Provided by: cons_2.3.0.1+2.2.0-1_all bug


       Cons - A Software Construction System


       A guide and reference for version 2.2.0

       Copyright (c) 1996-2000 Free Software Foundation, Inc.

       This program is free software; you can redistribute it and/or modify it under the terms of
       the GNU General Public License as published by the Free Software Foundation; either
       version 2 of the License, or (at your option) any later version.

       This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.

       You should have received a copy of the GNU General Public License along with this program;
       see the file COPYING.  If not, write to the Free Software Foundation, Inc., 59 Temple
       Place - Suite 330, Boston, MA 02111-1307, USA.


       Cons is a system for constructing, primarily, software, but is quite different from
       previous software construction systems. Cons was designed from the ground up to deal
       easily with the construction of software spread over multiple source directories. Cons
       makes it easy to create build scripts that are simple, understandable and maintainable.
       Cons ensures that complex software is easily and accurately reproducible.

       Cons uses a number of techniques to accomplish all of this. Construction scripts are just
       Perl scripts, making them both easy to comprehend and very flexible. Global scoping of
       variables is replaced with an import/export mechanism for sharing information between
       scripts, significantly improving the readability and maintainability of each script.
       Construction environments are introduced: these are Perl objects that capture the
       information required for controlling the build process. Multiple environments are used
       when different semantics are required for generating products in the build tree. Cons
       implements automatic dependency analysis and uses this to globally sequence the entire
       build. Variant builds are easily produced from a single source tree. Intelligent build
       subsetting is possible, when working on localized changes. Overrides can be setup to
       easily override build instructions without modifying any scripts. MD5 cryptographic
       signatures are associated with derived files, and are used to accurately determine whether
       a given file needs to be rebuilt.

       While offering all of the above, and more, Cons remains simple and easy to use. This will,
       hopefully, become clear as you read the remainder of this document.

Why Cons? Why not Make?

       Cons is a make replacement. In the following paragraphs, we look at a few of the
       undesirable characteristics of make--and typical build environments based on make--that
       motivated the development of Cons.

       Build complexity

       Traditional make-based systems of any size tend to become quite complex. The original make
       utility and its derivatives have contributed to this tendency in a number of ways. Make is
       not good at dealing with systems that are spread over multiple directories. Various work-
       arounds are used to overcome this difficulty; the usual choice is for make to invoke
       itself recursively for each sub-directory of a build. This leads to complicated code, in
       which it is often unclear how a variable is set, or what effect the setting of a variable
       will have on the build as a whole. The make scripting language has gradually been extended
       to provide more possibilities, but these have largely served to clutter an already
       overextended language. Often, builds are done in multiple passes in order to provide
       appropriate products from one directory to another directory. This represents a further
       increase in build complexity.

       Build reproducibility

       The bane of all makes has always been the correct handling of dependencies. Most often, an
       attempt is made to do a reasonable job of dependencies within a single directory, but no
       serious attempt is made to do the job between directories. Even when dependencies are
       working correctly, make's reliance on a simple time stamp comparison to determine whether
       a file is out of date with respect to its dependents is not, in general, adequate for
       determining when a file should be rederived. If an external library, for example, is
       rebuilt and then ``snapped'' into place, the timestamps on its newly created files may
       well be earlier than the last local build, since it was built before it became visible.

       Variant builds

       Make provides only limited facilities for handling variant builds. With the proliferation
       of hardware platforms and the need for debuggable vs. optimized code, the ability to
       easily create these variants is essential. More importantly, if variants are created, it
       is important to either be able to separate the variants or to be able to reproduce the
       original or variant at will. With make it is very difficult to separate the builds into
       multiple build directories, separate from the source. And if this technique isn't used,
       it's also virtually impossible to guarantee at any given time which variant is present in
       the tree, without resorting to a complete rebuild.


       Make provides only limited support for building software from code that exists in a
       central repository directory structure.  The VPATH feature of GNU make (and some other
       make implementations) is intended to provide this, but doesn't work as expected: it
       changes the path of target file to the VPATH name too early in its analysis, and therefore
       searches for all dependencies in the VPATH directory.  To ensure correct development
       builds, it is important to be able to create a file in a local build directory and have
       any files in a code repository (a VPATH directory, in make terms) that depend on the local
       file get rebuilt properly.  This isn't possible with VPATH, without coding a lot of
       complex repository knowledge directly into the makefiles.

Keeping it simple

       A few of the difficulties with make have been cited above. In this and subsequent
       sections, we shall introduce Cons and show how these issues are addressed.

       Perl scripts

       Cons is Perl-based. That is, Cons scripts--Conscript and Construct files, the equivalent
       to Makefile or makefile--are all written in Perl. This provides an immediate benefit: the
       language for writing scripts is a familiar one. Even if you don't happen to be a Perl
       programmer, it helps to know that Perl is basically just a simple declarative language,
       with a well-defined flow of control, and familiar semantics. It has variables that behave
       basically the way you would expect them to, subroutines, flow of control, and so on. There
       is no special syntax introduced for Cons. The use of Perl as a scripting language
       simplifies the task of expressing the appropriate solution to the often complex
       requirements of a build.

       Hello, World!

       To ground the following discussion, here's how you could build the Hello, World! C
       application with Cons:

         $env = new cons();
         Program $env 'hello', 'hello.c';

       If you install this script in a directory, naming the script Construct, and create the
       hello.c source file in the same directory, then you can type `cons hello' to build the

         % cons hello
         cc -c hello.c -o hello.o
         cc -o hello hello.o

       Construction environments

       A key simplification of Cons is the idea of a construction environment. A construction
       environment is an object characterized by a set of key/value pairs and a set of methods.
       In order to tell Cons how to build something, you invoke the appropriate method via an
       appropriate construction environment. Consider the following example:

         $env = new cons(
               CC      =>      'gcc',
               LIBS    =>      'libworld.a'

         Program $env 'hello', 'hello.c';

       In this case, rather than using the default construction environment, as is, we have
       overridden the value of `CC' so that the GNU C Compiler equivalent is used, instead. Since
       this version of Hello, World! requires a library, libworld.a, we have specified that any
       program linked in this environment should be linked with that library. If the library
       exists already, well and good, but if not, then we'll also have to include the statement:

         Library $env 'libworld', 'world.c';

       Now if you type `cons hello', the library will be built before the program is linked, and,
       of course, `gcc' will be used to compile both modules:

         % cons hello
         gcc -c hello.c -o hello.o
         gcc -c world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       Automatic and complete dependency analysis

       With Cons, dependencies are handled automatically. Continuing the previous example, note
       that when we modify world.c, world.o is recompiled, libworld.a recreated, and hello

         % vi world.c
         % cons hello
         gcc -c world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       This is a relatively simple example: Cons ``knows'' world.o depends upon world.c, because
       the dependency is explicitly set up by the `Library' method. It also knows that libworld.a
       depends upon world.o and that hello depends upon libworld.a, all for similar reasons.

       Now it turns out that hello.c also includes the interface definition file, world.h:

         % emacs world.h
         % cons hello
         gcc -c hello.c -o hello.o
         gcc -o hello hello.o libworld.a

       How does Cons know that hello.c includes world.h, and that hello.o must therefore be
       recompiled? For now, suffice it to say that when considering whether or not hello.o is up-
       to-date, Cons invokes a scanner for its dependency, hello.c. This scanner enumerates the
       files included by hello.c to come up with a list of further dependencies, beyond those
       made explicit by the Cons script. This process is recursive: any files included by
       included files will also be scanned.

       Isn't this expensive? The answer is--it depends. If you do a full build of a large system,
       the scanning time is insignificant. If you do a rebuild of a large system, then Cons will
       spend a fair amount of time thinking about it before it decides that nothing has to be
       done (although not necessarily more time than make!). The good news is that Cons makes it
       very easy to intelligently subset your build, when you are working on localized changes.

       Automatic global build sequencing

       Because Cons does full and accurate dependency analysis, and does this globally, for the
       entire build, Cons is able to use this information to take full control of the sequencing
       of the build. This sequencing is evident in the above examples, and is equivalent to what
       you would expect for make, given a full set of dependencies. With Cons, this extends
       trivially to larger, multi-directory builds. As a result, all of the complexity involved
       in making sure that a build is organized correctly--including multi-pass hierarchical
       builds--is eliminated. We'll discuss this further in the next sections.

