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=head1 NAME
makepp_statements -- Various statements in a makefile
=for vc $Id: makepp_statements.pod,v 1.36 2010/07/16 21:15:23 pfeiffer Exp $
=head1 DESCRIPTION
=for genindex '-?\w+' makepp_statements.pod
B<A:>E<nbsp>L<autoload|/autoload_filename>,E<nbsp>
B<B:>E<nbsp>L<build_cache|/build_cache_path_to_build_cache>,
  L<build_check|/build_check_build_check_method>,E<nbsp>
B<D:>E<nbsp>L</define>,E<nbsp>
B<E:>E<nbsp>L<export|/export_var>,E<nbsp>
B<G:>E<nbsp>L<global|/global_var>,E<nbsp>
B<I:>E<nbsp>L<ifdef|/ifdef_variable>,
  L<ifeq|/ifeq_string1_string2>,
  L<ifmakeperl|/ifperl_perlcode>,
  L<ifndef|/ifdef_variable>,
  L<ifneq|/ifeq_string1_string2>,
  L<ifnsys|/ifsys_wildcard>,
  L<ifntrue|/iftrue_expression>,
  L<ifperl|/ifperl_perlcode>,
  L<ifsys|/ifsys_wildcard>,
  L<iftrue|/iftrue_expression>,
  L<include|/include_makefile>,E<nbsp>
B<L:>E<nbsp>L<load_makefile|/load_makefile_some_directory_somewhere_makefile>,E<nbsp>
B<M:>E<nbsp>L<make|/prebuild_target>,
  L<makeperl|/perl_perlcode>,
  L<makesub|/sub>,E<nbsp>
B<N:>E<nbsp>L</no_implicit_load>,E<nbsp>
B<P:>E<nbsp>L<perl|/perl_perlcode>,
  L</perl_begin>,
  L<prebuild|/prebuild_target>,E<nbsp>
B<R:>E<nbsp>L<register_command_parser|/register_command_parser_command_word_class_name>,
  L<register_input_suffix|/register_input_suffix_command_word_suffix>,
  L<register_scanner|/register_scanner_command_word_scanner_name>,
  L<repository|/repository_directory>,
  L<runtime|/runtime_program_library>,E<nbsp>
B<S:>E<nbsp>L</signature>,
  L</sub>
A statement is any line beginning with a word which does not have a C<:> in
it.  (A colon implies that the line is a rule.)  For example, these are
statements:
    include extra_rules.mk
    load_makefile subdir
Makepp has a number of builtin statements which you may occasionally need to
use.
Note that wherever you see an underscore, you may also use a dash,
because makepp converts dashes to underscores in statement names.
=head2 Conditionals
Conditionals are special statements, which control what lines of the
Makeppfile are actually seen.  The simplest form (where C<ifxxx> stands for
any of the conditional statements documented below) is:
    ifxxx ...
        lines seen if the statement evaluates as true
    endif
or:
    ifxxx ...
        lines seen if the statement evaluates as true
    else
        lines seen if the statement evaluates as false
    endif
There is also the possibility to do complex combinations like this:
    ifxxx ...
      and ifxxx ...
      and ifxxx ...
    or ifxxx ...
      and ifxxx ...
        lines seen if the combined statements evaluate as true
    else ifxxx ...
    or ifxxx ...
      and ifxxx ...
        lines seen if the first combination evaluates as false
        and these combined statements evaluate as true
    else
        lines seen if the statements above evaluate as false
    endif
As is suggested by the indentation, B<C<and>> has higher precedence than
B<C<or>>.  In other words an B<C<or>> elects between two groups of
B<C<and>>`s.  There may be any number of C<and ifxxx>`s, C<or ifxxx>`s and
C<else ifxxx>`s.
The C<ifxxx> conditional statements are unique in that they may occur in the
middle of rule actions, as in the above example, without disrupting the rule.
=over 4
=item ifeq I<string1, string2>
=item ifneq I<string1, string2>
    ifeq ($(STR1),$(STR2))
       makefile lines if true
    else
       makefile lines if false
    endif
If the two strings match exactly (except for leading or trailing
whitespace), then the first set of lines is used; otherwise the second
is used.  The else clause is optional.
There are two other acceptable syntaxes for the C<ifeq> and C<ifneq>
statements:
    ifeq string1, string2
    ifeq string1 string2
which are equivalent.  Of course you can quote the strings as needed.
C<ifeq> and its friends C<ifneq>, C<ifdef>, C<ifndef>, C<ifperl>,
C<ifmakeperl>, C<ifsys> and C<iftrue> are primarily useful when you have
to build a program under several different conditions.  For example,
    BUILD_TYPE := debug    # "debug" or "production"
  
