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Re: safe way to add contents to a file ?


From: Óscar Fuentes
Subject: Re: safe way to add contents to a file ?
Date: Wed, 18 Dec 2019 01:36:49 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

Jean-Christophe Helary <address@hidden>
writes:

> I'm struggling with what is the ideal "elispy" way of adding contents
> to a file at a given position.
>
> What I want it:
> - create some content
> - put that content at a specific location in a file saved on disk
>  (current use case: add an xml block to an RSS file)
>
> I thought write-region would do the trick but there are 2 issues with
> it
>
> 1) if APPEND is a number, that's the position from which my contents
> will *overwrite* the rest of the file (I want to *insert* my contents)
>
> 2) there are weird things like the whole code where I call this
> function is actually copied to the target file buffer and I have no
> idea how to prevent that.
>
> So, I thought of something a bit convoluted:
>
> use a temporary buffer
> insert the contents of the file there

find-file-noselect takes care of creating the buffer and reading the
contents of the file. Later you must get rid of the buffer.

> find the point where I want to insert my contents

use `insert' or `insert-buffer-substring'

> write all that to the original file
>
> But I thought, it's not like such things don't happen all the time in
> emacs, so there must be a better workflow.
>
> But the reference is incredibly cryptic regarding that and the Intro
> does not address any file i/o at all...

The sequence of operations above is actually quite simple (although not
very efficient if the target file is large and your new content is near
the end).




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