Building large trees--still just as simple

       A hierarchy of build scripts

       A larger build, in Cons, is organized by creating a hierarchy of build scripts. At the top
       of the tree is a script called Construct. The rest of the scripts, by convention, are each
       called Conscript. These scripts are connected together, very simply, by the `Build',
       `Export', and `Import' commands.

       The Build command

       The `Build' command takes a list of Conscript file names, and arranges for them to be
       included in the build. For example:

         Build qw(

       This is a simple two-level hierarchy of build scripts: all the subsidiary Conscript files
       are mentioned in the top-level Construct file. Notice that not all directories in the tree
       necessarily have build scripts associated with them.

       This could also be written as a multi-level script. For example, the Construct file might
       contain this command:

         Build qw(

       and the Conscript file in the drivers directory might contain this:

         Build qw(

       Experience has shown that the former model is a little easier to understand, since the
       whole construction tree is laid out in front of you, at the top-level. Hybrid schemes are
       also possible. A separately maintained component that needs to be incorporated into a
       build tree, for example, might hook into the build tree in one place, but define its own
       construction hierarchy.

       By default, Cons does not change its working directory to the directory containing a
       subsidiary Conscript file it is including.  This behavior can be enabled for a build by
       specifying, in the top-level Construct file:

         Conscript_chdir 1;

       When enabled, Cons will change to the subsidiary Conscript file's containing directory
       while reading in that file, and then change back to the top-level directory once the file
       has been processed.

       It is expected that this behavior will become the default in some future version of Cons.
       To prepare for this transition, builds that expect Cons to remain at the top of the build
       while it reads in a subsidiary Conscript file should explicitly disable this feature as

         Conscript_chdir 0;

       Relative, top-relative, and absolute file names

       You may have noticed that the file names specified to the Build command are relative to
       the location of the script it is invoked from. This is generally true for other filename
       arguments to other commands, too, although we might as well mention here that if you begin
       a file name with a hash mark, ``#'', then that file is interpreted relative to the top-
       level directory (where the Construct file resides). And, not surprisingly, if you begin it
       with ``/'', then it is considered to be an absolute pathname. This is true even on systems
       which use a back slash rather than a forward slash to name absolute paths.

       Using modules in build scripts

       You may pull modules into each Conscript file using the normal Perl `use' or `require'

         use English;
         require My::Module;

       Each `use' or `require' only affects the one Conscript file in which it appears.  To use a
       module in multiple Conscript files, you must put a `use' or `require' statement in each
       one that needs the module.

       Scope of variables

       The top-level Construct file and all Conscript files begin life in a common, separate Perl
       package.  Cons controls the symbol table for the package so that, the symbol table for
       each script is empty, except for the Construct file, which gets some of the command line
       arguments.  All of the variables that are set or used, therefore, are set by the script
       itself--not by some external script.

       Variables can be explicitly imported by a script from its parent script. To import a
       variable, it must have been exported by the parent and initialized (otherwise an error
       will occur).

       The Export command

       The `Export' command is used as in the following example:

         $env = new cons();
         $INCLUDE = "#export/include";
         $LIB = "#export/lib";
         Export qw( env INCLUDE LIB );
         Build qw( util/Conscript );

       The values of the simple variables mentioned in the `Export' list will be squirreled away
       by any subsequent `Build' commands. The `Export' command will only export Perl scalar
       variables, that is, variables whose name begins with `$'. Other variables, objects, etc.
       can be exported by reference--but all scripts will refer to the same object, and this
       object should be considered to be read-only by the subsidiary scripts and by the original
       exporting script. It's acceptable, however, to assign a new value to the exported scalar
       variable--that won't change the underlying variable referenced. This sequence, for
       example, is OK:

         $env = new cons();
         Export qw( env INCLUDE LIB );
         Build qw( util/Conscript );
         $env = new cons(CFLAGS => '-O');
         Build qw( other/Conscript );

       It doesn't matter whether the variable is set before or after the `Export' command. The
       important thing is the value of the variable at the time the `Build' command is executed.
       This is what gets squirreled away. Any subsequent `Export' commands, by the way,
       invalidate the first: you must mention all the variables you wish to export on each
       `Export' command.

       The Import command

       Variables exported by the `Export' command can be imported into subsidiary scripts by the
       `Import' command. The subsidiary script always imports variables directly from the
       superior script. Consider this example:

         Import qw( env INCLUDE );

       This is only legal if the parent script exported both `$env' and `$INCLUDE'. It also must
       have given each of these variables values. It is OK for the subsidiary script to only
       import a subset of the exported variables (in this example, `$LIB', which was exported by
       the previous example, is not imported).

       All the imported variables are automatically re-exported, so the sequence:

         Import qw ( env INCLUDE );
         Build qw ( beneath-me/Conscript );

       will supply both `$env' and `$INCLUDE' to the subsidiary file. If only `$env' is to be
       exported, then the following will suffice:

         Import qw ( env INCLUDE );
         Export qw ( env );
         Build qw ( beneath-me/Conscript );

       Needless to say, the variables may be modified locally before invoking `Build' on the
       subsidiary script.

       Build script evaluation order

       The only constraint on the ordering of build scripts is that superior scripts are
       evaluated before their inferior scripts. The top-level Construct file, for instance, is
       evaluated first, followed by any inferior scripts. This is all you really need to know
       about the evaluation order, since order is generally irrelevant. Consider the following
       `Build' command:

         Build qw(

       We've chosen to put the script names in alphabetical order, simply because that's the most
       convenient for maintenance purposes. Changing the order will make no difference to the

A Model for sharing files

       Some simple conventions

       In any complex software system, a method for sharing build products needs to be
       established. We propose a simple set of conventions which are trivial to implement with
       Cons, but very effective.

       The basic rule is to require that all build products which need to be shared between
       directories are shared via an intermediate directory. We have typically called this
       export, and, in a C environment, provided conventional sub-directories of this directory,
       such as include, lib, bin, etc.

       These directories are defined by the top-level Construct file. A simple Construct file for
       a Hello, World! application, organized using multiple directories, might look like this:

         # Construct file for Hello, World!

         # Where to put all our shared products.
         $EXPORT = '#export';

         Export qw( CONS INCLUDE LIB BIN );

         # Standard directories for sharing products.
         $INCLUDE = "$EXPORT/include";
         $LIB = "$EXPORT/lib";
         $BIN = "$EXPORT/bin";

         # A standard construction environment.
         $CONS = new cons (
               CPPPATH => $INCLUDE,    # Include path for C Compilations
               LIBPATH => $LIB,        # Library path for linking programs
               LIBS => '-lworld',      # List of standard libraries

         Build qw(

       The world directory's Conscript file looks like this:

         # Conscript file for directory world
         Import qw( CONS INCLUDE LIB );

         # Install the products of this directory
         Install $CONS $LIB, 'libworld.a';
         Install $CONS $INCLUDE, 'world.h';

         # Internal products
         Library $CONS 'libworld.a', 'world.c';

       and the hello directory's Conscript file looks like this:

         # Conscript file for directory hello
         Import qw( CONS BIN );

         # Exported products
         Install $CONS $BIN, 'hello';

         # Internal products
         Program $CONS 'hello', 'hello.c';

       To construct a Hello, World! program with this directory structure, go to the top-level
       directory, and invoke `cons' with the appropriate arguments. In the following example, we
       tell Cons to build the directory export. To build a directory, Cons recursively builds all
       known products within that directory (only if they need rebuilding, of course). If any of
       those products depend upon other products in other directories, then those will be built,

         % cons export
         Install world/world.h as export/include/world.h
         cc -Iexport/include -c hello/hello.c -o hello/hello.o
         cc -Iexport/include -c world/world.c -o world/world.o
         ar r world/libworld.a world/world.o
         ar: creating world/libworld.a
         ranlib world/libworld.a
         Install world/libworld.a as export/lib/libworld.a
         cc -o hello/hello hello/hello.o -Lexport/lib -lworld
         Install hello/hello as export/bin/hello

       Clean, understandable, location-independent scripts

       You'll note that the two Conscript files are very clean and to-the-point. They simply
       specify products of the directory and how to build those products. The build instructions
       are minimal: they specify which construction environment to use, the name of the product,
       and the name of the inputs. Note also that the scripts are location-independent: if you
       wish to reorganize your source tree, you are free to do so: you only have to change the
       Construct file (in this example), to specify the new locations of the Conscript files. The
       use of an export tree makes this goal easy.

