fcntl problems

[mhearne808[insert-at-sign-here]gmail[insert-dot-here]com]

On Aug 31, 8:42 am, Miles <semantic... at gmail.com> wrote:
> On 8/31/07, mhearne808 wrote:
> > I have a script that will be run from a cron job once a minute.  One
> > of the things this script will do is open a file to stash some
> > temporary results.  I expect that this script will always finish its
> > work in less than 15 seconds, but I didn't want to depend on that.
> > Thus I started to look into file locking, which I had hoped I could
> > use in the following fashion:
>
> > Process A opens file foo
> > Process A locks file foo
> > Process A takes more than a minute to do its work
> > Process B wakes up
> > Process B determines that file foo is locked
> > Process B quits in disgust
> > Process A finishes its work
>
> That would look like (untested):
>
> importfcntl, sys
> f = open('foo', 'w+')
> try:
>    fcntl.flock(f.fileno(),fcntl.LOCK_EX |fcntl.LOCK_NB)
> except IOError, e:
>     if e.args[0] == 35:
>         sys.exit(1)
>     else:
>         raise
> f.seek(0, 2) # seek to end
> # do your thing with the file
> f.flush()fcntl.flock(f.fileno(),fcntl.LOCK_UN)
> f.close()
>
> -Miles

I tested that, and it works!  Thanks!

Looking at my flock(3) man page, I'm guessing that "35" is the error
code for EWOULDBLOCK.  Which system header file am I supposed to look
in to figure that magic number out?

I would make the argument that this module could be either more
pythonic, or simply documented more completely.  The open source
response, of course, would be "go for it!".

--Mike