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.