ? I just could think may be pthread_join help but still it just waits the created thread to be terminated and that won’t help my questions ( printing twice or maybe calling th_func twice ). So I am wishing someone might help …
Thanks in advance 🙂