Re: [PATCH 3/3] autoconf: refine quadrigraph test in _AC_DEFINE_UNQUOTED

[Eric Blake]

On 10/31/2016 06:39 AM, Paolo Bonzini wrote:
> It is a very common case that a quadrigraph appears in the argument of
> _AC_DEFINE_UNQUOTED, because "#define" is expanded through a quadrigraph.
> Therefore, restrict the quadrigraph tests to "$" (for "$(" and "${")
> and "(" (for "$(").
> 
> At the same time, "#" should not be used inside AC_ECHO because it confuses
> m4's comment parsing.  This pre-existing latent bug is caught by test 251:
> 
>    AC_DEFINE_UNQUOTED([bar], [[%!_!# X]])
> 
> Previously the quadrigraph in "@%:@define bar %!_!# X" made Autoconf fall back
> to cat anyway.  Now that Autoconf is not fooled by the quadrigraph, the test
> catches that "#" is not special-cased.  Kudos to Eric for coming up with it!
> 
> * lib/autoconf/general (_AC_DEFINE_UNQUOTED): Do not blindly
> use cat on all quadrigraphs.
> 
> Signed-off-by: Paolo Bonzini <address@hidden>
> ---
>  lib/autoconf/general.m4 | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)

ACK.

> 
> diff --git a/lib/autoconf/general.m4 b/lib/autoconf/general.m4
> index c15fb13..ef6285d 100644
> --- a/lib/autoconf/general.m4
> +++ b/lib/autoconf/general.m4
> @@ -2167,9 +2167,11 @@ m4_define([AC_DEFINE_UNQUOTED], [_AC_DEFINE_Q([_$0], 
> $@)])
>  # Append the pre-expanded STRING and a newline to confdefs.h, as if
>  # with an unquoted here-doc, but avoiding a fork in the common case of
>  # no backslash, no command substitution, no complex variable
> -# substitution, and no quadrigraphs.
> +# substitution (taking into account quadrigraphs as well).  Also
> +# avoid AS_ECHO if "#" is present to avoid confusing m4 with comments,
> +# but quadrigraphs are fine in that case.
>  m4_define([_AC_DEFINE_UNQUOTED],
> -[m4_if(m4_bregexp([$1], [\\\|`\|\$(\|\${\|@]), [-1],
> +[m4_if(m4_bregexp([$1], [#\|\\\|`\|\(\$\|@S|@\)\((|{|@{:@\)]), [-1],

Line noise for the win!

>         [AS_ECHO(["AS_ESCAPE([$1], [""])"]) >>confdefs.h],
>         [cat >>confdefs.h <<_ACEOF
>  [$1]
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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