=head1 NAME

makepp_incompatibilities -- Incompatibilities between makepp and GNU make

=for vc $Id: makepp_incompatibilities.pod,v 1.37 2012/03/25 18:34:48 pfeiffer Exp $

=head1 DESCRIPTION

Makepp was designed to be as close as possible to GNU make
(L<http://www.gnu.org/software/make/manual/make.html>).  GNU autotools
(L<http://www.gnu.org/software/automake/manual/automake.html>), CMake
(L<http://www.cmake.org/>), Premake (L<http://industriousone.com/premake> and
see L<remark below|/Compatibility via the option:
--traditional-recursive-make>) or handcrafted legacy build systems should be
buildable with makepp.  This is so you can either migrate projects
effortlessly.  Or if you don't want to enjoy all of makepp's advantages
(e.g. so others can still build your project with GNU make) while you profit
from the reliability advantage for your development.

However, because of the difference in L<philosophy|makepp_build_algorithm>,
some of GNU make's or POSIX make's
(L<http://pubs.opengroup.org/onlinepubs/009695399/utilities/make.html>)
features cannot be supported.  A few have not been implemented because we
haven't had time.  Most of the differences from GNU make are quite technical
and only rarely cause problems.  Alas the workarounds for the short-comings of
traditional make are becoming more and more complex, and are giving makepp a
hard time.

In a nutshell, if it doesn't build out of the box, try:

    makepp --no-warn makepp_simple_concatenation=1 makepp_percent_subdirs=1 \
 	--build-check=target_newer --last-chance-rules --no-remake-makefiles

If that succeeds, you can try to eliminate those arguments one by one.  But if
that fails, try adding either one or both of:

 	--defer-include
 	--traditional-recursive-make

If that also fails, the build system needs some tweaking to cooperate with
makepp.  Even if some options described here make something buildable, it is
still recommended to adapt things slightly, so they become compatible out of
the box with both makes.

=head1 Forcing more POSIX or GNU make compatibility

Here are some command line possibilities for getting many legacy build systems
to work without modification.  They cause makepp to emulate GNU make's
behavior precisely.

=head2 Compatibility via the option: C<--build-check=target_newer>

By default, makepp will attempt to rebuild all targets if any of the
dependencies have changed since the last build, or if the command has changed
(see L<makepp_build_check> for details).  This is normally what you want.
Sometimes, however, you don't want the target to be rebuilt if it has been
modified apart from the control of makepp (e.g., by editing it, or by running
a program manually to make the file).  You can force makepp to use the
traditional make algorithm, which only rebuilds if any of the targets are
newer than the dependencies, by adding this option to the command line.

=head2 Compatibility via the option: C<--dont-build=config.status>

There are packages which try to autoconfigure themselves, or do other things,
which gmake ignores unless being asked to, like:

    config.status : configure
 	./config.status --recheck
 
    configure : configure.in aclocal.m4
 	autoconf

Most people don't even have C<autoconf> installed, so conscientiously doing
everything by the rules, as makepp does, will fail.  This option prevents
that, if you figure out what not to build.

=head2 Compatibility via the option: C<--last-chance-rules>

Default rules (pattern rules with no pattern dependencies) are not normally
supported.  Makepp instantiates all rules based on the existing files, so that
it is aware of every file that could be generated.  Alas this way it does not
know how to instantiate a pattern rule with no pattern dependency.  The
L<:last_chance|makepp_rules/last_chance> mechanism partially remedies that.
Where this is good enough for legacy makefiles, this option allows turning it
on globally.

=head2 Compatibility via the option: C<--no-warn>

This one doesn't improve the result.  Makepp will give warning messages for
many things which the traditional Unix make accepts without flinching.  This
is because there are better ways to do them with makepp.  If these warnings
annoy you, you can turn them off with this option.

