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From: cross@spitfire.i.gajendra.net (Dan Cross)
Newsgroups: comp.os.vms
Subject: Re: FreeTDS port to VMS V9.x on x86?
Date: Fri, 4 Jul 2025 04:31:16 -0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
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In article <10470be$dalk$1@dont-email.me>,
Craig A. Berry <craigberry@nospam.mac.com> wrote:
>
>On 7/3/25 3:07 PM, Dan Cross wrote:
>> In article <1046hc5$62th$2@dont-email.me>,
>> Arne Vajhøj  <arne@vajhoej.dk> wrote:
>>> On 7/3/2025 1:40 PM, Dan Cross wrote:
>
>>>> You refer to `_PATH_DEVNULL` but do not `#include <paths.h>`, as
>>>> required by POSIX.
>>>
>>> This must support non *nix systems (as an example
>>> VMS !!) - config.h is expected to provide that with the rest.
>> 
>> If copyright dates are anything to judge by, `<paths.h>` has
>> been a thing since 1989, but I was wrong in that it is not
>> actually in POSIX: it is a BSD extension, though common.
>> 
>> In general, it is good practice and good hygiene in C programs
>> to `#include` the header files that are documented to define the
>> symbols and types that you use in your program, instead of
>> relying on transitive includes to populate things ambiantly via
>> e.g. `config.h`.
>> 
>> Regardless, in that case, you shouldn't use `_PATH_DEVNULL`.
>> Note the leading `_`: that signifies a reserved identifier,
>> which is not something you should be defining yourself.  If you
>> want to punt this to e.g. config.h, better would be to define a
>> new name (say, `TDS_PATH_DEVNULL`) and use that.
>
>The code ya'll are talking about replacing:
>
>https://github.com/FreeTDS/freetds/blob/Branch-1_5/src/replacements/vasprintf.c
>
>does the following:
>
>#if HAVE_PATHS_H
>#include <paths.h>
>#endif /* HAVE_PATHS_H */
>. . .
>#ifndef _PATH_DEVNULL
>#define _PATH_DEVNULL "/dev/null"
>#endif
>
>FreeTDS is relying on autoconf to sort out whether paths.h is available
>and will only include it if HAVE_PATHS_H is defined in config.h, and
>thus only defines _PATH_DEVNULL if it's known not to exist. Obviously
>one should avoid making up one's own reserved identifiers and clobbering
>documented and well-known names, but that's not really the same thing as
>providing an implementation for one that's known to be missing. This
>fallback approach could of course be added to Arne's implementation if
>desired; to me it seems a lot cleaner than defining yet another macro,
>which would need to be defined in terms of _PATH_DEVNULL when it exists
>but otherwise not.
>
>The problem for VMS is that the workaround doesn't work.  Or at least it
>didn't when I was first porting FreeTDS 20+ years ago because the CRTL
>at the time did not recognize '/dev/null' as a valid path.  I believe
>recentish CRTLs (maybe VMS 8.x and later?) do have special case code
>that translates it to the native null device.  But that wasn't
>available, so I added the following line to the template from which
>config.h is generated on VMS:
>
>#define _PATH_DEVNULL "_NLA0:"
>
>The main advantage of this approach is that it worked, and it did so
>with a one-line change to a file I was already maintaining. I didn't
>have to sprinkle '#ifdef __VMS' in files that already existed and were
>already working on other platforms. I didn't have to create a lot of new
>files, the presence of which in the repository I couldn't test without
>autoconf/automake, which I didn't have access to at the time.  All of
>which meant upstream maintainers were far more likely to accept my changes.
>
>I'm sure I'll get a lecture about everything I did wrong, but at least
>now the context is out there for why it was reasonable for Arne to
>depend on prior art, i.e., the presence of _PATH_DEVNULL in config.h.

Meh.  If it works and it's not new code, then that's fine.  But
I don't see how having a "compat.h" header with something like:

#ifndef TDS_PATH_DEVNULL
#if defined(HAVE_PATHS_H)
#include <paths.h>
#define TDS_PATH_DEVNULL _PATH_DEVNULL
#elif defined(VMS)
#define TDS_PATH_DEVNULL "_NLA0:"
#else
#define TDS_PATH_DEVNULL "/dev/null"
#endif
#endif

....in it is either appreciably different or much cleaner.

The point isn't to sprinkle VMS-isms all over, but rather, to
centralize platform-specifics and provide an application
specific portability layer.  Given that `_PATH_DEVNULL` is not
standard, but _is_ provided by system headers on a lot of
systems, using it directly is already injecting a kind of system 
dependency.  By isolating its use as above, you instead decouple
that from the rest of the system

Anyway, not meant to be a lecture, just an observation.

	- Dan C.