    ifeq ($(BUILD_TYPE), debug)
      CFLAGS := -g
    else
      CFLAGS := -O2
    endif
  
    program : *.o
        $(CC) $(CFLAGS) $(inputs) -o $(output) $(LIBS)
    ifeq ($(BUILD_TYPE), production)
        strip $(output)
    endif
  
    %.o : %.c
        $(CC) $(CFLAGS) -c $(input) -o $(output)
If this is a production build, all files are compiled with the C<-O2>
option instead of the C<-g> option.  Furthermore, the program C<strip>
is run on the resulting binary (in case you happened to link with some
libraries that were compiled in debug mode).
Sometimes it is easier to use the L<C<$(if)>
function|makepp_functions/if_string_result_if_string_not_blank_result_if_string_blank>
or L<C<$(perl)> function|makepp_functions/perl_perlcode> function instead of a
C<ifeq> statement.
If you just want to see whether a symbol is blank or not, you only need
to supply a single argument, like this:
    ifneq $(EXE_SUFFIX)
      # what to do if $(EXE_SUFFIX) is not blank
    endif
=item ifdef I<VARIABLE ...>
=item ifndef I<VARIABLE ...>
These statements work analogously to the C<ifeq> and C<ifneq> statements,
except that they test whether any of the variables is defined or not any is
(i.e. none is defined).  A variable is defined if:
=over 4
=item *
It was given a value with an assignment earlier in the makefile.  See
L<makepp_variables> for details.
=item *
It was given a value as a perl variable in a C<perl_begin> block.
=item *
The variable is present in the environment.
=item *
The variable is present on the command line, e.g., to invoke your
makefile, you typed
    makepp CFLAGS=-O2
=back
For example,
    ifndef CFLAGS
      CFLAGS := -g
    endif
In this case, C<CFLAGS> is set to C<-g> only if it wasn't already
defined.  Note that this statement could just as easily have been
written using the C<?=> assignment, like this:
    CFLAGS ?= -g
=item ifperl I<perlcode>
=item ifmakeperl I<perlcode>
These statements work analogously to the C<ifeq> and C<ifneq> statements,
except that the tests are in Perl.  The first variant is plain Perl code,
while the second variant first passes the statement through Make-style
variable expansion.
    VERSION := 3.0
    # VERSION is automatically also a Perl variable:
    ifperl $VERSION <= 2
      CPPFLAGS := -DNEW
    endif
    # quotes necessary for CFLAGS, because Perl sees only the value:
    ifmakeperl my $$x = '$(CFLAGS)'; $$x =~ /-g/
      CFLAGS := -g -O2
    endif
=item ifsys I<wildcard ...>
=item ifnsys I<wildcard ...>
Tests if the current system makepp is running on matches any of the wildcards
or not any (i.e. none).
    ifsys i[3-6]86
      and ifsys Linux SunOS
      ...               # An Intel platform with Linux or Solaris
    else ifnsys sparc power*
      ...               # Nor Sparc or PowerPC
    endif
There are up to six different strings you can match against.  The actual