       Note, too, how Cons takes care of little details for you. All the export directories, for
       example, were made automatically. And the installed files were really hard-linked into the
       respective export directories, to save space and time. This attention to detail saves
       considerable work, and makes it even easier to produce simple, maintainable scripts.

Separating source and build trees

       It's often desirable to keep any derived files from the build completely separate from the
       source files. This makes it much easier to keep track of just what is a source file, and
       also makes it simpler to handle variant builds, especially if you want the variant builds
       to co-exist.

       Separating build and source directories using the Link command

       Cons provides a simple mechanism that handles all of these requirements. The `Link'
       command is invoked as in this example:

         Link 'build' => 'src';

       The specified directories are ``linked'' to the specified source directory. Let's suppose
       that you setup a source directory, src, with the sub-directories world and hello below it,
       as in the previous example. You could then substitute for the original build lines the

         Build qw(

       Notice that you treat the Conscript file as if it existed in the build directory. Now if
       you type the same command as before, you will get the following results:

         % cons export
         Install build/world/world.h as export/include/world.h
         cc -Iexport/include -c build/hello/hello.c -o build/hello/hello.o
         cc -Iexport/include -c build/world/world.c -o build/world/world.o
         ar r build/world/libworld.a build/world/world.o
         ar: creating build/world/libworld.a
         ranlib build/world/libworld.a
         Install build/world/libworld.a as export/lib/libworld.a
         cc -o build/hello/hello build/hello/hello.o -Lexport/lib -lworld
         Install build/hello/hello as export/bin/hello

       Again, Cons has taken care of the details for you. In particular, you will notice that all
       the builds are done using source files and object files from the build directory. For
       example, build/world/world.o is compiled from build/world/world.c, and
       export/include/world.h is installed from build/world/world.h. This is accomplished on most
       systems by the simple expedient of ``hard'' linking the required files from each source
       directory into the appropriate build directory.

       The links are maintained correctly by Cons, no matter what you do to the source directory.
       If you modify a source file, your editor may do this ``in place'' or it may rename it
       first and create a new file. In the latter case, any hard link will be lost. Cons will
       detect this condition the next time the source file is needed, and will relink it

       You'll also notice, by the way, that no changes were required to the underlying Conscript
       files. And we can go further, as we shall see in the next section.

Variant builds

       Hello, World! for baNaNa and peAcH OS's

       Variant builds require just another simple extension. Let's take as an example a
       requirement to allow builds for both the baNaNa and peAcH operating systems. In this case,
       we are using a distributed file system, such as NFS to access the particular system, and
       only one or the other of the systems has to be compiled for any given invocation of
       `cons'. Here's one way we could set up the Construct file for our Hello, World!

         # Construct file for Hello, World!

         die qq(OS must be specified) unless $OS = $ARG{OS};
         die qq(OS must be "peach" or "banana")
               if $OS ne "peach" && $OS ne "banana";

         # Where to put all our shared products.
         $EXPORT = "#export/$OS";

         Export qw( CONS INCLUDE LIB BIN );

         # Standard directories for sharing products.
         $INCLUDE = "$EXPORT/include";
         $LIB = "$EXPORT/lib";
         $BIN = "$EXPORT/bin";

         # A standard construction environment.
         $CONS = new cons (
               CPPPATH => $INCLUDE,    # Include path for C Compilations
               LIBPATH => $LIB,        # Library path for linking programs
               LIBS => '-lworld',      # List of standard libraries

         # $BUILD is where we will derive everything.
         $BUILD = "#build/$OS";

         # Tell cons where the source files for $BUILD are.
         Link $BUILD => 'src';

         Build (

       Now if we login to a peAcH system, we can build our Hello, World!  application for that

         % cons export OS=peach
         Install build/peach/world/world.h as export/peach/include/world.h
         cc -Iexport/peach/include -c build/peach/hello/hello.c -o build/peach/hello/hello.o
         cc -Iexport/peach/include -c build/peach/world/world.c -o build/peach/world/world.o
         ar r build/peach/world/libworld.a build/peach/world/world.o
         ar: creating build/peach/world/libworld.a
         ranlib build/peach/world/libworld.a
         Install build/peach/world/libworld.a as export/peach/lib/libworld.a
         cc -o build/peach/hello/hello build/peach/hello/hello.o -Lexport/peach/lib -lworld
         Install build/peach/hello/hello as export/peach/bin/hello

       Variations on a theme

       Other variations of this model are possible. For example, you might decide that you want
       to separate out your include files into platform dependent and platform independent files.
       In this case, you'd have to define an alternative to `$INCLUDE' for platform-dependent
       files. Most Conscript files, generating purely platform-independent include files, would
       not have to change.

       You might also want to be able to compile your whole system with debugging or profiling,
       for example, enabled. You could do this with appropriate command line options, such as
       `DEBUG=on'. This would then be translated into the appropriate platform-specific
       requirements to enable debugging (this might include turning off optimization, for
       example). You could optionally vary the name space for these different types of systems,
       but, as we'll see in the next section, it's not essential to do this, since Cons is pretty
       smart about rebuilding things when you change options.


       MD5 cryptographic signatures

       Whenever Cons creates a derived file, it stores a signature for that file. The signature
       is stored in a separate file, one per directory. After the previous example was compiled,
       the .consign file in the build/peach/world directory looked like this:

         world.o:834179303 23844c0b102ecdc0b4548d1cd1cbd8c6
         libworld.a:834179304 9bf6587fa06ec49d864811a105222c00

       The first number is a timestamp--for a UNIX systems, this is typically the number of
       seconds since January 1st, 1970. The second value is an MD5 checksum. The Message Digest
       Algorithm is an algorithm that, given an input string, computes a strong cryptographic
       signature for that string. The MD5 checksum stored in the .consign file is, in effect, a
       digest of all the dependency information for the specified file. So, for example, for the
       world.o file, this includes at least the world.c file, and also any header files that Cons
       knows about that are included, directly or indirectly by world.c. Not only that, but the
       actual command line that was used to generate world.o is also fed into the computation of
       the signature. Similarly, libworld.a gets a signature which ``includes'' all the
       signatures of its constituents (and hence, transitively, the signatures of their
       constituents), as well as the command line that created the file.

       The signature of a non-derived file is computed, by default, by taking the current
       modification time of the file and the file's entry name (unless there happens to be a
       current .consign entry for that file, in which case that signature is used).

       Notice that there is no need for a derived file to depend upon any particular Construct or
       Conscript file--if changes to these files affect the file in question, then this will be
       automatically reflected in its signature, since relevant parts of the command line are
       included in the signature. Unrelated changes will have no effect.

       When Cons considers whether to derive a particular file, then, it first computes the
       expected signature of the file. It then compares the file's last modification time with
       the time recorded in the .consign entry, if one exists. If these times match, then the
       signature stored in the .consign file is considered to be accurate. If the file's previous
       signature does not match the new, expected signature, then the file must be rederived.

       Notice that a file will be rederived whenever anything about a dependent file changes. In
       particular, notice that any change to the modification time of a dependent (forward or
       backwards in time) will force recompilation of the derived file.

       The use of these signatures is an extremely simple, efficient, and effective method of
       improving--dramatically--the reproducibility of a system.

