Provided by: makepp_2.0.98.5-2.1_all 
NAME
makepp_build_check -- How makepp decides to rebuild files
DESCRIPTION
A: "architecture_independent", E: "exact_match", I: "ignore_action", O: "only_action",
T: "target_newer"
Makepp stores a variety of information about how any given file was built the last time. This
information includes the build command, the architecture, and the signatures of all the file's
dependencies. (All the stored information is in the subdirectory .makepp of each directory.) If any of
this information has changed, makepp usually decides to rebuild the file. The build check method is what
controls makepp's decision to rebuild. It decides which information to look at, and which to ignore.
Makepp usually picks the correct build check method automatically. However, you can change the signature
method for an individual rule by using :build_check modifier on the rule, or for all rules in a makefile
by using the build_check statement, or for all makefiles at once using the -m or --build-check-method
command line option.
The data used to decide about a rebuild or a repository or build cache import is stored in the internal
build info file. You can display it with makeppinfo, mppi. Below each method gives an example of how to
see its keys.
Build check methods included in the distribution
At present, there are five build check methods included in the distribution:
exact_match
This method uses the modification dates on the file as signatures. It rebuilds the targets unless
all of the following conditions are true:
• The signature of each dependency is the same as it was on the last build.
• The signature of each target is the same as it was on the last build (i.e., no one has messed
with the targets since makepp built them).
• The build command has not changed.
• The machine architecture (or what Perl thinks it is) has not changed.
"exact_match" is the default method unless you are rebuilding a Makefile (see below). This is a
highly reliable way of ensuring correct builds, and is almost always what you want. However, it does
have a few side effects that may be surprising:
• If you've been compiling with the traditional make, and then switch to makepp, everything is
recompiled the first time you run makepp.
• If you damage makepp's information about what happened on the last build (e.g., you delete the
subdirectory ".makepp", or don't copy it when you copy everything else), then a rebuild is
triggered.
• If you replace a file with an older version, a rebuild is triggered. This is normally what you
want, but it might be surprising.
• If you modify a file outside of the control of makepp (e.g., you run the compilation command
yourself), then makepp will rebuild the file next time. (If you want to avoid this, check out
the "--dont-build" command line option.)
• Architecture-independent files are rebuilt when you switch to a different architecture. This is
usually not a problem, because they often don't take long to build. The reason why all files are
tagged with the architecture, instead of just binary files, is that often times even ASCII files
are architecture-dependent. For example, output from the Solaris "lex" program won't compile on
Linux (or at least this used to be true the last time I tried it).
Concretely, a file will not be rebuilt, or can be fetched from repository or build cache, if the
following command output stays the same, i.e. matches the signatures of the dependencies:
mppi -k'COMMAND ARCH SORTED_DEPS DEP_SIGS ENV_{DEP,VAL}S' file
architecture_independent
The "architecture_independent" method is the same as "exact_match" except that it does not check the
architecture. This can be useful for architecture-independent files, that don't need to be rebuilt
when you switch to a different architecture. For example, you probably don't need to rerun "bison"
on Solaris if you already ran it on Linux.
The "architecture_independent" method is best used by specifying it using the
":build_check architecture_independent" modifier to the each rule that produces architecture
independent files. Makepp by default never assumes any files are architecture independent, because
even .c files can be architecture dependent. For example, the output of Solaris lex will not compile
under Linux, or at least it wouldn't last time I tried. So you must manually specify this build
check method for any files which are truly architecture-independent.
Concretely, a file will not be rebuilt, or can be fetched from repository or build cache, if the
following command output stays the same, i.e. matches the signatures of the dependencies:
mppi -k'COMMAND SORTED_DEPS DEP_SIGS ENV_{DEP,VAL}S' file
ignore_action
The "ignore_action" method is the same as "exact_match" except that it does not check the action
string (the command). Sometimes a command can change and you don't want to force a rebuild.
For example, you might want to explicitly put a date into your command to log when the build was
done, but you don't want to force a rebuild every time the command is executed. For example,
BUILD_DATE := $(shell date)
my_program : $(MODULES).o
$(CXX) $(inputs) -DBUILD_DATE="\"$(BUILD_DATE)\"" date_stamp.c -o $(output)
This will compile date_stamp.c with the last build date stamp, but won't force a recompile when the
date changes. Unfortunately, if something else about the link command changes (e.g., you change
linker options), it also won't trigger a rebuild.