strings are not standardized.  Three of them reflect what the Perl instance
was built for (not necessarily the same as where it is running), and the
others come from the system and all vary wildly in form.  You can find all of
what the current platform matches by typing the following command at the
Shell:
    perl -MConfig -e'print "$^O @Config{qw(archname myarchname)} "'; uname -mps
=item iftrue I<expression>
=item ifntrue I<expression>
Tests if the expression evaluates to some value other than zero or the empty
string.
=back
=head2 Other Multiline Statements
Conditionals may control a whole multiline statement, but they cannot be
inside such a statement.
=over 4
=item define X<endef> X<define> X<enddef>
    define VARIABLE
    variable value line 1
    variable value line 2
    endef
Defines $(VARIABLE)'s value to be all the lines between the C<define>
statement and the C<endef> statement.  See L<setting
variables|makepp_variables/Setting variables inside a makefile>.
=item perl_begin X<perl_begin> X<perl_end>
This is the same as C<perl>, but using GNU make style statement syntax.
This statement introduces a block of code which is interpreted verbatim
by perl.  It can be useful for defining functions, but you can do this
more concisely with the C<sub> statement.  A block of perl code in your
makefile can be useful to perform actions that are easier in perl than
with makepp functions and rules.
The remainder of the line following the C<perl_begin> statement is
ignored.  All text up until a line that begins at the left margin with
C<perl_end> is sent verbatim to the perl interpreter.  There can be no
spaces before C<perl_end>.
One example that I use this for is to make directories that might not
necessarily exist.  It's common in makefiles to put all the .o files in
a subdirectory (e.g., a directory with a name F<i386>, or F<sparc>, or
something that depends on the machine type).  But what if the directory
does not exist yet?  You can make each .o file depend on the
subdirectory, and put a rule in to build the subdirectory.  But it's a
lot easier just to do this:
    OBJDIR := $(ARCH)               # Where we put .o files.
    perl_begin
    -d $OBJDIR or mkdir $OBJDIR;    # Make sure the directory exists.
    perl_end
This way, every time the makefile is run, the subdirectory will be
created if it does not exist.
Some operations are better expressed in terms of regular expressions
than makepp's text functions.  For example,
    perl_begin
    if ($ARCH =~ /^i[56]86/) {          # You could do this with: ifsys i[56]86
      $CFLAGS = '-O6 -malign-double';   # On intel machines > 486, there
                                        # is a substantial speed penalty
                                        # for doubles that aren't quadword
                                        # aligned.
    } else {
      $CFLAGS = '-O6';
    }
    perl_end
  
