[Beowulf] Stupid MPI programming question

Michael Will mwill at penguincomputing.com
Thu Sep 28 10:03:42 PDT 2006

That's wierd. On my scyld cluster it worked fine once I had created /tmp/oooo/ on all compute nodes  before running the job.


 -----Original Message-----
From: 	Clements, Brent M (SAIC) [mailto:brent.clements at bp.com]
Sent:	Thu Sep 28 07:11:27 2006
To:	Jakob Oestergaard; Robert G. Brown
Cc:	beowulf at beowulf.org
Subject:	RE: [Beowulf] Stupid MPI programming question

What I ended up doing was just stripping the program down to like 10 lines of code and I have a simple sprintf to create the directory name.
What is wierd is that(I haven't done error reporting yet):
mkdir("NEWDIRNAME"); works(creates a directory in the current working directory)
but mkdir("/tmp/oooo/NEWDIRNAME") doesn't work, I even tried a chdir(which came back succcessful) and then wrote the above mkdir("NEWDIRNAME");
Anyway..I'm starting to get off-topic.
Nevertheless, I got it working minimally to what I wanted to do, so I have a great template for a simple MPI program.
Thanks to everyone who helped out!!!
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From: Jakob Oestergaard [mailto:jakob at unthought.net]
Sent: Thu 9/28/2006 8:09 AM
To: Robert G. Brown
Cc: Clements, Brent M (SAIC); beowulf at beowulf.org
Subject: Re: [Beowulf] Stupid MPI programming question

On Thu, Sep 28, 2006 at 08:57:28AM -0400, Robert G. Brown wrote:
> On Thu, 28 Sep 2006, Jakob Oestergaard wrote:
> Ah, that's it.  I'd forgotten this one and missed the write to a static
> string, although it has bitten me in the past (partly because back in
> the remote K&R past one could nearly always get away with it).  This is
> also a way that buffer overwrite attacks can begin if any nefarious
> human can control the string that is overwritten IIRC...
> Although in this particular case, that should have produced a very
> different error than -1 on the mkdir call, should it not?

Well, if the write doesn't give him a segfault and he's allowed to write
to memory that shouldn't be written to, then I guess pretty much
anything can happen after that.

> And he was
> writing out the results string per node right before calling as well, so
> his compiler was probably letting him get away with it or failing in
> some odd way later.


I wonder if valgrind would have caught it...

> >What you probably want to do is:
> >---
> >char foo[1024];          // 1KiB on the stack - writable
> >strncpy(foo, sizeof foo, "my test"); // Assign contents by copying
> >...
> >foo[0] = ' ';             // <- fine
> >---
> Absolutely.

Uh, except I meant strncpy(foo, "my test", sizeof foo) of course...



 / jakob

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