Re: [PATCH] autoconf: warn if AC_*_IFELSE lacks complete program

[Eric Blake]

On 08/26/2010 11:24 PM, Ralf Wildenhues wrote:
> Testing shows me that
>    AC_INIT
>    AC_PROG_CC
>    AC_COMPILE_IFELSE([AC_LANG_DEFINES_PROVIDED s])
>    AC_COMPILE_IFELSE([t])
>
> still manages to correctly warn about the second instance (might be a
> good testsuite addition), but I fail to see how that is ensured.  Is
>    m4_pushdef([M]) m4_define([M]) m4_popdef([M])
>
> documented to work and do the expected (what would that be?) thing?

Yes - GNU m4 is documented where m4_define([a],[b]) is strictly 
equivalent to m4_popdef([a])m4_pushdef([b]) (with the special case that 
if [a] was not already defined, the m4_popdef([a]) action will not warn 
about popping an undefined variable).  So it is always safe to use 
m4_define() to redefine the existing value of a variable; and when done 
inside an outer m4_pushdef/m4_popdef sequence, it has the net result of 
changing what the m4_pushdef did.  M4sugar uses this concept in several 
places.  (Some other m4 implementations are documented where m4_define 
wipes out the entire pushdef stack, rather than the top-most definition 
on the stack, and POSIX allows that alternate semantic, but it would 
break m4sugar.)

So, AC_COMPILE_IFELSE([AC_LANG_DEFINES_PROVIDED]) effectively expands to:

m4_pushdef([a],[warn])m4_define([a],[nowarn])a()m4_popdef([a])

and by the time a() is called, it has the [nowarn] definition.  On the 
other hand, AC_COMPILE_IFELSE(AC_LANG_DEFINES_PROVIDED) (note the 
missing quoting) expands to:

m4_define([a],[nowarn])m4_pushdef([a],[warn])a()m4_popdef([a])

and still issues a warning, because the AC_LANG_DEFINES_PROVIDED was 
expanded too early, and the expansion of a is still tied to the [warn] 
from the pushdef.

As for adding your example to the testsuite (of warning exactly once, 
and only on the second instance because the first was fine), yeah, that 
would make sense, so I'll try to get to that today.

--
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org