    %.o: %.c
        $(CC) $(CFLAGS) -c $(input) -o $(output)
Any make variable can be accessed directly as a perl scalar.
In this case, we've set the value of C<CFLAGS>
differently based on a regular expression match on the
architecture flags.
As a final example, some pieces of information are easier to access
directly from perl than from makepp.  For example, you can access all of
the configuration information that perl knows about your system,
including how to build shared libraries, etc.  (Type
S<C<perldoc Config>> if you want to see what configuration information
perl has available.)
    perl_begin
  
    use Config;
  
    $ARCH = $Config{'archname'};    # Use perl's knowledge of the architecture.
    $CC = $Config{'cc'};            # Use the same C compiler as perl did.
    $SHARED_OBJ_CFLAGS = $Config{'cccdlflags'};
                                # Flags needed to compile objects which will
                                # go into a shared library.
    $SHARED_OBJ_LDFLAGS = $Config{'ccdlflags'} . " " . $Config{'lddlflags'};
                                # Linker flags to make a shared library.
    $SHARED_CC_LINK = $Config{'ld'}; # Command to produce shared libraries.
  
    $SHARED_EXTENSION = $Config{'dlext'}; # Extension of shared libraries.
    perl_end
  
    %.o: %.c
        $(CC) $(CFLAGS) $(SHARED_OBJ_CFLAGS) -c $(input) -o $(output)
  
    libmylib.$(DLEXT): *.o
        $(SHARED_CC_LINK) $(inputs) -o $(output) $(SHARED_OBJ_LDFLAGS)
Note how we define a bunch of variables in the perl block, and then we
use them afterwards in the rest of the makefile.  You can use the full
power of the perl interpreter to set your variables in arbitrarily
complicated ways.  You can run shell commands from your perl code,
access a database, or whatever you want.
=item perl I<perlcode>
=item makeperl I<perlcode>
This is the same as C<perl_begin>, but using Perl-style braces.  The first
variant is plain Perl code, while the second variant first passes the
statement through Make-style variable expansion.  Note that the difficulty of
parsing Perl's braces has lead to the following simple heuristic:
=over
=item *
If a double opening brace is found on the same or next line, a double closing
brace will terminate the block.  It must be at the beginning of a line, but
may be preceded by whitespace.
=item *
Else, if the closing brace is at the very end of the C<perl> line this is a
one liner.
=item *
Otherwise the closing brace must be at the very beginning of a following line,
i.e. no leading whitespace.
=back
For an efficient way to call Perl scripts, see
L<C<run>|makepp_extending/run_script_argumentsmakepp_extending/run>.  Unlike
the L<C<$(perl)>|makepp_functions/perl_perlcode> function, the return value of
this block is ignored.
    perl { print "passed this point in the makefile\n" }
  
    perl
    {
      print "and this one too\n";
    }
  
    ifdef NOISY
      perl {{
        print "as well as this one\n"
      }}
    endif
You can use the Perl debugger for your embedded code, by running makepp itself
in the debugger, where ... are the arguments, if any, you normally pass:
    perl -d -S mpp ...
It is hard to set breakpoints in Perl code that has not been loaded.  You can
work around this by putting this line into your embedded Perl, just before
where you want to break:
    $DB::single = 1;
Then you can type C<c> at the debugger's prompt, to continue till that point.
=item sub
=item makesub
This statement provides a way to define a perl subroutine inside your
makefile.  The first variant is plain Perl code, while the second variant
first passes the statement through Make-style variable expansion.  The syntax
is identical to that of the perl sub statement, except that prototypes are
meaningless.
For the three possibilities of putting the braces of the body, see the
explanation at the C<perl> statement.
A perl subroutine is invoked whenever a statement is seen, or when an
expression like S<C<$(name words)>> is seen.  For example, suppose that
for some reason you need to load the contents of a file into a make
variable.  (You could do this by saying S<C<$(shell cat filename)>> but
it's possible to do it without ever invoking the shell.)  This can be
done by placing the following into your makefile:
    sub f_file_contents {
      my ($file) = @_;          # Name the argument.
      open my $fh, $file or die "$file: $!\n";
      local $/ = undef;         # Slurp file in one read.
      <$fh>;
    }
  
    ifdef NEWSUB
      makesub f_VAR2
      {{
        $(VAR) * 2;
      }}
    endif
  