       We'll demonstrate this with a simple example:

         # Simple "Hello, World!" Construct file
         $CFLAGS = '-g' if $ARG{DEBUG} eq 'on';
         $CONS = new cons(CFLAGS => $CFLAGS);
         Program $CONS 'hello', 'hello.c';

       Notice how Cons recompiles at the appropriate times:

         % cons hello
         cc -c hello.c -o hello.o
         cc -o hello hello.o
         % cons hello
         cons: "hello" is up-to-date.
         % cons DEBUG=on hello
         cc -g -c hello.c -o hello.o
         cc -o hello hello.o
         % cons DEBUG=on hello
         cons: "hello" is up-to-date.
         % cons hello
         cc -c hello.c -o hello.o
         cc -o hello hello.o

Code Repositories

       Many software development organizations will have one or more central repository directory
       trees containing the current source code for one or more projects, as well as the derived
       object files, libraries, and executables.  In order to reduce unnecessary recompilation,
       it is useful to use files from the repository to build development software--assuming, of
       course, that no newer dependency file exists in the local build tree.


       Cons provides a mechanism to specify a list of code repositories that will be searched,
       in-order, for source files and derived files not found in the local build directory tree.

       The following lines in a Construct file will instruct Cons to look first under the
       /usr/experiment/repository directory and then under the /usr/product/repository directory:

         Repository qw (

       The repository directories specified may contain source files, derived files (objects,
       libraries and executables), or both.  If there is no local file (source or derived) under
       the directory in which Cons is executed, then the first copy of a same-named file found
       under a repository directory will be used to build any local derived files.

       Cons maintains one global list of repositories directories.  Cons will eliminate the
       current directory, and any non-existent directories, from the list.

       Finding the Construct file in a Repository

       Cons will also search for Construct and Conscript files in the repository tree or trees.
       This leads to a chicken-and-egg situation, though: how do you look in a repository tree
       for a Construct file if the Construct file tells you where the repository is?  To get
       around this, repositories may be specified via `-R' options on the command line:

         % cons -R /usr/experiment/repository -R /usr/product/repository .

       Any repository directories specified in the Construct or Conscript files will be appended
       to the repository directories specified by command-line `-R' options.

       Repository source files

       If the source code (include the Conscript file) for the library version of the Hello,
       World! C application is in a repository (with no derived files), Cons will use the
       repository source files to create the local object files and executable file:

         % cons -R /usr/src_only/repository hello
         gcc -c /usr/src_only/repository/hello.c -o hello.o
         gcc -c /usr/src_only/repository/world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       Creating a local source file will cause Cons to rebuild the appropriate derived file or

         % pico world.c
         % cons -R /usr/src_only/repository hello
         gcc -c world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       And removing the local source file will cause Cons to revert back to building the derived
       files from the repository source:

         % rm world.c
         % cons -R /usr/src_only/repository hello
         gcc -c /usr/src_only/repository/world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       Repository derived files

       If a repository tree contains derived files (usually object files, libraries, or
       executables), Cons will perform its normal signature calculation to decide whether the
       repository file is up-to-date or a derived file must be built locally.  This means that,
       in order to ensure correct signature calculation, a repository tree must also contain the
       .consign files that were created by Cons when generating the derived files.

       This would usually be accomplished by building the software in the repository (or,
       alternatively, in a build directory, and then copying the result to the repository):

         % cd /usr/all/repository
         % cons hello
         gcc -c hello.c -o hello.o
         gcc -c world.c -o world.o
         ar r libworld.a world.o
         ar: creating libworld.a
         ranlib libworld.a
         gcc -o hello hello.o libworld.a

       (This is safe even if the Construct file lists the /usr/all/repository directory in a
       `Repository' command because Cons will remove the current directory from the repository

       Now if we want to build a copy of the application with our own hello.c file, we only need
       to create the one necessary source file, and use the `-R' option to have Cons use other
       files from the repository:

         % mkdir $HOME/build1
         % cd $HOME/build1
         % ed hello.c
         % cons -R /usr/all/repository hello
         gcc -c hello.c -o hello.o
         gcc -o hello hello.o /usr/all/repository/libworld.a

       Notice that Cons has not bothered to recreate a local libworld.a library (or recompile the
       world.o module), but instead uses the already-compiled version from the repository.

       Because the MD5 signatures that Cons puts in the .consign file contain timestamps for the
       derived files, the signature timestamps must match the file timestamps for a signature to
       be considered valid.

       Some software systems may alter the timestamps on repository files (by copying them,
       e.g.), in which case Cons will, by default, assume the repository signatures are invalid
       and rebuild files unnecessarily.  This behavior may be altered by specifying:

         Repository_Sig_Times_OK 0;

       This tells Cons to ignore timestamps when deciding whether a signature is valid.  (Note
       that avoiding this sanity check means there must be proper control over the repository
       tree to ensure that the derived files cannot be modified without updating the .consign

       Local copies of files

       If the repository tree contains the complete results of a build, and we try to build from
       the repository without any files in our local tree, something moderately surprising

         % mkdir $HOME/build2
         % cd $HOME/build2
         % cons -R /usr/all/repository hello
         cons: "hello" is up-to-date.

       Why does Cons say that the hello program is up-to-date when there is no hello program in
       the local build directory?  Because the repository (not the local directory) contains the
       up-to-date hello program, and Cons correctly determines that nothing needs to be done to
       rebuild this up-to-date copy of the file.

       There are, however, many times in which it is appropriate to ensure that a local copy of a
       file always exists.  A packaging or testing script, for example, may assume that certain
       generated files exist locally.  Instead of making these subsidiary scripts aware of the
       repository directory, the `Local' command may be added to a Construct or Conscript file to
       specify that a certain file or files must appear in the local build directory:

         Local qw(

       Then, if we re-run the same command, Cons will make a local copy of the program from the
       repository copy (telling you that it is doing so):

         % cons -R /usr/all/repository hello
         Local copy of hello from /usr/all/repository/hello
         cons: "hello" is up-to-date.

       Notice that, because the act of making the local copy is not considered a "build" of the
       hello file, Cons still reports that it is up-to-date.

       Creating local copies is most useful for files that are being installed into an
       intermediate directory (for sharing with other directories) via the `Install' command.
       Accompanying the `Install' command for a file with a companion `Local' command is so
       common that Cons provides a `Install_Local' command as a convenient way to do both:

         Install_Local $env, '#export', 'hello';

       is exactly equivalent to:

         Install $env '#export', 'hello';
         Local '#export/hello';

       Both the `Local' and `Install_Local' commands update the local .consign file with the
       appropriate file signatures, so that future builds are performed correctly.

       Repository dependency analysis

       Due to its built-in scanning, Cons will search the specified repository trees for included
       .h files.  Unless the compiler also knows about the repository trees, though, it will be
       unable to find .h files that only exist in a repository.  If, for example, the hello.c
       file includes the hello.h file in its current directory:

         % cons -R /usr/all/repository hello
         gcc -c /usr/all/repository/hello.c -o hello.o
         /usr/all/repository/hello.c:1: hello.h: No such file or directory

       Solving this problem forces some requirements onto the way construction environments are
       defined and onto the way the C `#include' preprocessor directive is used to include files.

       In order to inform the compiler about the repository trees, Cons will add appropriate `-I'
       flags to the compilation commands.  This means that the `CPPPATH' variable in the
       construct environment must explicitly specify all subdirectories which are to be searched
       for included files, including the current directory.  Consequently, we can fix the above
       example by changing the environment creation in the Construct file as follows:

         $env = new cons(
               CC      => 'gcc',
               CPPPATH => '.',
               LIBS    => 'libworld.a',

       Due to the definition of the `CPPPATH' variable, this yields, when we re-execute the

         % cons -R /usr/all/repository hello
         gcc -c -I. -I/usr/all/repository /usr/all/repository/hello.c -o hello.o
         gcc -o hello hello.o /usr/all/repository/libworld.a

       The order of the `-I' flags replicates, for the C preprocessor, the same repository-
       directory search path that Cons uses for its own dependency analysis.  If there are
       multiple repositories and multiple `CPPPATH' directories, Cons will append the repository
       directories to the beginning of each `CPPPATH' directory, rapidly multiplying the number
       of `-I' flags.  As an extreme example, a Construct file containing:

         Repository qw(

         $env = new cons(
               CPPPATH => 'a:b:c',

       Would yield a compilation command of:

         cc -Ia -I/u1/a -I/u2/a -Ib -I/u1/b -I/u2/b -Ic -I/u1/c -I/u2/c -c hello.c -o hello.o

       Because Cons relies on the compiler's `-I' flags to communicate the order in which
       repository directories must be searched, Cons' handling of repository directories is
       fundamentally incompatible with using double-quotes on the `#include' directives in your C
       source code:

         #include "file.h"     /* DON'T USE DOUBLE-QUOTES LIKE THIS */

       This is because most C preprocessors, when faced with such a directive, will always first
       search the directory containing the source file.  This undermines the elaborate `-I'
       options that Cons constructs to make the preprocessor conform to its preferred search

       Consequently, when using repository trees in Cons, always use angle-brackets for included

         #include <file.h>     /* USE ANGLE-BRACKETS INSTEAD */


       Cons provides a `Repository_List' command to return a list of all repository directories
       in their current search order.  This can be used for debugging, or to do more complex Perl

         @list = Repository_List;
         print join(' ', @list), "\n";

       Repository interaction with other Cons features

       Cons' handling of repository trees interacts correctly with other Cons features--which is
       to say, it generally does what you would expect.

