[Wellington-pm] question about 'system', and other methods for calling os functions

Richard Hector richard at walnut.gen.nz
Tue Apr 19 17:14:09 PDT 2011

On Tue, 2011-04-19 at 22:37 +1200, Ewen McNeill wrote:

> You may be able to explicitly set the (unix) FD_CLOEXEC flag on the 
> relevant file descriptors before the system() call (which should be 
> doing fork()/exec()/other stuff under the hood on a Linux system), sort 
> of like the reverse of the example towards the end of this page:
> http://aaroncrane.co.uk/talks/pipes_and_processes/
> but I'm not sure it's worth the effort.

Yes, I found that (FD_CLOEXEC, not the link) last night, and was
planning to try it ...

> That said, I'm unable to find a combination of perl+LVM tools amongst, 
> eg, my Debian systems that reproduces that behaviour with:
> sudo perl -e 'system("lvdisplay");'
> sudo perl -e 'system("lvcreate");'

... but I figured I'd play with reproducing it in a smaller program, and
eventually managed by adding the line:

my $vmm = Sys::Virt->new(uri => "qemu:///system");

Sys::Virt is an XS wrapper around libvirt, for managing various types of
virtual machines on linux.

So I guess it's opening the extra filehandles - which may well not be
the ones I mentioned earlier. I don't like the idea of hiding warnings,
so I guess I need to re-order the code (so it creates the lvs before
calling Sys::Virt), or confirm which filehandles they are and close them
in a separate process with FD_CLOEXEC. Or something.

Any tips on identifying open filehandles? Google wasn't too helpful on
that :-(



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