    makesub f_VAR1 { $(VAR) + 1 }
Now, with this function defined, you can write
    X = $(file_contents filename) # equivalent to builtin $(&cat filename)
and the variable C<$(X)> will fetch the contents of the given file every time
it gets expanded.  Use C<:=> to do this exactly once, or C<;=> to do this at
most once.
See L<makepp_extending> for more details and examples.
=back
=head2 Simple Statements
=over 4
=item autoload I<filename ...>
Specifies one or more makefiles to load should an attempt to find a rule for
a file in this directory otherwise fail.
This is useful when the makefile has rules whose definitions depend (possibly
indirectly) on a file in another directory that depends (possibly indirectly)
on other files in this directory (built by rules that do I<not> depend
on the file in the other directory).
For example, your F<Makeppfile> might look like this:
    rules-to-build-files-that-otherdir/x-depends-on
    more_rules.makeppfile: otherdir/x
        action-to-build-more_rules.makeppfile
    autoload more_rules.makeppfile
Note that we cannot reliably replace C<autoload> with C<include> here,
because if something other than the rule for F<more_rules.makeppfile>
tries to build F<otherdir/x> first, then F<more_rules.makeppfile> will
probably fail because F<otherdir/x> won't exist yet, because there is
already an attempt to build it underway when F<Makeppfile> is implicitly
loaded on its behalf.
WARNING: Be very careful about doing things in an autoloaded makefile that
change the behavior of rules in the directory's other makefileZ<>(s), as this
will cause that behavior to depend on whether or not some previously built
target caused makefiles to be autoloaded.
=item build_cache I</path/to/build/cache>
Specifies a path to a build cache.  See L<makepp_build_cache> for details.
The build cache must already exist; see
L<makepp_build_cache/How to manage a build cache> for how to make it in the
first place.  A C<build_cache> statement in a makefile overrides the
C<--build-cache> command line option for rules in the makefile, but it may be
overridden by the C<:build_cache> rule modifier on a per-rule basis.
Specify C<none> instead of a path to a directory if you want to disable
the build cache for all rules in this makefile.
=item build_check I<build_check_method>
Specifies the default build check method for all rules in this
makefile.  See L<makepp_build_check> for details.  The C<build_check>
statement overrides the C<--build-check-method> command line option for
all rules in the makefile, but may be overridden by the C<:build_check>
modifier on a per-rule basis.
=item export I<VAR ...>
=item export I<assignment>
    export PATH := $(PWD):$(PATH)
Marks the given variables for export to subprocesses.  See L<setting
variables|makepp_variables/Setting variables inside a makefile>.
=item global I<VAR ...>
=item global I<assignment>
    global MYPROJECT.INFO = info to be seen in all makefiles
Marks the given variables as global to all makefiles.  See L<setting
variables|makepp_variables/Setting variables inside a makefile>.
=item include I<makefile>
This inserts the contents of another makefile into the current makefile.  It
can be useful if you have boilerplate files with a number of rules or
variables, and each directory only needs to make a few modifications.  The
C<include> statement also used to be commonly used in traditional makes in
conjunction with automatic include file scanners, but this is no longer
necessary with makepp.
C<include> first considers the current directory, then the parent of the
current directory, then its parent, etc.  It stops considering
directories when it reaches the root of the file system or when the file
system device ID changes.  (This means that it will not find files
located in other NFS mounts.  This is to prevent problems with network
file systems or automounters and dead servers.)  If it does not find a
file of the given name by the time its search is stopped, then it looks
in the makepp data directory (F</usr/local/share/makepp> if you
installed makepp in F</usr/local>) for one of the include files that
comes with makepp.
If you want to include a template file in every makefile in a whole
directory hierarchy, you can place your makefile template at the top
directory.  The makefiles do not have to know exactly where they are in
the hierarchy; each makefile can contain a line like this:
    include standard_definitions.mk
instead of something more complicated, like this:
    include ../../../standard_definitions.mk  # Is this the right number of ..?
You can specify as many files as you want, and variables are allowed:
    include file1 file2 file3 $(other_include_files)
If you're working on a build that needs to work with both GNU make and
makepp, sometimes it's convenient to have exactly identical makefiles
but a different include file.  For example, all of your makefiles may
contain a line like this:
    include $(TOPDIR)/standard_rules.mk