       Most notably, repository trees interact correctly, and rather powerfully, with the 'Link'
       command.  A repository tree may contain one or more subdirectories for version builds
       established via `Link' to a source subdirectory.  Cons will search for derived files in
       the appropriate build subdirectories under the repository tree.

Default targets

       Until now, we've demonstrated invoking Cons with an explicit target to build:

         % cons hello

       Normally, Cons does not build anything unless a target is specified, but specifying '.'
       (the current directory) will build everything:

         % cons                # does not build anything

         % cons .              # builds everything under the top-level directory

       Adding the `Default' method to any Construct or Conscript file will add the specified
       targets to a list of default targets.  Cons will build these defaults if there are no
       targets specified on the command line.  So adding the following line to the top-level
       Construct file will mimic Make's typical behavior of building everything by default:

         Default '.';

       The following would add the hello and goodbye commands (in the same directory as the
       Construct or Conscript file) to the default list:

         Default qw(

       The `Default' method may be used more than once to add targets to the default list.

Selective builds

       Cons provides two methods for reducing the size of given build. The first is by specifying
       targets on the command line, and the second is a method for pruning the build tree. We'll
       consider target specification first.

       Selective targeting

       Like make, Cons allows the specification of ``targets'' on the command line. Cons targets
       may be either files or directories. When a directory is specified, this is simply a short-
       hand notation for every derivable product--that Cons knows about--in the specified
       directory and below. For example:

         % cons build/hello/hello.o

       means build hello.o and everything that hello.o might need. This is from a previous
       version of the Hello, World! program in which hello.o depended upon
       export/include/world.h. If that file is not up-to-date (because someone modified
       src/world/world.h), then it will be rebuilt, even though it is in a directory remote from

       In this example:

         % cons build

       Everything in the build directory is built, if necessary. Again, this may cause more files
       to be built. In particular, both export/include/world.h and export/lib/libworld.a are
       required by the build/hello directory, and so they will be built if they are out-of-date.

       If we do, instead:

         % cons export

       then only the files that should be installed in the export directory will be rebuilt, if
       necessary, and then installed there. Note that `cons build' might build files that `cons
       export' doesn't build, and vice-versa.

       No ``special'' targets

       With Cons, make-style ``special'' targets are not required. The simplest analog with Cons
       is to use special export directories, instead. Let's suppose, for example, that you have a
       whole series of unit tests that are associated with your code. The tests live in the
       source directory near the code. Normally, however, you don't want to build these tests.
       One solution is to provide all the build instructions for creating the tests, and then to
       install the tests into a separate part of the tree. If we install the tests in a top-level
       directory called tests, then:

         % cons tests

       will build all the tests.

         % cons export

       will build the production version of the system (but not the tests), and:

         % cons build

       should probably be avoided (since it will compile tests unecessarily).

       If you want to build just a single test, then you could explicitly name the test (in
       either the tests directory or the build directory). You could also aggregate the tests
       into a convenient hierarchy within the tests directory. This hierarchy need not
       necessarily match the source hierarchy, in much the same manner that the include hierarchy
       probably doesn't match the source hierarchy (the include hierarchy is unlikely to be more
       than two levels deep, for C programs).

       If you want to build absolutely everything in the tree (subject to whatever options you
       select), you can use:

         % cons .

       This is not particularly efficient, since it will redundantly walk all the trees,
       including the source tree. The source tree, of course, may have buildable objects in
       it--nothing stops you from doing this, even if you normally build in a separate build

Build Pruning

       In conjunction with target selection, build pruning can be used to reduce the scope of the
       build. In the previous peAcH and baNaNa example, we have already seen how script-driven
       build pruning can be used to make only half of the potential build available for any given
       invocation of `cons'. Cons also provides, as a convenience, a command line convention that
       allows you to specify which Conscript files actually get ``built''--that is, incorporated
       into the build tree. For example:

         % cons build +world

       The `+' argument introduces a Perl regular expression. This must, of course, be quoted at
       the shell level if there are any shell meta-characters within the expression. The
       expression is matched against each Conscript file which has been mentioned in a `Build'
       statement, and only those scripts with matching names are actually incorporated into the
       build tree. Multiple such arguments are allowed, in which case a match against any of them
       is sufficient to cause a script to be included.

       In the example, above, the hello program will not be built, since Cons will have no
       knowledge of the script hello/Conscript. The libworld.a archive will be built, however, if
       need be.

       There are a couple of uses for build pruning via the command line. Perhaps the most useful
       is the ability to make local changes, and then, with sufficient knowledge of the
       consequences of those changes, restrict the size of the build tree in order to speed up
       the rebuild time. A second use for build pruning is to actively prevent the recompilation
       of certain files that you know will recompile due to, for example, a modified header file.
       You may know that either the changes to the header file are immaterial, or that the
       changes may be safely ignored for most of the tree, for testing purposes.With Cons, the
       view is that it is pragmatic to admit this type of behavior, with the understanding that
       on the next full build everything that needs to be rebuilt will be. There is no equivalent
       to a ``make touch'' command, to mark files as permanently up-to-date. So any risk that is
       incurred by build pruning is mitigated. For release quality work, obviously, we recommend
       that you do not use build pruning (it's perfectly OK to use during integration, however,
       for checking compilation, etc. Just be sure to do an unconstrained build before committing
       the integration).

Temporary overrides

       Cons provides a very simple mechanism for overriding aspects of a build. The essence is
       that you write an override file containing one or more `Override' commands, and you
       specify this on the command line, when you run `cons':

         % cons -o over export

       will build the export directory, with all derived files subject to the overrides present
       in the over file. If you leave out the `-o' option, then everything necessary to remove
       all overrides will be rebuilt.

       Overriding environment variables

       The override file can contain two types of overrides. The first is incoming environment
       variables. These are normally accessible by the Construct file from the `%ENV' hash
       variable. These can trivially be overridden in the override file by setting the
       appropriate elements of `%ENV' (these could also be overridden in the user's environment,
       of course).

       The Override command

       The second type of override is accomplished with the `Override' command, which looks like

         Override <regexp>, <var1> => <value1>, <var2> => <value2>, ...;

       The regular expression regexp is matched against every derived file that is a candidate
       for the build. If the derived file matches, then the variable/value pairs are used to
       override the values in the construction environment associated with the derived file.

       Let's suppose that we have a construction environment like this:

         $CONS = new cons(
               COPT => '',
               CDBG => '-g',
               CFLAGS => '%COPT %CDBG',

       Then if we have an override file over containing this command:

         Override '\.o$', COPT => '-O', CDBG => '';

       then any `cons' invocation with `-o over' that creates .o files via this environment will
       cause them to be compiled with `-O 'and no `-g'. The override could, of course, be
       restricted to a single directory by the appropriate selection of a regular expression.

       Here's the original version of the Hello, World! program, built with this environment.
       Note that Cons rebuilds the appropriate pieces when the override is applied or removed:

         % cons hello
         cc -g -c hello.c -o hello.o
         cc -o hello hello.o
         % cons -o over hello
         cc -O -c hello.c -o hello.o
         cc -o hello hello.o
         % cons -o over hello
         cons: "hello" is up-to-date.
         % cons hello
         cc -g -c hello.c -o hello.o
         cc -o hello hello.o

       It's important that the `Override' command only be used for temporary, on-the-fly
       overrides necessary for development because the overrides are not platform independent and
       because they rely too much on intimate knowledge of the workings of the scripts. For
       temporary use, however, they are exactly what you want.

