Multithreaded qsort(3)

Dan Nelson dnelson at
Mon Mar 19 08:45:14 UTC 2007

In the last episode (Mar 18), Giorgos Keramidas said:
> On 2007-03-17 23:43, Kip Macy <kip.macy at> wrote:
> > Reminds me of how Solaris blindly uses vfork for implementing
> > system(3). It was very easy for a naive user (me) to call system
> > from a multi-threaded python application. I had numerous failures
> > that were impossible to track back to system(3).
> It seems like an 'obvious' optimization, though.  vfork() will block
> the parent process until the child runs exec(), and the whole purpose
> of system is to exec() a shell and run an external command.
> Can you elaborate on the problems you were seeing?  It sounds like
> something both interesting and educational :)

Btw - the Solaris 10 system manpage only mentions signal interactions
as the cause for MT-unsafeness:

    "The system() function manipulates the signal handlers for SIGINT,
     SIGQUIT, and SIGCHLD.  It is therefore not safe to call system()
     in a multithreaded process, since some other thread that
     manipulates these signal handlers and a thread that concurrently
     calls system() can interfere with each other in a destructive
     manner.  If, however, no such other thread is active, system() can
     safely be called concurrently from multiple threads.  See
     popen(3C) for an alternative to system() that is thread-safe."

It looks like there were some vfork-related system() bugs in older
versions of Solaris, but they appear to have been fixed:

	Dan Nelson
	dnelson at

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