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Re: [PATCH 0/2] Modernize header checks

From: Zack Weinberg
Subject: Re: [PATCH 0/2] Modernize header checks
Date: Fri, 31 May 2013 13:51:25 -0400

On Fri, May 31, 2013 at 12:39 PM, Russ Allbery <address@hidden> wrote:
> Zack Weinberg <address@hidden> writes:
> I *think* your patch would remove strings.h from the list of headers that
> are probed by default by Autoconf, and hence remove HAVE_STRINGS_H from
> the preprocessor directives set by Autoconf.

That's right.

> If so, note that removing strings.h from the list of headers that are
> probed by default will cause backwards compatibility issues.  One still
> must include strings.h (not string.h) according to POSIX in order to get
> strcasecmp and friends, and some operating systems (specifically at least
> some versions of FreeBSD) do actually enforce that and do not prototype
> those functions in string.h.  I'm quite sure there is code out there that
> assumes that Autoconf will probe for strings.h as a side effect of other
> probes and set HAVE_STRINGS_H, and therefore doesn't probe for it
> explicitly.  (I maintain some of it, in fact.)

I had been under the impression that everything one still wanted out
of strings.h was also specified to be in string.h (in particular both
strcasecmp and ffs).  I see now that this is wrong.

For the short term I'm good with putting a conditional #include of
strings.h and a probe for it back into AC_INCLUDES_DEFAULT (MSVC is
the only common platform that doesn't seem to have it, but that's true
even in the very latest version).  However, I'd like to come up with a
transition plan so we don't have to treat this as an
almost-always-wanted header forever.  We don't have any tooling for a
"you need to start probing for this manually" warning, do we?  An
unconditional AC_DIAGNOSE would be spurious for most people...

I'm also good with restoring compatibility AC_DEFINEs for the C90
headers (HAVE_STRING_H, HAVE_STDDEF_H, HAVE_STDLIB_H, I think that was
all) just to be safe.

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