Re: proposed gnulib-related additions to Autoconf

[Eric Blake]

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According to Paul Eggert on 3/1/2006 11:28 AM:
> "Gary V. Vaughan" <address@hidden> writes:
> 
>>   i) Let's just improve AC_CHECK_HEADERS (et al) to always make configure
>>      smaller and faster by despatching to (undocumented internal)
>>      _AC_CHECK_HEADERS_ONCE when it can.
> 
> Won't that break old scripts?  If they do something like this:

Not just old scripts.  Read on.

> 
>   if test $ac_feature_foo = yes; then
>     AC_CHECK_HEADERS([sys/param.h])
>   fi
> 
> then we'd hoist the test for sys/param.h out of the then-part, which
> is not what was intended.

Indeed, my recent patch for gnulib's m4/sys_socket_h.m4 relies on the
AC_CHECK_HEADERS *NOT* being run when expanded inside a conditional, since
on cygwin, the header winsock2.h exists but is incompatible with
sys/socket.h.  If we used AC_CHECK_HEADERS_ONCE in that case, then cygwin
would get annoying warning messages from CVS head coreutils that a header
is present but cannot be compiled.

- --
Life is short - so eat dessert first!

Eric Blake             address@hidden
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