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Detect Linux Runlevel

Marko Rauhamaa marko at pacujo.net
Wed Dec 7 00:30:04 EST 2016 
Tim Chase <python.list at tim.thechases.com>:

> On 2016-12-07 00:29, Marko Rauhamaa wrote:
>> A word a warning: your code doesn't lock /var/run/utmp before
>> access, which is a race condition. The file may be updated at any
>> time, and ordinary file reads may yield corrupted records.
>
> Since the code is reading in record-sized blocks and never writing,
> I'm not sure how much possibility there is for a race condition. At
> worst, I imagine that it would result in reading the old data which
> isn't a horrible condition.

If you read a full record at an offset, you might be right. However,
your code uses Python's buffered I/O:

      with open(utmp_fname, "rb") as f:
          while True:
              bytes = f.read(XTMP_STRUCT_SIZE)

instead of os.open() and os.read().

> For under a certain block-size (PIPE_BUF, which I think used to be
> the minimum POSIX requirement of 512b, but is now 4096b on Linux),
> *nix operating systems were atomic in their reading and writing.

That particular guarantee is true for pipes and FIFOs only.


Marko
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