Question 38996 in Ubuntu Question posted in
The official Ubuntu documentation can be found here.

A simple threaded program: questions and doubts about detach() – Ubuntu

Here is my threaded program, which is very simple:

#include <iostream>
#include <thread>

using namespace std;


void myprint(int a)
{
    cout << a << endl;
}
int main()
{
    thread obj(myprint, 3);
    obj.detach();
    cout << "end!!!" << endl;
}

What I can’t understand is that the program occasionally has sub-threads that print 3 multiple times, such as:

3
3
end!!!

I have browsed through many sources but have not found a good answer to this question.

I know that std::out is thread-unsafe and understand the garbled output due to race. The issue of duplicate output, however, doesn’t seem to have much to do with race, as it doesn’t occur in join() contexts.

The relevant configuration of the programming environment is as follows

Linux version 5.15.0-48-generic (buildd@lcy02-amd64-080)
(gcc (Ubuntu 11.2.0-19ubuntu1) 11.2.0,
 GNU ld (GNU Binutils for Ubuntu) 2.38)

GCC version:

gcc version 11.2.0
Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Aha，this morning I had a stroke of genius and solve the problem successfully. You just need to test the cpu time that join() and detach() executed, you will get the answers.

    I think @Someprogrammerdude is right, it's just that the join takes longer to execute, so the problem of duplicate output only occurs in detach()


  2. 0

    It’s a buffering issue. Because the process itself only have one std::cout object and to that one connected buffer, it will be shared between all the threads.

    When probably happens is that the thread prints the output, but doesn’t manage to clear the buffer before the thread is preempted and the main thread runs to print its "end!!!" message. Since the buffer isn’t clear it will also contain the 3 output from the thread, so it’s written again. This is a race condition, and those are bad.

    The simplest solution is to not detach the thread, and join it before you print the "end!!!" message and exit the process.

    A more complex solution is to add some kind of synchronization between the threads in your processes, so all access to std::cout is protected and can run only mutually exclusive from all other access to it.

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