=head2 Compatibility via the option: C<--hybrid-recursive-make>

Recursive invocations of make are often considered to be an unsafe practice
(see L<makepp/Better system for hierarchical builds> for details), but they
are extremely common in existing makefiles.  Makepp supports recursive make
for backward compatibility; for new makefiles, it is much better to use the
C<load_makefile> statement, or makepp's implicit makefile loading mechanism.

In order to be able to use repositories for variant builds, and to help make
recursive invocations of make safer, makepp normally does not actually invoke
itself recursively even if you tell it to.  Instead, a subprocess communicates
with the parent process, and the actual build is done by the parent process.

This works in most cases, but you may not invoke several makefiles from the
same directory, e.g., the following will not work:

    target: dependencies
    	$(MAKE) -f other_makefile targets

In this case makepp notices it is loading a 2nd makefile and complains.  With
this option instead it will fall back to the traditional way of building from
additional makefiles in a separate makepp process each.

Note: Technically loading several makefiles would be no problem, but they
usually have the same phony target names.  Keeping that apart would mean a
complete redesign of makepp internals.  However, this will work, but it is not
equivalent:

    target: dependencies
    	cd subdir && $(MAKE) -f other_makefile targets

=head2 Compatibility via the option: C<--traditional-recursive-make>

Sometimes the previous option is not enough, especially if the recursive
invocations use contradictory options.  Makepp uses only one set of global
options, so a submake is not allowed to modify them, as that would also
pertain to other makefiles.

Adding this option to the command line, has the following undesirable side
effects:

=over 4

=item *

Recursive makes do not internally execute in parallel, even if the parent
does.  Unlike gmake there is no overall coordination of the number of
processes.  This will not be implemented because this way of working is not a
design goal of makepp.

=item *

Recursive make processes do not know anything about repositories.

=item *

Each recursive make process produces its own log file, in the directory it is
invoked in, instead of producing one log file for the entire build.

=item *

Since makepp usually builds more than traditional make deems necessary, and
since many build systems provide recursive calls in all directions, this may
lead to endless recursion.  Makepp will pull the brake after 50 rounds and
tell you how to increase that, in case you really have such deep nesting.

=back

Even with the C<--traditional-recursive-make> option, the environment
variables C<MAKEOVERRIDES> and C<MFLAGS> are not set up, and ignored, so
makefiles that depend on those will not work.

A B<Premake> generated F<Makefile> is only a funny wrapper to a sub-make
invocation in the same directory.  If you have some project target I<XYZ> it
will have a line like

 	@${MAKE} --no-print-directory -C . -f XYZ.make

In this case you can avoid the C<--traditional-recursive-make> option by
directly invoking makepp with that C<-f I<XYZ>.make> option.

=head2 Compatibility without the option: C<--jobs=I<n>>

Legacy makefiles will sometimes not list all dependencies, relying on the
order of execution to make them in time.  In this situation makepp may manage
to call a rule before its dependencies have all been made.  Then results may
be better with less, or even no parallel execution.

=head2 Compatibility via the variable: C<makepp_simple_concatenation=1>

L<Rc-style substitution|makepp_variables/rc-style substitution> is the default
way makepp performs variable substitution into text strings because it very
rarely breaks legacy makefiles and is often useful in new makefiles.  However,
it does introduce occasional incompatibilities in the substitution of
variables not surrounded by spaces.  For example,

    INCLUDE_PREFIX := -I/some/include/dir -I
    INCLUDES := $(INCLUDE_PREFIX)/other/include/dir

will set C<INCLUDES> to
S<C<-I/some/include/dir/other/include/dir -I/other/include/dir>>
if rc-style substitution is enabled, whereas GNU make would set it
to S<C<-I/some/include/dir -I/other/include/dir>>.

There is also an incompatibility in the handling of whitespace in a variable:

    null :=
    T := -o $(null)		# T contains -o followed by one space.
    OUTFILE = $(T)outfile


will set C<OUTFILE> to C<-ooutfile> if rc-style substitution is enabled,
whereas GNU make would set it to S<C<-o outfile>>.

