已退出但可连接线程不在/ proc / [pid] / task下吗？

Question

As far as I know, threads created by pthread_create with NULL thread attribute will make the thread joinable after the thread exits if its host process is still alive. So, I wrote a demo to verify this:

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
void* thread_function(void* argv)
{
    printf("thread_function invoked\n");
    return NULL;
}

int main(int argc, char* argv[])
{
    pthread_t tid;
    pthread_create(&tid, NULL, thread_function, NULL);
    pause();
    return 0;
}

Compile it:

gcc -o 01_detach 01_detach.c -pthread

Then run:

./01_detach
thread_function invoked

Find the process:

ps -aux | grep 01_detach
liucong 5856 0.0 0.0 14708 1032 pts/7 S+ 22:59 0:00 ./01_detach

Find the joinable thread:

ls /proc/5856/task
5856

I think there should be two entries in /proc/5856/task , one for main thread and the other for the joinable thread which exited. How to explain this? Is exited threads always not under /proc/[pid]/task? How to find out the exited but joinable threads in Linux?

Platform:

Linux liucong-vaio 4.4.0-98-generic #121~14.04.1-Ubuntu SMP Wed Oct 11 11:54:55 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

gcc version:

gcc version 4.8.5 (Ubuntu 4.8.5-2ubuntu1~14.04.1)

Answer 1

As far as I know, threads created by pthread_create with NULL thread attribute will make the thread joinable after the thread exits if its host process is still alive.

Pretty much yes: by default, POSIX threads are joinable until and unless they are explicitly detached.

I think there should be two entries in /proc/5856/task , one for main thread and the other for the joinable thread which exited.

You seem to be jumping to a conclusion there. The Linux manual page for /proc says:

This is a directory that contains one subdirectory for each thread in the process.

You have assumed that a joinable thread that has terminated but not been joined still counts as a "thread in the process" for /proc purposes. That would be plausible, but the docs don't lend much support to it.

How to explain this? Is exited threads always not under /proc/[pid]/task?

You have pretty much answered your own question: no, your testing shows that they are not. This is presumably covered somewhere in the kernel docs, but it is not stated explicitly in any of the docs that I turned up in a few searches. It is suggestive, however, that the same manual page I already referenced says this, too:

In a multithreaded process, the contents of the /proc/[pid]/task directory are not available if the main thread has already terminated.

Note well that the main thread having terminated does not imply that the whole process has terminated.

How to find out the exited but joinable threads in Linux?

I am unaware of any general-purpose mechanism to extract that information. Certainly the pthreads API does not provide for it -- as far as pthreads is concerned, the process that wants to know about threads belonging to it is expected keep track of that information itself.

But all of your efforts seem to be extremely roundabout. If you want to demonstrate that a thread launched with default thread attributes is joinable, then why not just join it?