makepp_faq -- Frequently asked questions about makepp
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 cookbook.
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
$@ and neither new style
$(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.
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
--gullible option. But then the next command which depends on the produced file might still fail.
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
acregmin=0, making attributes visible immediately.
This can also happen with repositories, e.g. if someone else has built in the repository with
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. /opt/repository and /opt/sometool, in which case makepp will remember the path once as relative, and once as absolute, looking like changed dependencies.
In this rule why does makepp make output depend on input1, but not on 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 input1 and 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 (
cc -llib), include paths (
cc -Idir) and input files. The task of a
zcat parser would be to know that
-S takes an argument, but all other non option words are filenames (optionally suffixed by .gz), and that
-- 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.
You can put
$(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 if you want) makefile with the
--dump-makefile=file option, to see how makepp sees it.
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 makeppinfo, mppi for one or more files, usually gives a clue what is wrong.
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).
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
--dry-run (which does not run the rules, but evaluates everything else). That might be something like this:
bad_boy := $(shell rm *)
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 repository. And they are not extensible to other languages, as makepp's scanner 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 :include.
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 build cache, which has the added advantage that it can handle all kinds of files.
Daniel Pfeiffer (email@example.com)