       Note that it is still useful to provide, say, the ability to create a fully optimized
       version of a system for production use--from the Construct and Conscript files. This way
       you can tailor the optimized system to the platform. Where optimizer trade-offs need to be
       made (particular files may not be compiled with full optimization, for example), then
       these can be recorded for posterity (and reproducibility) directly in the scripts.

More on construction environments

       Default construction variables

       We have mentioned, and used, the concept of a construction environment, many times in the
       preceding pages. Now it's time to make this a little more concrete. With the following

         $env = new cons();

       a reference to a new, default construction environment is created. This contains a number
       of construction variables and some methods. At the present writing, the default list of
       construction variables is defined as follows:

         CC            => 'cc',
         CFLAGS        => '',
         CCCOM         => '%CC %CFLAGS %_IFLAGS -c %< -o %>',
         INCDIRPREFIX  => '-I',
         CXX           => '%CC',
         CXXFLAGS      => '%CFLAGS',
         CXXCOM        => '%CXX %CXXFLAGS %_IFLAGS -c %< -o %>',
         LINK          => '%CXX',
         LINKCOM       => '%LINK %LDFLAGS -o %> %< %_LDIRS %LIBS',
         LINKMODULECOM => '%LD -r -o %> %<',
         LIBDIRPREFIX  => '-L',
         AR            => 'ar',
         ARFLAGS       => 'r',
         ARCOM         => "%AR %ARFLAGS %> %<\n%RANLIB %>",
         RANLIB        => 'ranlib',
         AS            => 'as',
         ASFLAGS       => '',
         ASCOM         => '%AS %ASFLAGS %< -o %>',
         LD            => 'ld',
         LDFLAGS       => '',
         PREFLIB       => 'lib',
         SUFLIB        => '.a',
         SUFLIBS       => '.so:.a',
         SUFOBJ        => '.o',
         ENV           => { 'PATH' => '/bin:/usr/bin' },

       On Win32 systems (Windows NT), the following construction variables are overridden in the

         CC            => 'cl',
         CFLAGS        => '/nologo',
         CCCOM         => '%CC %CFLAGS %_IFLAGS /c %< /Fo%>',
         CXXCOM        => '%CXX %CXXFLAGS %_IFLAGS /c %< /Fo%>',
         INCDIRPREFIX  => '/I',
         LINK          => 'link',
         LINKCOM       => '%LINK %LDFLAGS /out:%> %< %_LDIRS %LIBS',
         LINKMODULECOM => '%LD /r /o %> %<',
         LIBDIRPREFIX  => '/LIBPATH:',
         AR            => 'lib',
         ARFLAGS       => '/nologo ',
         ARCOM         => "%AR %ARFLAGS /out:%> %<",
         RANLIB        => '',
         LD            => 'link',
         LDFLAGS       => '/nologo ',
         PREFLIB       => '',
         SUFEXE        => '.exe',
         SUFLIB        => '.lib',
         SUFLIBS       => '.dll:.lib',
         SUFOBJ        => '.obj',

       These variables are used by the various methods associated with the environment, in
       particular any method that ultimately invokes an external command will substitute these
       variables into the final command, as appropriate. For example, the `Objects' method takes
       a number of source files and arranges to derive, if necessary, the corresponding object
       files. For example:

         Objects $env 'foo.c', 'bar.c';

       This will arrange to produce, if necessary, foo.o and bar.o. The command invoked is simply
       `%CCCOM', which expands through substitution, to the appropriate external command required
       to build each object. We will explore the substitution rules further under the `Command'
       method, below.

       The construction variables are also used for other purposes. For example, `CPPPATH' is
       used to specify a colon-separated path of include directories. These are intended to be
       passed to the C preprocessor and are also used by the C-file scanning machinery to
       determine the dependencies involved in a C Compilation. Variables beginning with
       underscore, are created by various methods, and should normally be considered ``internal''
       variables. For example, when a method is called which calls for the creation of an object
       from a C source, the variable `_IFLAGS' is created: this corresponds to the `-I' switches
       required by the C compiler to represent the directories specified by `CPPPATH'.

       Note that, for any particular environment, the value of a variable is set once, and then
       never reset (to change a variable, you must create a new environment. Methods are provided
       for copying existing environments for this purpose). Some internal variables, such as
       `_IFLAGS' are created on demand, but once set, they remain fixed for the life of the

       The `CFLAGS', `LDFLAGS', and `ARFLAGS' variables all supply a place for passing options to
       the compiler, loader, and archiver, respectively.  Less obviously, the `INCDIRPREFIX'
       variable specifies the option string to be appended to the beginning of each include
       directory so that the compiler knows where to find .h files.  Similarly, the
       `LIBDIRPREFIX' variable specifies the option string to be appended to the beginning of
       each directory that the linker should search for libraries.

       Another variable, `ENV', is used to determine the system environment during the execution
       of an external command. By default, the only environment variable that is set is `PATH',
       which is the execution path for a UNIX command. For the utmost reproducibility, you should
       really arrange to set your own execution path, in your top-level Construct file (or
       perhaps by importing an appropriate construction package with the Perl `use' command). The
       default variables are intended to get you off the ground.

       Interpolating construction variables

       Construction environment variables may be interpolated in the source and target file names
       by prefixing the construction variable name with `%'.

         $env = new cons(
               DESTDIR =>      'programs',
               SRCDIR  =>      'src',
         Program $env '%DESTDIR/hello', '%SRCDIR/hello.c';

       Expansion of construction variables is recursive--that is, the file name(s) will be re-
       expanded until no more substitutions can be made. If a construction variable is not
       defined in the environment, then the null string will be substituted.

Default construction methods

       The list of default construction methods includes the following:

       The `new' constructor

       The `new' method is a Perl object constructor. That is, it is not invoked via a reference
       to an existing construction environment reference, but, rather statically, using the name
       of the Perl package where the constructor is defined. The method is invoked like this:

         $env = new cons(<overrides>);

       The environment you get back is blessed into the package `cons', which means that it will
       have associated with it the default methods described below. Individual construction
       variables can be overridden by providing name/value pairs in an override list. Note that
       to override any command environment variable (i.e. anything under `ENV'), you will have to
       override all of them. You can get around this difficulty by using the `copy' method on an
       existing construction environment.

       The `clone' method

       The `clone' method creates a clone of an existing construction environment, and can be
       called as in the following example:

         $env2 = $env1->clone(<overrides>);

       You can provide overrides in the usual manner to create a different environment from the
       original. If you just want a new name for the same environment (which may be helpful when
       exporting environments to existing components), you can just use simple assignment.

       The `copy' method

       The `copy' method extracts the externally defined construction variables from an
       environment and returns them as a list of name/value pairs. Overrides can also be
       provided, in which case, the overridden values will be returned, as appropriate. The
       returned list can be assigned to a hash, as shown in the prototype, below, but it can also
       be manipulated in other ways:

         %env = $env1->copy(<overrides>);

       The value of `ENV', which is itself a hash, is also copied to a new hash, so this may be
       changed without fear of affecting the original environment. So, for example, if you really
       want to override just the `PATH' variable in the default environment, you could do the

         %cons = new cons()->copy();
         $cons{ENV}{PATH} = "<your path here>";
         $cons = new cons(%cons);

       This will leave anything else that might be in the default execution environment

       The `Install' method

       The `Install' method arranges for the specified files to be installed in the specified
       directory. The installation is optimized: the file is not copied if it can be linked. If
       this is not the desired behavior, you will need to use a different method to install the
       file. It is called as follows:

         Install $env <directory>, <names>;

       Note that, while the files to be installed may be arbitrarily named, only the last
       component of each name is used for the installed target name. So, for example, if you
       arrange to install foo/bar in baz, this will create a bar file in the baz directory (not

       The `InstallAs' method

       The `InstallAs' method arranges for the specified source file(s) to be installed as the
       specified target file(s). Multiple files should be specified as a file list. The
       installation is optimized: the file is not copied if it can be linked. If this is not the
       desired behavior, you will need to use a different method to install the file. It is
       called as follows:

       `InstallAs' works in two ways:

       Single file install:

         InstallAs $env TgtFile, SrcFile;

       Multiple file install:

         InstallAs $env ['tgt1', 'tgt2'], ['src1', 'src2'];

       Or, even as:

         @srcs = qw(src1 src2 src3);
         @tgts = qw(tgt1 tgt2 tgt3);
         InstallAs $env [@tgts], [@srcs];

       Both the target and the sources lists should be of the same length.

