[Baypiggies] File IO question
ken at seehart.com
Wed Jul 26 03:54:00 CEST 2006
steve hindle wrote:
>> On 7/25/06, Ken Seehart <ken at seehart.com> wrote:
>>> My question is this: If I write a single byte to a file that contains
>>> exactly one byte, can I be sure that a power outage during the write
>>> will not render the file unreadable?
> Given its a binary switch - the file contents are immaterial.
> You could simply use the (non-)existance of the file as the status indicator.
That was my original plan, but I was worried that it would be easier to
corrupt a directory by interrupting file creation/deletion than it would
be to interrupt writing a byte to a file. I'm not certain about this
> Also, in these situations, its common to write the indicator LAST, after all
> updates have completed normally. So if the power fails, etc - you can
> still boot
> the original known-good copy.
Yes, that's the basic idea. The pseudocode is something like this:
set indicator 1 to false
write to file 1
set indicator 1 to true
set indicator 2 to false
write to file 2
set indicator 2 to true
At boot up, the indicators tell me which file is valid, and the
appropriate file copy is performed.
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