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[sr #110518] Document that code snippets undergo shell variable expansio

From: Bruno Haible
Subject: [sr #110518] Document that code snippets undergo shell variable expansion
Date: Sun, 18 Jul 2021 04:51:07 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:88.0) Gecko/20100101 Firefox/88.0


                 Summary: Document that code snippets undergo shell variable
                 Project: Autoconf
            Submitted by: haible
            Submitted on: Sun 18 Jul 2021 10:51:05 AM CEST
                Category: None
                Priority: 5 - Normal
                Severity: 3 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: None



Code snippets passed to AC_RUN_IFELSE, AC_LANG_IFELSE, and more generally
AC_LANG_CONFTEST, undergo the expansions inside unquoted here documents. This
comes from

cat > conftest.$ac_ext <<_ACEOF

There is already user code that relies on this (in Gnulib, for example:
btowc.m4, fchownat.m4, gettext.m4, gnulib-common.m4, iswdigit.m4,
iswxdigit.m4, mbrlen.m4, mbrtoc32.m4, mbrtowc.m4, mbsnrtowcs.m4, mbsrtowcs.m4,
socklen.m4, stdint.m4, strfmon_l.m4, warn-on-use.m4, wcrtomb.m4,
wcsnrtombs.m4, wcsrtombs.m4, wctob.m4). Therefore it's unlikely that this can
ever be changed.

But people who write invocations AC_RUN_IFELSE, AC_LANG_IFELSE, etc. need to
be aware of it, otherwise it's easy to introduce bugs. See the first patch
from https://lists.gnu.org/archive/html/bug-gnulib/2021-07/msg00025.html

The Autoconf documentation should mention that in such code snippets, $
expansion references environment variables, and $ and \ meant as literal
values need to be backslash-escaped.


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