Both of these incompatibilities are removed by setting the
C<makepp_simple_concatenation> variable.  Note, however, that even with
C<makepp_simple_concatenation>, makepp still treats whitespace incompatibly in
some situations:

    T := -o # Don't delete this comment.

GNU make sets C<T> to contain C<-o> followed by a space, whereas makepp strips
out the trailing space anyway.  If you want the trailing space, you must set
C<makepp_simple_concatenation> and also set C<T> using the technique involving
a dummy variable such as C<null>, as shown above.

=head2 Workaround option C<--defer-include>

Makepp reads makefiles in a single pass, where GNU make is multipass.  Hence
this will not work, because F<GIT-VERSION-FILE> must be built to process the
C<include> statement.  But it depends on something that is unknown at this
point.  To solve this, the C<.PHONY> declaration must be moved up before the
C<include> statement:

    GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
    include GIT-VERSION-FILE
    .PHONY: FORCE

This option internally reorders the include statements to the end of the makefile.
That may in turn cause variables to have different values, in which case you
should set the problematic ones on the command line.

=head2 Workaround option C<--no-remake-makefiles>

Typical open source requires calling C<configure> to create the makefiles.
But then these makefiles can contain rules to remake the makefile, by calling
some command.  Makepp will happily comply and update it according to the rule.
But sometimes this is harmful, so just skip it.

=head2 Compatibility via the variable: C<makepp_percent_subdirs=1>

By default, C<%> in a pattern rule does not match directories.  This means
that a rule like this:

    %.o: %.c
    	$(CC) $(CFLAGS) -c $(input) -o $(output)

will not be applied to files like C<../shared/xyz.c>.  If you want it to match
files in subdirectories too, then set the variable C<makepp_percent_subdirs=1>
on the command line or near the beginning of a makefile.

=head2 Compatibility via the environment variable: C<$MAKEPP_IGNORE_OPTS>

Sometimes legacy recursive invocations pass options that makepp doesn't
understand.  Hopefully the option is not important, but it prevents makepp
from running.  With this environment variable you can ask makepp to silently
ignore certain options.  The value shall be a space separated list of options,
which can come in 4 variants:

=over 4

=item --I<long>=x

A long option that expects an argument.  This fact must be declared through
the equals sign, though the actual use may also separated by whitespace,
either C<--long=bla> or C<--long bla>.

=item --I<long>

A long option without an argument.

=item -I<s>x

A short option that expects an argument.  This fact must be declared by adding
something directly after the option, though the actual use may also separated
by whitespace, either C<-sbla> or C<-s bla>.

=item -I<s>

A short option without an argument.

=back

E.g. override makepp's -R option by one without an argument and accept gmake's
debug option with an argument:

    export MAKEPP_IGNORE_OPTS='-R --debug=x'

=head1 Incompatibilities that require Makefile changes

=over 4

=item *

Makefiles that explicitly call make prevent makepp from building everything
itself.  Alas Perl's own C<ExtUtils::MakeMaker> commits the second of the
following two forms of this mistake up to version 6.56 (Perl 5.12.1):

    subdir:
 	cd subdir; make

    MAKE = make

=item *

Setting the C<VPATH> variable to some value implicitly calls C<vpath % value>.
C<vpath> statements are emulated with the L<repository|makepp_repositories>
mechanism.  So, where gmake substitutes the path to the file found in the
vpath, makepp will instead link it symbolically to where it is needed.  Thus
makepp will provide an unmodified string, which is usually not a problem.

Targets in a vpath are not supported.  (Gmake considers them if they are newer
than their dependencies, but if not, the target will be recreated in the
current directory -- rather inconsistent.)  Unsetting vpaths is not supported.

=item *

A pattern rule present later in a makefile overrides one that is present
earlier.  This is backwards from GNU make.

