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2002 Jun 24 15:39

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Re: [vox-tech] Perl question: file handling
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Re: [vox-tech] Perl question: file handling



On Mon, 24 Jun 2002, Rod Roark wrote:

> I'm looking at some Perl code (not mine) that does this 
> (simplified for clarity):
> 
>   open(INFILE, "<$filename");
>   flock INFILE, LOCK_EX;
>   open(OUTFILE, ">$tempname");
>   flock OUTFILE, LOCK_EX;
>   ...
>   (read from INFILE, change stuff, write to OUTFILE)
>   ...
>   rename $tempname, $filename;
>   flock OUTFILE, LOCK_UN;
>   close(OUTFILE);
>   flock INFILE, LOCK_UN;
>   close(INFILE);
> 
> Notice that it's renaming/deleting files that are open and
> locked.  Is this as insane as I think it is?

No, it is not insane at all, if you are using a *nix-style filesystem.

Think of rename as "mv"... it doesn't affect the file, rather it alters
the directory entries that point at the file (inode).  No deletion is
occurring, and any processes accessing the file are doing so through the
inode, which is unaffected.

This behaviour allows you to mv a logfile even while it is being written
to, for example.  The process writing to the logfile cares not what you
call it, and the next process to attempt to open the logfile finds no
inode for that filename, and creates the new inode as needed.

I hate the tied data/filename aspect of MSDOS/Windows that makes such
tasks a b***h to implement.

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