and you want F<standard_rules.mk> to be different for GNU make and
makepp.  To facilitate this, the C<include> statement B<first> looks for
a file with the suffix of F<.makepp> before looking for the file you
asked for.  In this case, it would first look for a file called
F<standard_rules.mk.makepp>, and if that exists, it would load it
instead of F<standard_rules.mk>.  This way, when you run the makefile
with GNU make, it loads F<standard_rules.mk>, but with makepp, it loads
F<standard_rules.mk.makepp>.
=item _include I<makefile>
A minor variant on C<include>, the C<_include> statement includes the file if
it exists but doesn't generate a fatal error if it does not.  The C<_include>
statement used to be important for include file scanning with GNU make, but is
seldom useful for makepp.
=item load_makefile I</some/directory/somewhere/Makefile>
=item load_makefile I<subdir>
=item load_makefile I<VAR1=value1 VAR2=value2 subdir>
This statement causes makepp to cd to the directory containing the makefile
and load its rules into makepp's internal database.  If you specify just a
directory instead of a makefile, C<load_makefile> looks for C<Makeppfile>,
C<makefile>, or C<Makefile> in that directory.
Any variables you specify with the syntax C<VAR=value> (or S<C<VAR="value1
value2">>) are passed to the loaded makefiles.  They override any settings in
those makefiles, just as if you had typed them on the command line.
Using C<load_makefile> is different from the command
    include dir/makefile
in two ways.  First, C<load_makefile> does not transfer any variables from the
top-level makefile into the subordinate makefile; each makefile exists in its
own namespace.  The subordinate makefile cannot influence the variables in the
top-level makefile in any way.
Second, each build command is tagged with the directory of the makefile that
it came from.  When makepp executes a rule from a different makefile, it first
cd's to the directory containing that makefile before executing the command.
Makefiles which are seen with the C<include> statement are actually treated as
part of the makefile that included them, and therefore their rules are not
tagged with a different directory.
You usually do not have to load a makefile explicitly, unless it has an
unusual name, or it has targets which are not contained in the same
directory as the makefile itself, or you have disabled implicit makefile
loading.  By default, if makepp is trying to build a file and doesn't
have a rule to build it, or if it is evaluating a wildcarded filename in
a directory, it will automatically attempt to load a makefile from that
directory.  See L<makepp_cookbook/Tips for multiple directories> for
info on building with multiple directories.
You cannot use C<load_makefile> to load several makefiles that apply to the
same directory.  Use C<include> for several pieces of the makefile that apply
to the same directory, and C<load_makefile> for makefiles that apply to
different directories.
=item no_implicit_load
This statement turns off L<implicit loading|makepp_build_algorithm/Implicit
loading> of makefiles from a set of directories.  This can be useful if you
want to load makefiles automatically from most directories, but there are some
directories which for various reasons you do not want makepp to attempt to
update.  (E.g., maybe the directory has a makefile for some other version of
make which makepp does not understand.)  For example,
    no_implicit_load dir1 dir2/*
The above statement will turn off implicit loading for makefiles
in C<dir1> B<and all of its subdirectories>.  It
will also turn of implicit makefile loading for all subdirectories
of C<dir2> (and all of their subdirectories), but not
for C<dir2> itself.
You may use wildcards in the statement.  Non-directory files that
match the wildcard are ignored.  You can also use functions to further
specify the directories that you are interested in, e.g.,
    no_implicit_load $(filter-out dir1 dir2, *)
will turn off implicit loading for all subdirectories except F<dir1> and
F<dir2> and their subdirectories.
=item prebuild I<target>
=item make I<target>
The arguments (which undergo Make-style variable expansion) are built
immediately.  This is useful when the list of targets that the Makefile can
build depends on a generated file in another directory.
Currently, it will quietly fail to build targets if there is a dependency
loop among the prebuilt targets and the Makefiles that must be loaded to
build them, but that ought to be treated as an error.
=item register_scanner I<command_word scanner_name>
Registers the C<scanner_name> (which usually begins with prefix "scanner_"
that can be omitted here) as the command parser for commands whose base name
is C<command_word>.  In the name you can use C<-> interchangeably with C<_>.
The predefined scanners are documented in L<makepp_scanning>.
=item register_command_parser I<command_word class_name>
