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Top Forums Programming Multiple instances of pthread Post 302644375 by clerew on Monday 21st of May 2012 05:54:05 PM
Old 05-21-2012
Multiple instances of pthread

Suppose I declare
pthread_t clear_thread;
and then
pthread_create(&clear_thread, &detach, clear_message, this);
the thread is supposed to go away, perform the service it is intended to procide, and then kill itself.

A little while later, I require this service again, so I say
pthread_create(&clear_thread, &detach, clear_message, this);
Now if, whether by accident or design, the first thread is still running, what is supposed to happen? The man pages seem to be silent on this issue (one would have hoped that creating a new thread on an already running identifier would return an error, but it seems not to be so).

So, are there now two threads running? and if I try to do a pthread_kill on it which thread (or both?) gets killed?

And, for that matter, is there any way to tell whether a thread that I have created is still running? Or do I have to declare a variable to keep track of that myself?

I suspect that the answer is that I have made a programming blunder by allowing this situation to arise, but it is an easy mistake to make, especially if you did not foresee the necessity to demand this service frequently (I got into this mess because my progma ran amok and started demanding service continuously, and eventually I think I got into some deadlock). But if that is the answer, then I suspect it is a bug in the design of the pthread system - or is it a feature?

For the record, I am trying to put messages up on an LCD, and this thread is created to take them down again after 30 seconds, or when the user acknowledges them, or whatever, and there are associated threads for putting the backlight on, and turning it off after a while.
 

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PTHREAD_DETACH(3)					     Linux Programmer's Manual						 PTHREAD_DETACH(3)

NAME
pthread_detach - detach a thread SYNOPSIS
#include <pthread.h> int pthread_detach(pthread_t thread); Compile and link with -pthread. DESCRIPTION
The pthread_detach() function marks the thread identified by thread as detached. When a detached thread terminates, its resources are automatically released back to the system without the need for another thread to join with the terminated thread. Attempting to detach an already detached thread results in unspecified behavior. RETURN VALUE
On success, pthread_detach() returns 0; on error, it returns an error number. ERRORS
EINVAL thread is not a joinable thread. ESRCH No thread with the ID thread could be found. CONFORMING TO
POSIX.1-2001. NOTES
Once a thread has been detached, it can't be joined with pthread_join(3) or be made joinable again. A new thread can be created in a detached state using pthread_attr_setdetachstate(3) to set the detached attribute of the attr argument of pthread_create(3). The detached attribute merely determines the behavior of the system when the thread terminates; it does not prevent the thread from being terminated if the process terminates using exit(3) (or equivalently, if the main thread returns). Either pthread_join(3) or pthread_detach() should be called for each thread that an application creates, so that system resources for the thread can be released. (But note that the resources of all threads are freed when the process terminates.) EXAMPLE
The following statement detaches the calling thread: pthread_detach(pthread_self()); SEE ALSO
pthread_attr_setdetachstate(3), pthread_cancel(3), pthread_create(3), pthread_exit(3), pthread_join(3), pthreads(7) COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2008-11-27 PTHREAD_DETACH(3)
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