This is also useful in conjunction with the $(changed_inputs) or $? variable for actions that merely
update a target, rather than rebuilding it from scratch. For example, you could update a .a file
like this:
libmine.a : *.o : build_check ignore_action
$(AR) ru $(output) $(changed_inputs)
This will still mostly work if you forget to specify the ": build_check ignore_action". However,
suppose that none of the .o files have changed. The command will now be "ar ru libmine.a" which is
probably different from what it was last time (e.g., "ar ru libmine.a buggy_module.o"), so makepp
will run the command. In this case, the command won't do anything except waste time.
Building .a files like this is discouraged, because it can leave stale .o files inside the archive.
If you delete a source file, the .o file is still inside the .a file, and this can lead to incorrect
builds. It's better to build a .a file like this:
libmine.a : *.o
&rm $(output)
$(AR) ru $(output) $(inputs)
Concretely, a file will not be rebuilt, or can be fetched from repository or build cache, if the
following command output stays the same, i.e. matches the signatures of the dependencies:
mppi -k'ARCH SORTED_DEPS DEP_SIGS ENV_{DEP,VAL}S' file
target_newer
The "target_newer" method looks only at the file date. If any dependency is more recent than the
target, the target is rebuilt. This is the algorithm that the traditional Unix make utility uses.
The "target_newer" method isn't as safe as the "exact_match" method because it won't trigger a
rebuild if you change the build command, or if you replace a file with an older version. Sometimes
also it can get confused if clocks are not properly synchronized. For example, if a file somehow
gets a date of June 4, 2048, then between now and 2048, every file that depends on that file will be
rebuilt even though the file doesn't change. Also switching to a different architecture won't
trigger a rebuild. It prevents fetching a rule's target from a build cache, because there is no
unique signature that can be associated to the endless set of pairs fulfilling the relationship newer
than.
But there are a few cases where you may want to use the "target_newer" method:
• When it is reasonable for a user to build a file outside of the control of makepp. Perhaps the
most common example are the commands that generate the makefile itself, i.e., the autoconfigure
procedure. Users commonly issue the configure command manually, but makefiles often have a way
to update themselves automatically. In this case, we don't want to force the makefile to rebuild
itself if the user typed the command in manually, so the "target_newer" method is more
appropriate than the "exact_match" method. In fact, if makepp is trying to build a makefile, it
makes "target_newer" the default method until it has finished building the makefile.
• When it is reasonable for a user to modify a file after makepp has built it. For example, if a
file does not exist, you may want to copy it from a central location, or check it out from a
repository; but the user should be allowed to modify it. If you use the default "exact_match"
build check method, makepp will detect that the user has changed the file and so it will force a
fresh copy from the central location or a fresh checkout, wiping out the user's changes.
If you need to manually check the timestamps, see makeppinfo examples for how to get the path of each
dependency.
only_action
The very specific "only_action" method will only execute the action if the command string differs
from the last time it was executed. For example,
$(ROOT)/include/%.h : %.h
&ln -fr $(input) $(output)
publishes a file, but does not repeat this when the file changes. Note that the &ln command is
builtin and thus cheap, but makepp still has to fork off and monitor a process to perform the whole
action. So if you have lots of files to publish, there is still a benefit. Actually we did not
specify the method, because, when the target is a symbolic link, this build check gets used
automatically. You only need to specify it for other commands that depend solely on the command
(which usually contains the names of the inputs):
%.list : %.x : build_check only_action
&echo $(inputs) -o $(output)
Concretely, a file will not be rebuilt, or can be fetched from repository or build cache, if the
following command output stays the same, i.e. matches the signatures of the dependencies:
mppi -kCOMMAND file
Other build check methods are possible. You can write your own build check method by creating a module
"Mpp::BuildCheck::MyMethod". Read the documentation in Mpp/BuildCheck.pm in the makepp distribution.
Most likely, you will want your build check method to inherit from "Mpp::BuildCheck::exact_match", so
read its documentation too.
It's more commonly useful modify the signature mechanism than to modify the build check mechanism
directly. Before you change the build check mechanism, see if your problem is better served by changing
signatures (see makepp_signatures for details).
Here are some reasons why a custom build check method might be useful:
• If you want makepp to ignore part of the command. For example, if you have commands in your makefile
like this:
x.o : x.c
ssh $(REMOTE_MACHINE) cc $< -o $@
you might want makepp not to force a rebuild if "$(REMOTE_MACHINE)" changes. You could modify the
"exact_match" method so it knows about ssh commands and ignores the machine name. Check :dispatch
for another way to achieve that.
perl v5.32.0 2021-01-06 MAKEPP_BUILD_CHECK(1)