retitle 593838 AX_CFLAGS_GCC_OPTION misuses AS_VAR_PUSHDEF variable
reassign 593838 autoconf-archive
thanks
Jerome BENOIT <jgm...@rezozer.net> initially reported that
AX_CFLAGS_GCC_OPTION(-std=c99) fails with Autoconf 2.67. Vincent
Bernat <ber...@debian.org> determined that the bug could be fixed
by replacing a literal shell assignment by AS_VAR_SET.
Just now, I took a closer look. I was able to reduce the problem
to the following trivial case:
--8<--------------------------cut here-------------------------->8--
AC_DEFUN([MY_MACRO],
[AS_VAR_PUSHDEF([VAR],[prefix_$1])
dnl This is the version used by AX_CFLAGS_GCC_OPTION.
dnl I believe that it is incorrect. It does not work on Autoconf >= 2.67.
VAR=the_value
dnl This version works.
dnl AS_VAR_SET([VAR], [the_value])
AS_VAR_POPDEF([VAR])])
AC_INIT
MY_MACRO([a=b])
--8<--------------------------cut here-------------------------->8--
If I comment out the first version and uncomment the second
version, it works.
The documentation in the Autoconf manual for AS_VAR_PUSHDEF says
this:
-- Macro: AS_VAR_PUSHDEF (M4-NAME, VALUE)
-- Macro: AS_VAR_POPDEF (M4-NAME)
A common M4sh idiom involves composing shell variable names from
an m4 argument (for example, writing a macro that uses a cache
variable). VALUE can be an arbitrary string, which will be
transliterated into a valid shell name by `AS_TR_SH'. In order to
access the composed variable name based on VALUE, it is easier to
declare a temporary m4 macro M4-NAME with `AS_VAR_PUSHDEF', then
use that macro as the argument to subsequent `AS_VAR' macros as a
polymorphic variable name, and finally free the temporary macro
with `AS_VAR_POPDEF'. These macros are often followed with `dnl',
to avoid excess newlines in the output.
Here is an involved example, that shows the power of writing
macros that can handle composed shell variable names:
m4_define([MY_CHECK_HEADER],
[AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl
AS_VAR_IF([my_Header], [yes], [echo "header $1 available"])dnl
AS_VAR_POPDEF([my_Header])dnl
])
MY_CHECK_HEADER([stdint.h])
for header in inttypes.h stdlib.h ; do
MY_CHECK_HEADER([$header])
done
This description says one may "use that macro as the argument to
subsequent `AS_VAR' macros as a polymorphic variable name", not
that one may use the M4-NAME literally. The example that it
gives also shows the specific M4-NAME of my_Header used only as
a polymorphic variable argument, not as a literal shell variable.
On that basis, I infer that one is not supposed to use the
M4-NAME literally, and thus I believe that this is a bug in
AX_CFLAGS_GCC_OPTION. So, I am reassigning this bug to
autoconf-archive (with this email).
If I am making any bad assumptions here, please correct me.
Thanks,
Ben.
--
Ben Pfaff
http://benpfaff.org
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