[racket] open-output-file #:exist

Hi Marijn,

At least on Unix/Linux, a file (inode) has some distinct identity
apart from its content.

If a file is truncated, this is observed by all processes which have the file
open. In addition, all hard-links to the file are also truncated (since
these are just references to the same inode).

If a file is replaced, the old inode is unaffected: processes which
had the file open continue to see the old data, other hard links
keep referring to the old file.

If you don't care about these subtleties you can use truncate/replace.

must-truncate is, I think, mostly useful if the file is to be used
for some form of inter-process communication. must-replace doesn't seem
useful in any way to me and moreover cannot be implemented reliably
without introducing a race condition between checking for existence of
the file and overwriting it with a new one.

Stephan

On 08/24/2011 03:01 PM, Marijn wrote:
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> Hi,
>
> In the reference in section 12.1.5 File Ports[1], open-output-file is
> defined as having an optional keyword argument #:exist which can have
> one of 8 different values. The same options are used for a number of
> wrapper functions, among which with-output-to-file[2].
> If you go to [2] you need to follow 3 internal links to similar
> functions to get to open-output-file[1] to find out what the different
> values of the #:exists argument mean. I think it would be good if a link
> was inserted such that you can go straight there.
> Unfortunately I find some of the explanations not so clear. I will
> comment on the explanation of each option in the order they appear in
> the docs.
> 1) error: this one is clear
> 2) replace: how is this different from 3) truncate?
> 3) truncate: see 2)
> 4) must-truncate: what about must-replace?
> 5) truncate/replace: clear if 2) and 3) are clarified
> 6) update: what is the current read/write position (this is hinted at in
> 8), but should really be explained fully here) and what happens when you
> write to the file (does it overwrite, insert, either depending on some
> other option)?
> 7) can-update: clear if 6) clarified
> 8) append: why is it only equivalent to update on Windows?
>
> Marijn
>
>
> [1]:http://docs.racket-lang.org/reference/file-ports.html?#%28def._%28%28lib._racket/private/base..rkt%29._open-output-file%29%29
> [2]:http://docs.racket-lang.org/reference/file-ports.html?#%28def._%28%28lib._racket/private/base..rkt%29._with-output-to-file%29%29
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