Register an anonymous subroutine that constructs and returns an object of
exact type Mpp::CommandParser::C<class_name> as the command parser
for commands whose base name is C<command_word>.
=item register_input_suffix I<command_word suffix ...>
Add C<suffix> ... to the list of input file suffixes recognized when an
action beginning with C<command_word> is scanned.  The scanner would
normally pick this up via Mpp::CommandParser::input_filename_regexp, but it
might instead ignore this entirely.
Mpp::Scanners don't normally pick up all the arguments that aren't recognized as
options, because they might be arguments of unrecognized options.
(For example, F<i386v> is I<not> an input file of the command C<gcc -b i386v foo.c>.)
Instead, they pick up only positional arguments that look like input filenames.
It is not unusual to use standard tools with site-specific nonstandard suffixes
in order to signify that those files require special handling, such as
different command options and/or postprocessing steps.  For example:
    register_input_suffix cpp .vpp
    %.v: %.vpp
        cpp $< > $@
=item repository I<directory>
=item repository I<destdir=srcdir>
Specifies one or more L<repository directories|makepp_repositories>.
The first repository specified has precedence over the others if the
same file exists in multiple repositories and there is no build command
for it.  See L<makepp_repositories> for more details about repositories.
If you specify just a directory after C<repository>, its contents are
linked into the current directory.  You can link its contents into any
arbitrary place in the file system by specifying the location before an
equals sign, e.g,
    repository subdir1/subdir2=/users/joe/joes_nifty_library
You should put the repository statement near the top of your makefile, before
any rules that may need to use it.
=item runtime I<program,library>
Store C<library> as a runtime dependency of C<program>.
Both C<program> and C<library> may contain multiple words, in which case
each word in C<library> is stored as a runtime dependency of each word in
C<program>.
When C<program> is added automatically as the executable dependency
of a command by the C<Mpp::CommandParser> base class, its runtime dependencies
(if any) are added as well.
In order for this to happen, C<program> must be specified in the rule
with a directory component, and without any shell meta characters.
The purpose of this statement is to capture dependencies on libraries and
other executables that are often loaded by the program, without having to
specify them explicitly as dependencies of each rule that invokes C<program>,
or to scan C<program> to determine those dependencies (which could be
prohibitively difficult.)
Runtime dependencies are traversed recursively, so if C<a> has a runtime
dependency on C<b> and C<b> has a runtime dependency on C<c>, then any rule
that uses C<./a> will have implicit dependencies on both C<b> and C<c> (unless
it uses a special C<Mpp::CommandParser> class that overrides this behavior).
Note that missing dependencies won't necessarily be added after you add this
statement to a makefile, unless the rule is re-scanned.
Use the C<--force-rescan> command line option to ensure that this happens.
=item signature
    signature md5
    signature c_compilation_md5
    signature default
Overrides the default signature method for all rules following the
C<signature> statement.  This overrides the signature method specified
on the command line with C<-m> or C<--signature-method>, but does not
override signature methods specified with the C<:signature> rule
modifier.
Specify S<C<signature default>> to return to makepp's default, either
the builtin default or the default specified on the command line.
For more information about signature methods, see L<makepp_signatures>.
=back
=head2 Commands
All builtin and self defined commands (see L<builtin commands|makepp_builtins>
and L<extending makepp|makepp_extending>), as well as external cleanly
programmed perl scripts can be used like statements.  In this case they differ
from rule actions in that they run in the same process as makepp and any input
or output files are not noted as dependencies or as having been built by
makepp.
As with all statements, they are considered as such, if they are indented less
than the actions of the previous rule, if any.
This can be used for messages to be output while reading the makefile:
    &echo The value of $$(VAR) is $(VAR)
Or instead of making many rules each depend on a directory creation rule, you
can simply create it on the fly.  Note that commands which create files are
processed again every time the makefile is read., That's why we protect this
one with a test -- though in this special case that would not be necessary, as
this command would do no harm when repeated:
    ifperl !-d 'include'
        &mkdir -p include           # Create only if not present
    endif
=head1 AUTHOR
Gary Holt (holt-makepp@gholt.net)