=item *

The set of L<builtin implicit rules|makepp_builtin> is somewhat different from
those for GNU make, though the variable names are largely compatible.  The
builtin rules should successfully compile C/C++/Fortran programs, and in fact
may be able to guess the proper libraries in some cases too.  Support for
Modula-2 and RatFor and other rare languages is deliberately not present,
because I kept running into problems with GNU make's rules when I accidentally
reused the extensions for those languages.

=item *

An action prefix of C<+> is silently ignored.

=item *

Archive members are not supported, and neither are the associated automatic
variables C<$%>, C<$(%D)>, and C<$(%F)>.

=item *

There is no SCCS support.

=item *

Leading and trailing whitespace in variable assignments is ignored (even if
the whitespace is followed by a comment).  For more details on whitespace
handling incompatibilities, see L<makepp_variables/Whitespace in variables>.

=item *

Makepp does not attempt to rebuild files included with the C<include>
statement unless the makefile contains a rule for building them before the
include statement is seen.  (It will attempt to rebuild the makefile itself,
however.)  This is normally used for handling include file dependencies, and
is not as useful with makepp since you don't need to do that anyway.

=item *

The C<SHELL> variable is currently partially ignored.  Makepp always uses
F</bin/sh> unless F</usr/xpg4/bin/sh> or F</sbin/xpg4/sh> is found or unless
you export the C<SHELL> variable in your makefile.  But if you do, the command
parser might not fully understand what your shell command does.  On Windows
Strawberry or ActiveState Perl you must instead set your SHELL variable
B<before> calling makepp.

=item *

Dependencies of anything on the Makefile still work, but are usually
unnecessary.  This is usually used to force a rebuild when compilation options
change.  Makepp knows when build commands have changed without anything
special in the makefile; it stores this on a file-by-file basis.  If you
change the makefile, it knows exactly which files need recompilation.

=item *

Intermediate files are not deleted.  (Because makepp insists on having all of
the file dates be the same as they were on the last build, intermediate files
must all be present or else rebuilds will occur.)  There is no special status
accorded to intermediate files.

=item *

The only special target that is supported is C<.PHONY> and partially
C<.SUFFIXES>.  The remaining are simply ingored.

Specifically, GNU make has the following special targets:

=over 4

=item .SUFFIXES

Makepp ignores C<.SUFFIXES> except for the special case of C<.SUFFIXES> with
no dependencies, like this:

    .SUFFIXES:

which tells it not to load any of its default rules.

=item .INTERMEDIATE, .SECONDARY, .PRECIOUS

No special status is accorded to intermediate files and so these targets are
not meaningful.

=item .IGNORE

This target is ignored.  If you want to ignore errors, put the word
C<ignore_error> (or a minus sign) in front of the command whose exit status is
to be ignored.

=item .SILENT

This target is ignored.  If you want commands not to echo, put the word
C<noecho> (or the C<@> character) in front of the command which is not
supposed to be echoed, or use the C<--silent> option to makepp.

=item .DELETE_ON_ERROR

=item .EXPORT_ALL_VARIABLES

=item .NOEXPORT

=item .POSIX

=item .DEFAULT

These targets are not supported and are simply ignored.

=back

=item *

The GNU make functions C<eval>, C<flavor> and C<value> are not currently
supported.  You can achieve the same thing as eval in a more straight-forward
way with C<$[...]> variable or function expansion.

=item *

Double colon rules are not fully supported.  (They cannot be: in
makepp's paradigm, there cannot be more than one way to update a
target.)  Currently, each successive double colon rule for a given
target simply appends its command string and dependency list to the
command string and dependency list for this target.  For example, if you
write this:

    a :: b
    	&cat b -o a

    # Later in your makefile:
    a :: c
    	&cat c -o >>a

it is B<exactly> the same as if you had written

    a : b c
    	&cat b -o a
    	&cat c -o >>a

This is certainly not what double colon rules are intended for, and it
will not always work, but it does work for targets like C<clean> or for
all the stuff that ExtUtils::MakeMaker puts into its makefiles.  Don't
count on it for anything other than legacy makefiles.

