Re: Knowing which tool is being run, from M4

[Eric Blake]

On 8/5/20 2:35 PM, Zack Weinberg wrote:
> This feels like a stupid question but I’ve been failing to find the answer
> all day. Is there a way for M4 code (specifically in autoconf/general.m4,
> if it matters) to tell which command-line tool (autoconf, autoheader, etc.)
> is being run?

Nothing obvious jumps out at me.  Each command-line tool calls autom4te 
--language=XYZ, where the choice of language determines which other 
arguments are pre-loaded.  Perhaps seeing if a particular witness macro 
is defined from a given language's presets is enough to deduce things, 
though?  For example, the 'M4sugar' language is the base of the 
inheritance tree, so it is always loaded; the 'M4sh' language loads a 
definition of AS_INIT from m4sh.m4f (so if that macro is not defined, 
you are in m4sugar rather than m4sh); the 'Autotest' language loads 
autotest.m4f on top of M4sh (so seeing if AT_INIT is defined is a good 
witness); the 'Autoconf' language loads in autoconf.m4f on top of M4sh 
(so seeing if AC_INIT is defined is a good witness).  On the other hand, 
autom4te doesn't have an 'Autoheader' language per se, but does define 
'Autoheader-preselections', but that doesn't cause any extra files to be 
loaded or new macros to be defined, so offhand I don't see an easy 
witness to tell when autoheader is being run, and the autoheader binary 
calls autom4te --language=autoconf.

It might be an interesting idea to add support for a well-documented 
macro (probably at the m4sugar namespace) that can be queried to learn 
the current language, and then updating autom4te or each binary that 
calls autom4te to use -Dm4_language=XYZ to then make that macro be a 
reliable indicator of which language flavor is being run under autom4te. 
 But given the difference in release cycles between autoconf and 
automake, that may still not help you in the short term for learning 
when automake is running things (other than seeing if AM_INIT is defined).

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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