Subj : Re: process vs. thread ids on linux
To   : comp.programming.threads
From : Loic Domaigne
Date : Fri Jun 03 2005 12:38 am

Salut,

> However, a ps -am gives me this:
> 
> 19103 pts/2    00:00:00 ttest
> 19104 pts/2    00:00:00 ttest
> ...
> 19113 pts/2    00:00:00 ttest

As pointed out by some other posters, the "PID column" is in fact the 
KSE id.

David (Schwartz) is right: this is an issue of 'ps'. Newer 'ps' get this 
right. On my Fedora Core 3:

[loic@neumann ~]$ ps -am
   PID TTY          TIME CMD
  5455 pts/2    00:00:00 ttest
     - -        00:00:00 -
     - -        00:00:00 -
     - -        00:00:00 -
     - -        00:00:00 -
     - -        00:00:00 -
  5480 pts/3    00:00:00 ps
     - -        00:00:00 -

Do you note the '-' ?
We can even go a step further and print the KSE id using the '-L' flag:

[loic@neumann ~]$ ps -aL
   PID   LWP TTY          TIME CMD
  4463  4463 pts/2    00:00:05 xemacs
  5455  5455 pts/2    00:00:00 ttest
  5455  5456 pts/2    00:00:00 ttest
  5455  5457 pts/2    00:00:00 ttest
  5455  5458 pts/2    00:00:00 ttest
  5455  5459 pts/2    00:00:00 ttest
  5483  5483 pts/3    00:00:00 ps

The column "PID" is the same for all KSEs. The Column "LWP" corresponds 
to the KSE id (with your version of 'ps', that column is wrongly called 
"PID").


> So, what is going on here?   If the kernel is indeed assigning a new
> pid to each thread, how can I get to that id (since getpid() doesn't
> work).  Why doesn't getpid() work, anyway?    

Oh, getpid() does work on NPTL. As pointed out by David (Butenhof), 
getpid() returns in fact the "thread group id", which happens to be the 
KSE id of the main thread. As a result getpid() has the same value in 
each thread. That conforms to POSIX.

In order to get the KSE id, you must use the Linux specific syscall 
gettid().


> And can I then use these pids to send signals directly to a
> particular  thread (which I was under the impression was not possible)?

Actually, yes you can. If you know the KSE id, you can directly send a 
signal to a particular thread. ( And as a matter of fact, that's exactly 
the mechanism used by NPTL to send a signal to a specific thread with 
pthread_kill() : it sends the signal to the corresponding KSE id. )

Now if you have understood the whole picture, you should be able to send 
a signal to a particular thread from the shell...


Needless to say, but all those "features" are highly Linux-specific!
But I shouldn't have tell you all of this ;-)


Cheers,
Loic.

.