=item *

The S<C<$(wildcard )>> function matches not only files which exist, but also
files which do not yet exist, but which have a rule which makepp has seen at
the time the S<C<$(wildcard )>> function is evaluated.

=item *

The C<define> statement is supported, but handling of C<@> preceding it
is done differently.  Currently in makepp, C<@> in front of a variable
which has a multi-line value will only suppress echoing of the first
line.  For example,

    define echo-lines
    &echo line1 -o $@
    &echo line2 -o>>$@
    endef

    x:
    	@$(echo-lines)

will not suppress printing of S<C<&echo line2>> as it does in GNU make;
it will only suppress printing of S<C<&echo line1>>.

=item *

Makepp does not support the following environment variables (it does not set
them up, and it just ignores them):

=over 4

=item MAKEOVERRIDES

=item MFLAGS

=back

=back

=head2 Incompatibilities in order of expression expansion


=over 4

=item *

In makepp, rule actions are expanded before all of the dependencies are
guaranteed to have been built.  You can work around this by changing rules
such as this:

    foo: bar
        genfoo < $(shell cat bar)

to this:

    foo: bar
        genfoo < `cat bar`

or this, which will make the file during the expansion:

    foo: bar
        genfoo < $(&cat $(make bar))

This is preferable here, because the file listed in F<bar> is also a
dependency of this rule, and makepp can now catch it when lexically analyzing
the redirection.

=item *

Though I have not seen this used, GNU make allows the following:

    colon = :
    a$(colon) b
 	echo $^

Makepp expands C<$(colon)> too late for this to work.  However it offers the
alternative C<$[colon]> syntax, which can do much more than GNU make, because
it is expanded very early.

=back

=head2 C<$(MAKE)> may include spaces

In an uninstalled makepp or if the platform doesn't seem to support starting a
Perl script by magic number or with C<--traditional-recursive-make> this
variable will include at least one space.  That is not a problem when using it
as a command.  But when passing it as an unquoted parameter to a script (as
the Perl 5.14.0 build system does), it will tear it apart into separate
parameters, leading to confusion.  So as a parameter it is safer to quote it
as C<'$(MAKE)'>. which doesn't break backward compatibility.

=head2 Target-specific assignments don't propagate

Makepp's target-specific variables are slightly different from GNU make's in
that they only apply to the rule for the one file mentioned, and not to any of
its predecessors; see
L<Target-specific assignments|makepp_variables/target_specific_assignments>.

=head2 Parentheses or braces don't nest

Makepp ends expressions at the first matching parenthesis or brace.  Instead
of this

    $(somefunction ... ( ) ...)	# GNU make style

you must use either of these

    ${somefunction ... ( ) ...}	# GNU make compatible
    $((somefunction ... ( ) ...)) # Makepp extension

This will probably be fixed in version 2.1, maybe optionally.

=head2 Minor points

=over

=item Pattern dependencies don't match phony targets

    %.a: %.b; ...
    $(phony x.b): ; ...		# does not provide a way to build x.a

=item Comments don't have continuation lines

    # This is \
    NOT a 2-line comment

=back

=head1 Command line incompatibilities

Makepp supports a few of make's more useful command line options.  The
following, however, are not supported:

=over 4

=item -d or --debug

=item -f -

Makepp's internal makefile objects are linked to file objects, so it can't
handle stdin.

=item -i

=item -l or --load-average or --max-load

=item -m

Makepp's C<-m> option has to do with signature method selection, whereas GNU
make ignores -m.

=item -p or --print-data-base

=item -q or --question

=item -R or --no-builtin-variables

Makepp's C<-R> option actually does something completely different.

=item -S --no-keep-going or --stop

The C<--stop> option stops (puts to sleep) makepp after learning all the
rules, so you can continue editing.

=item -t or --touch

=item -w or --print-directory

This happens automatically.

=item --warn-undefined-variables

=back

Some of these can be easily supported if anyone cares.
