Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.unix.wizards
Subject: Re: How can a file be truncated on System V??
Keywords: link, unlink, directory
Message-ID: <1539@auspex.auspex.com>
Date: 3 May 89 06:47:52 GMT
References: <651@mitisft.Convergent.COM> <13725@steinmetz.ge.com>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 15

>  There was a method in wizards a few years ago I believe. It had to do
>with using O_TRUNC, which seems to truncate to the position of the
>current file pointer.

Not on S5R3.1, and not on 4.3BSD.  O_TRUNC is used *only* on "open", and
since the whole point of "open" is to *create* a file pointer, there
*is* no current file pointer....  O_TRUNC isn't used on "fcntl".

>There was a trick with reopen or dup or ??? which
>allowed opening of an already open file.

Well, "dup" will give you a second file descriptor that refers to an
already-open file, but there's no trick that lets you open an
already-open file with "open", unless you have something like the
"/dev/fd" mechanism.