Subj : Re: killing thread
To   : comp.programming.threads
From : David Butenhof
Date : Wed Aug 10 2005 03:08 pm

Vitale Ferruccio wrote:
> Hi all,
> 
> I've two threads: the former blocked in an sleep(seconds) and the latter
> which have to kill the former. Even if I call pthread_cancel or deliver
> a SIGTERM signal with pthread_kill, the former thread continues to
> run and it exits only after timer expires.
> 
> Any ideas?

What OS? If Linux, which version and thread implementation 
(specifically, Linuxthreads or NPTL)?

What do you mean by "continues to run"? It was sleeping, right, not 
running? So do you really mean that it "continues to sleep", or do you 
mean that it wakes from the sleep but fails to TERMINATE as you expected?

Note that if you don't have a signal action for SIGTERM, "sending 
SIGTERM", even with pthread_kill(), will terminate the PROCESS, not a 
thread. The process control signals are an integral part of POSIX/UNIX 
job control, and changing the semantics from process level to thread 
level would have broken a lot of things.

Now, if you do have a signal action for SIGTERM, fine -- though SIGTERM 
is an odd choice. (Better to reserve that for process termination as 
intended, and use something else like SIGUSR1 or a realtime signal to 
interrupt a thread... or even better, avoid signals entirely.) Delivery 
of the signal, presuming the signal isn't blocked by the target, will 
interrupt any interruptable call. In particular, sleep() ought to return 
to the caller after interruption, with the remaining sleep time. (Could 
the sleep() call be in a loop? Otherwise, what would the thread do when 
sleep returned prematurely?)

Similarly, if cancellation is enabled for the sleeping thread, 
pthread_cancel() will interrupt the sleep and cancel the thread. But do 
you know that cancel is enabled? Are there POSIX cleanup handlers (or 
C++ destructors or catch(...) blocks) in scope... and if so, are you 
sure that they'll complete and allow the thread to terminate?

-- 
Dave Butenhof, David.Butenhof@hp.com
HP Utility Pricing software, POSIX thread consultant
Manageability Solutions Lab (MSL), Hewlett-Packard Company
110 Spit Brook Road, ZK2/3-Q18, Nashua, NH 03062

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