       The `Precious' method

       The `Precious' method asks cons not to delete the specified file or list of files before
       building them again.  It is invoked as:

         Precious <files>;

       This is especially useful for allowing incremental updates to libraries or debug
       information files which are updated rather than rebuilt anew each time.  Cons will still
       delete the files when the `-r' flag is specified.

       The `Command' method

       The `Command' method is a catchall method which can be used to arrange for any external
       command to be called to update the target. For this command, a target file and list of
       inputs is provided. In addition a construction command line, or lines, is provided as a
       string (this string may have multiple commands embedded within it, separated by new
       lines). `Command' is called as follows:

         Command $env <target>, <inputs>, <construction command>;

       The target is made dependent upon the list of input files specified, and the inputs must
       be built successfully or Cons will not attempt to build the target.

       Within the construction command, any variable from the construction environment may be
       introduced by prefixing the name of the construction variable with `%'. This is recursive:
       the command is expanded until no more substitutions can be made. If a construction
       variable is not defined in the environment, then the null string will be substituted.  A
       doubled `%%' will be replaced by a single `%' in the construction command.

       There are several pseudo variables which will also be expanded:

       %>        The target file name (in a multi-target command, this is always the first target

       %0        Same as `%>'.

       %1, %2, ..., %9
                 These refer to the first through ninth input file, respectively.

       %<        The full set of inputs. If any of these have been used anywhere else in the
                 current command line (via `%1', `%2', etc.), then those will be deleted from the
                 list provided by `%<'. Consider the following command found in a Conscript file
                 in the test directory:

                   Command $env 'tgt', qw(foo bar baz), qq(
                         echo %< -i %1 > %>
                         echo %< -i %2 >> %>
                         echo %< -i %3 >> %>

                 If tgt needed to be updated, then this would result in the execution of the
                 following commands, assuming that no remapping has been established for the test

                   echo test/bar test/baz -i test/foo > test/tgt
                   echo test/foo test/baz -i test/bar >> test/tgt
                   echo test/foo test/bar -i test/baz >> test/tgt

       Any of the above pseudo variables may be followed immediately by one of the following
       suffixes to select a portion of the expanded path name:

         :a    the absolute path to the file name
         :b    the directory plus the file name stripped of any suffix
         :d    the directory
         :f    the file name
         :s    the file name suffix
         :F    the file name stripped of any suffix

       Continuing with the above example, `%<:f' would expand to `foo bar baz', and `%':d> would
       expand to `test'.

       It is possible to programmatically rewrite part of the command by enclosing part of it
       between `%[' and `%]'.  This will call the construction variable named as the first word
       enclosed in the brackets as a Perl code reference; the results of this call will be used
       to replace the contents of the brackets in the command line.  For example, given an
       existing input file named

         @keywords = qw(foo bar baz);
         $env = new cons(X_COMMA => sub { join(",", @_) });
         Command $env 'tgt', '', qq(
               echo '# Keywords: %[X_COMMA @keywords %]' > %>
               cat %< >> %>

       This will execute:

         echo '# Keywords: foo,bar,baz' > tgt
         cat >> tgt

       After substitution occurs, strings of white space are converted into single blanks, and
       leading and trailing white space is eliminated. It is therefore not possible to introduce
       variable length white space in strings passed into a command, without resorting to some
       sort of shell quoting.

       If a multi-line command string is provided, the commands are executed sequentially. If any
       of the commands fails, then none of the rest are executed, and the target is not marked as
       updated, i.e. a new signature is not stored for the target.

       Normally, if all the commands succeed, and return a zero status (or whatever platform-
       specific indication of success is required), then a new signature is stored for the
       target. If a command erroneously reports success even after a failure, then Cons will
       assume that the target file created by that command is accurate and up-to-date.

       The first word of each command string, after expansion, is assumed to be an executable
       command looked up on the `PATH' environment variable (which is, in turn, specified by the
       `ENV' construction variable). If this command is found on the path, then the target will
       depend upon it: the command will therefore be automatically built, as necessary. It's
       possible to write multi-part commands to some shells, separated by semi-colons. Only the
       first command word will be depended upon, however, so if you write your command strings
       this way, you must either explicitly set up a dependency (with the `Depends' method), or
       be sure that the command you are using is a system command which is expected to be
       available. If it isn't available, you will, of course, get an error.

       If any command (even one within a multi-line command) begins with `[perl]', the remainder
       of that command line will be evaluated by the running Perl instead of being forked by the
       shell.  If an error occurs in parsing the Perl or if the Perl expression returns 0 or
       undef, the command will be considered to have failed.  For example, here is a simple
       command which creates a file `foo' directly from Perl:

         $env = new cons();
         Command $env 'foo',
           qq([perl] open(FOO,'>foo');print FOO "hi\\n"; close(FOO); 1);

       Note that when the command is executed, you are in the same package as when the Construct
       or Conscript file was read, so you can call Perl functions you've defined in the same
       Construct or Conscript file in which the `Command' appears:

         $env = new cons();
         sub create_file {
               my $file = shift;
               open(FILE, ">$file");
               print FILE "hi\n";
               return 1;
         Command $env 'foo', "[perl] &create_file('%>')";

       The Perl string will be used to generate the signature for the derived file, so if you
       change the string, the file will be rebuilt.  The contents of any subroutines you call,
       however, are not part of the signature, so if you modify a called subroutine such as
       `create_file' above, the target will not be rebuilt.  Caveat user.

       Cons normally prints a command before executing it.  This behavior is suppressed if the
       first character of the command is `@'.  Note that you may need to separate the `@' from
       the command name or escape it to prevent `@cmd' from looking like an array to Perl quote
       operators that perform interpolation:

         # The first command line is incorrect,
         # because "@cp" looks like an array
         # to the Perl qq// function.
         # Use the second form instead.
         Command $env 'foo', '', qq(
               @cp %< tempfile
               @ cp tempfile %>

       If there are shell meta characters anywhere in the expanded command line, such as `<',
       `>', quotes, or semi-colon, then the command will actually be executed by invoking a
       shell. This means that a command such as:

         cd foo

       alone will typically fail, since there is no command `cd' on the path. But the command

         cd $<:d; tar cf $>:f $<:f

       when expanded will still contain the shell meta character semi-colon, and a shell will be
       invoked to interpret the command. Since `cd' is interpreted by this sub-shell, the command
       will execute as expected.

       To specify a command with multiple targets, you can specify a reference to a list of
       targets. In Perl, a list reference can be created by enclosing a list in square brackets.
       Hence the following command:

         Command $env ['foo.h', 'foo.c'], 'foo.template', q(
               gen %1

       could be used in a case where the command `gen' creates two files, both foo.h and foo.c.

       The `Objects' method

       The `Objects' method arranges to create the object files that correspond to the specified
       source files. It is invoked as shown below:

         @files = Objects $env <source or object files>;

       Under Unix, source files ending in .s and .c are currently supported, and will be compiled
       into a name of the same file ending in .o. By default, all files are created by invoking
       the external command which results from expanding the `CCCOM' construction variable, with
       `%<' and `%>' set to the source and object files, respectively (see the `Command' method
       for expansion details).  The variable `CPPPATH' is also used when scanning source files
       for dependencies. This is a colon separated list of pathnames, and is also used to create
       the construction variable `_IFLAGS,' which will contain the appropriate list of -`I'
       options for the compilation. Any relative pathnames in `CPPPATH' is interpreted relative
       to the directory in which the associated construction environment was created (absolute
       and top-relative names may also be used). This variable is used by `CCCOM'. The behavior
       of this command can be modified by changing any of the variables which are interpolated
       into `CCCOM', such as `CC', `CFLAGS', and, indirectly, `CPPPATH'. It's also possible to
       replace the value of `CCCOM', itself. As a convenience, this file returns the list of
       object filenames.

