=head1 NAME

makepp_faq -- Frequently asked questions about makepp

=for vc $Id: makepp_faq.pod,v 1.12 2012/03/25 18:34:48 pfeiffer Exp $

=head1 DESCRIPTION

Here you may find some points which are not obvious from the rest of the
documentation.  This shows stumbling blocks, whereas howto type questions will
be found in the L<cookbook|makepp_cookbook>.

=head2 Build Questions

=head3 Why does it run this rule 3 times?

GNU make has no makepp style multi target rules.  Instead it interprets this
as a shortcut for three separate rules:

    a b c:
 	echo $@
 	touch a b c

However, it doesn't check why a file is there.  If a file exists (and is newer
than any dependencies) it is happy.  Whichever of the three files gets built
first, provides the other two, so this behaves somewhat like a multitarget
rule -- but can cause race conditions in parallel builds.

A similar rule might have been:

    a b c:
 	touch $@

Gmake indeed runs this one once per required file.  Without knowing what the
command does (it might be a script which internally creates some files), the
two cases can't easily be told apart by makepp.

So as a special compatibility fallback, if a multi target rule action mentions
only old style C<$@> and neither new style C<$(output)> nor C<$(target)> nor their
plural forms, it is treated as separate rules.  This however means running it
repeatedly, as makepp ignores randomly appearing files for which it has no
metadata.

=head3 Why does it complain that a created file is phony?

If you have a command that continues working asynchronously, after it came
back with a success return code, makepp will notice the promised file as
missing and complain.  This can also typically happen on some network file
systems, which may physically write only several seconds later.

If you cannot evite such an unsatisfactory situation, you can ask makepp to be
sloppy about this check with the C<--gullible> option.  But then the next
command which depends on the produced file might still fail.

=head3 Why does it recreate files unnecessarily?

I have observed this on NFS, where due to file attribute caching the timestamp
of the produced file was not yet the one the it finally had.  On the next run
makepp noticed the difference and considered the file unduly modified.  This
got resolved with a mount option of C<acregmin=0>, making attributes visible
immediately.

This can also happen with repositories, e.g. if someone else has built in the
repository with C<umask 066> or using a compiler that bars others from reading
the produced file.  This will also happen if either the repository or your
build tree shares a common path prefix with some dependencies
(e.g. F</opt/repository> and F</opt/sometool>, in which case makepp will
remember the path once as relative, and once as absolute, looking like changed
dependencies.

=head2 Miscellaneous

=head3 Why does makepp selectively detect dependencies?

In this rule why does makepp make F<output> depend on F<input1>, but not on
F<input2>?

    output:
 	zcat <input1 >output
 	zcat input2 >>output

There are three levels to scanning.  The first is the lexer, which tries to
understand the Shell part of the execution.  I.e. which commands get called
and what I/O redirections take place.  This notices F<input1> and F<output>
(even if it had not been declared a target of this rule).

The next step are the command parsers.  Makepp has a few for typical
compilation commands.  These check the command line options to understand what
the command will do.  In the process they pick up dependencies like libraries
(C<cc -llib>), include paths (C<cc -Idir>) and input files.  The task of a
C<zcat> parser would be to know that C<-S> takes an argument, but all other
non option words are filenames (optionally suffixed by F<.gz>), and that C<-->
ends options.  Alas there is no such parser, no more than for hundreds of
other commands.

The third step for some languages is the scanning of input files, to detect
includes as further dependencies.  This does not apply to this example.

=head3 How can I debug makepp?

You can put C<$(print )> around a suspicious expression.  This returns the
unchanged expression, while printing it as a side effect.

You can dump the current directory's (multiply after C<-C> if you want)
makefile with the C<--dump-makefile=file> option, to see how makepp sees it.

Makepp writes a log of everything it does and why.  You can look at that with
L<makepplog, mppl|makepplog> or L<makeppgraph, mppg|makeppgraph>.

Makepp records all it knows about a file, for reuse on the next run.  Though
it takes some understanding of makepp's internals, dumping it with
L<makeppinfo, mppi|makeppinfo> for one or more files, usually gives a clue
what is wrong.

=head3 Is it safe to use?

B<Yes>, it will do exactly what your makefiles say (which many programmers
find hard to understand, since rule based inference is very different from
most programming paradigms).

B<And no>, if you don't trust the makefiles you got, definitely not!  A
makefile is a funny kind of script, the purpose of which is to run commands
that are expected to modify your file system.  Makepp has no means of checking
what they will do.

Worse, there are execute always syntaxes, which are performed even with
C<--dry-run> (which does not run the rules, but evaluates everything else).
That might be something like this:

    bad_boy := $(shell rm *)

=head2 External tools

=head3 Can I use cc -M or gcc -MM?

The short answer is yes.  The long answer is that they have the advantage of
knowing the effect of even the last weird compiler option, and sub-includes
hidden in some compiler internal directory, where makepp only comes pretty
close.  The disadvantage is that they have no idea of the build rules, so they
can not reliably depend on yet to-be-built files, which includes files to be
fetched from a L<repository|makepp_repositories>.  And they are not extensible
to other languages, as makepp's L<scanner|makepp_scanning> is.  Usually you
are at least as well off, not resorting to these tools.

Nonetheless, some compilers can produce this as a by-product.  If you'd rather
use this see L<:include|makepp_rules/include_file_or_pattern>.

=head3 Can I use CCache, Compilercache or cachecc1?

The short answer is yes.  The long answer is that these programs need to
repeat the work makepp does, to get a reliable fingerprint of files.  With
traditional makes this even comes too late, because those miss many situations
calling for a recompilation.  With makepp it is just easier to use the built
in L<build cache|makepp_build_cache>, which has the added advantage that it
can handle all kinds of files.

=head1 AUTHOR

Daniel Pfeiffer (occitan@esperanto.org)