On 09/14/2016 07:26 AM, Russell Shaw wrote: > Hi, > > In binutils/configure.ac there is: > > m4_divert_text([PARSE_ARGS], > [case $srcdir in > *" "*) > m4_pushdef([AS_MESSAGE_LOG_FD], [])dnl > AC_MSG_ERROR([path to source, $srcdir, contains spaces]) > m4_popdef([AS_MESSAGE_LOG_FD])dnl > ;; > esac > ac_subdirs_all=`cd $srcdir && echo */configure | sed 's,/configure,,g'` > ]) > > > I can't find where it's used, and was wondering how one would use this > definition. AS_MESSAGE_LOG_FD is documented; https://www.gnu.org/software/autoconf/manual/autoconf.html#AS_005fMESSAGE_005fLOG_005fFD. But let's look at what happens here. AC_MSG_ERROR is a synonym for AS_ERROR, and the definition of AS_ERROR is: m4_defun_init([AS_ERROR], [m4_append_uniq([_AS_CLEANUP], [m4_divert_text([M4SH-INIT-FN], [_AS_ERROR_PREPARE[]])])], [as_fn_error m4_default([$2], [$?]) "_AS_QUOTE([$1])"m4_ifval(AS_MESSAGE_LOG_FD, [ "$LINENO" AS_MESSAGE_LOG_FD])]) That is, the result of invoking AC_MSG_ERROR is a shell-script call to the function as_fun_error, with either 2 arguments (the error code and message) or 4 arguments (the error code, message, current line number, and log fd), based on whether the current macro definition of AS_MESSAGE_LOG_FD is blank. Looking at that shell function, it behaves differently according to whether the third and fourth arguments are present, by additionally logging to the log fd. So what binutils is doing is writing a low-level macro where they DON'T want the message logged to config.log, so they temporarily redefine AS_MESSAGE_LOG_FD to the empty string around the duration of the expansion of AC_MSG_ERROR, in order to change how that macro is expanded. > > Doesn't the pushdef/popdef pair make it useless? No, the pushdef/popdef pair is what makes it so that the temporary non-logging nature of AC_MSG_ERROR() lasts only for the duration of the expansion of this snippet, rather than the rest of configure.ac. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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