From: Joel VanderWerf <vjoel@...>
Date: 2003-03-02T06:51:38+09:00
Subject: mixing IO#read and IO#syswrite


The docs clearly warn against mixing buffered and unbuffered reads, or 
mixing buffered and unbuffered writes. But is it ok to open a file, read 
its contents, truncate it, and syswrite new contents? It seems to work 
fine, but ....

The reason I'm asking is that I want a file with marshalled data to be 
consistent (as in the ACID properties) regardless of failures. I'm 
guessing a dump to a string followed by a syswrite is the best way to do 
this, because the single OS call won't be interrupted by ruby, and so if 
the file system has the consistency property (I guess a journaled fs 
would suffice), the marshalled data will be consistent.

But I want to avoid allocating strings, if possible. So I use

   Marshal.load(file)

to get the objects without allocating an extra string, then

   file.rewind
   file.truncate(0)
   file.syswrite(Marshal.dump(obj))

to write out the new contents.

I _could_ just use an intermediate string to load the data:

   Marshal.load(file.sysread(...))

But I'd like to avoid the extra allocation, if possible.

Is my approach safe?