Re: [MacPerl] MacPerl::SetFileInfo Help (fwd)

["Brian L. Matthews" <blm@halcyon.com>]

Well, it's time to put a stop to this bit of misinformation. When a file is
created, it exists as soon as the create succeeds, and can be manipulated
or opened again just the same as a file that's been around since 1984.

Consider:

open F1, ">testfile" or die "can't create testfile: $!\n";
open F2, "<testfile" or die "can't open testfile: $!\n";

This works fine. If testfile didn't exist until it was closed, it wouldn't
work, the second open would fail with a "No such file or directory".

There are a couple of reasons it might look like the file doesn't exist:

Disk cache - Yes, disk accesses are cached, so it might be that after the
file is created, it's directory entry isn't yet written to disk. However,
the whole file system goes through the cache, so to an application, it
looks like the file exists on disk like every other file (unless you turn
off your computer without shutting down or it crashes before the directory
entry gets written to disk). A system that didn't maintain the consistency
of a cache would be badly broken.

Finder goofiness part 1 - The Finder doesn't trap file creates (I think it
should, but that's for another rant :-)), it simply examines the
modification times of the directories it has open. This could mean that
when a file is created, it won't appear in its proper Finder window for up
to 5 seconds (and I've seen cases where the Finder doesn't notice a new
file until you close and reopen the window for the folder it's in). Of
course files are often open for less than 5 seconds, so this could look
like the file doesn't exist until it's closed.

Finder goofiness part deux - When the Finder first notices a new file, it
grabs the type and creator and sets the file's Initted bit. Once a file's
Initted bit has been set, the Finder doesn't grab the file's creator and
type again (except for unusual circumstances, like rebuilding the desktop).
So consider the following scenario:

    perl creates a file in response to open HANDLE, ">file"
    perl gives some time to other applications
    the Finder gets to run, notices the new file, grabs the type and
	creator, and sets the Initted bit
    perl calls SetFileInfo to change the type and creator

Because the type and creator are set after the Finder has set the file's
Initted bit, the Finder won't notice the change, so it will look like it
didn't happen. Note that this is very timing dependent. If the Finder
doesn't happen to notice the new file until after it's type and creator are
set, it will see the change. That's why it may appear to work sometimes and
not others.

The two bits of Finder goofiness are why you should always update the
modification date of a file's containing directory whenever you create a
new file, and why you should always clear a file's Initted bit *after*
manipulating information about the file (like creator and type, dates, etc.)

Brian



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