       The `Program' method

       The `Program' method arranges to link the specified program with the specified object
       files. It is invoked in the following manner:

         Program $env <program name>, <source or object files>;

       The program name will have the value of the `SUFEXE' construction variable appended (by
       default, `.exe' on Win32 systems, nothing on Unix systems) if the suffix is not already

       Source files may be specified in place of objects files--the `Objects' method will be
       invoked to arrange the conversion of all the files into object files, and hence all the
       observations about the `Objects' method, above, apply to this method also.

       The actual linking of the program will be handled by an external command which results
       from expanding the `LINKCOM' construction variable, with `%<' set to the object files to
       be linked (in the order presented), and `%>' set to the target (see the `Command' method
       for expansion details). The user may set additional variables in the construction
       environment, including `LINK', to define which program to use for linking, `LIBPATH', a
       colon-separated list of library search paths, for use with library specifications of the
       form -llib, and `LIBS', specifying the list of libraries to link against (in either -llib
       form or just as pathnames. Relative pathnames in both `LIBPATH' and `LIBS' are interpreted
       relative to the directory in which the associated construction environment is created
       (absolute and top-relative names may also be used). Cons automatically sets up
       dependencies on any libraries mentioned in `LIBS': those libraries will be built before
       the command is linked.

       The `Library' method

       The `Library' method arranges to create the specified library from the specified object
       files. It is invoked as follows:

         Library $env <library name>, <source or object files>;

       The library name will have the value of the `SUFLIB' construction variable appended (by
       default, `.lib' on Win32 systems, `.a' on Unix systems) if the suffix is not already

       Source files may be specified in place of objects files--the `Objects' method will be
       invoked to arrange the conversion of all the files into object files, and hence all the
       observations about the `Objects' method, above, apply to this method also.

       The actual creation of the library will be handled by an external command which results
       from expanding the `ARCOM' construction variable, with `%<' set to the library members (in
       the order presented), and `%>' to the library to be created (see the `Command' method for
       expansion details). The user may set variables in the construction environment which will
       affect the operation of the command. These include `AR', the archive program to use,
       `ARFLAGS', which can be used to modify the flags given to the program specified by `AR',
       and `RANLIB', the name of a archive index generation program, if needed (if the particular
       need does not require the latter functionality, then `ARCOM' must be redefined to not
       reference `RANLIB').

       The `Library' method allows the same library to be specified in multiple method
       invocations. All of the contributing objects from all the invocations (which may be from
       different directories) are combined and generated by a single archive command. Note,
       however, that if you prune a build so that only part of a library is specified, then only
       that part of the library will be generated (the rest will disappear!).

       The `Module' method

       The `Module' method is a combination of the `Program' and `Command' methods. Rather than
       generating an executable program directly, this command allows you to specify your own
       command to actually generate a module. The method is invoked as follows:

         Module $env <module name>, <source or object files>, <construction command>;

       This command is useful in instances where you wish to create, for example, dynamically
       loaded modules, or statically linked code libraries.

       The `Depends' method

       The `Depends' method allows you to specify additional dependencies for a target.  It is
       invoked as follows:

         Depends $env <target>, <dependencies>;

       This may be occasionally useful, especially in cases where no scanner exists (or is
       writable) for particular types of files. Normally, dependencies are calculated
       automatically from a combination of the explicit dependencies set up by the method
       invocation or by scanning source files.

       A set of identical dependencies for multiple targets may be specified using a reference to
       a list of targets. In Perl, a list reference can be created by enclosing a list in square
       brackets. Hence the following command:

         Depends $env ['foo', 'bar'], 'input_file_1', 'input_file_2';

       specifies that both the foo and bar files depend on the listed input files.

       The `Ignore' method

       The `Ignore' method allows you to ignore explicitly dependencies that Cons infers on its
       own.  It is invoked as follows:

         Ignore <patterns>;

       This can be used to avoid recompilations due to changes in system header files or
       utilities that are known to not affect the generated targets.

       If, for example, a program is built in an NFS-mounted directory on multiple systems that
       have different copies of stdio.h, the differences will affect the signatures of all
       derived targets built from source files that `#include <stdio.h>'.  This will cause all
       those targets to be rebuilt when changing systems.  If this is not desirable behavior,
       then the following line will remove the dependencies on the stdio.h file:

         Ignore '^/usr/include/stdio\.h$';

       Note that the arguments to the `Ignore' method are regular expressions, so special
       characters must be escaped and you may wish to anchor the beginning or end of the
       expression with `^' or `$' characters.

       The `Salt' method

       The `Salt' method adds a constant value to the signature calculation for every derived
       file.  It is invoked as follows:

         Salt $string;

       Changing the Salt value will force a complete rebuild of every derived file.  This can be
       used to force rebuilds in certain desired circumstances.  For example,

         Salt `uname -s`;

       Would force a complete rebuild of every derived file whenever the operating system on
       which the build is performed (as reported by `uname -s') changes.

       The `UseCache' method

       The `UseCache' method instructs Cons to maintain a cache of derived files, to be shared
       among separate build trees of the same project.

         UseCache("cache/<buildname>") ⎪⎪ warn("cache directory not found");

       The `SourcePath' method

       The `SourcePath' mathod returns the real source path name of a file, as opposted to the
       path name within a build directory.  It is invoked as follows:

         $path = SourcePath <buildpath>;

       The `ConsPath' method

       The `ConsPath' method returns true if the supplied path is a derivable file, and returns
       undef (false) otherwise.  It is invoked as follows:

         $result = ConsPath <path>;

       The `SplitPath' method

       The `SplitPath' method looks up multiple path names in a string separated by the default
       path separator for the operating system (':' on UNIX systems, ';' on Windows NT), and
       returns the fully-qualified names.  It is invoked as follows:

         @paths = SplitPath <pathlist>;

       The `SplitPath' method will convert  names prefixed '#' to the appropriate top-level build
       name (without the '#') and will convert relative names to top-level names.

       The `DirPath' method

       The `DirPath' method returns the build path name(s) of a directory or list of directories.
       It is invoked as follows:

         $cwd = DirPath <paths>;

       The most common use for the `DirPath' method is:

         $cwd = DirPath '.';

       to fetch the path to the current directory of a subsidiary Conscript file.

       The `FilePath' method

       The `FilePath' method returns the build path name(s) of a file or list of files.  It is
       invoked as follows:

         $file = FilePath <path>;

       The `Help' method

       The `Help' method specifies help text that will be displayed when the user invokes `cons
       -h'.  This can be used to provide documentation of specific targets, values, build
       options, etc. for the build tree.  It is invoked as follows:

         Help <helptext>;

       The `Help' method may only be called once, and should typically be specified in the top-
       level Construct file.

Extending Cons

       Overriding construction variables

       There are several ways of extending Cons, which vary in degree of difficulty. The simplest
       method is to define your own construction environment, based on the default environment,
       but modified to reflect your particular needs. This will often suffice for C-based
       applications. You can use the `new' constructor, and the `clone' and `copy' methods to
       create hybrid environments. These changes can be entirely transparent to the underlying
       Conscript files.

       Adding new methods

       For slightly more demanding changes, you may wish to add new methods to the `cons'
       package. Here's an example of a very simple extension, `InstallScript', which installs a
       tcl script in a requested location, but edits the script first to reflect a platform-
       dependent path that needs to be installed in the script:

         # cons::InstallScript - Create a platform dependent version of a shell
         # script by replacing string ``#!your-path-here'' with platform specific
         # path $BIN_DIR.

         sub cons::InstallScript {
               my ($env, $dst, $src) = @_;
               Command $env $dst, $src, qq(
                       sed s+your-path-here+$BIN_DIR+ %< > %>
                       chmod oug+x %>

       Notice that this method is defined directly in the `cons' package (by prefixing the name
       with `cons::'). A change made in this manner will be globally visible to all environments,
       and could be called as in the following example:

         InstallScript $env "$BIN/foo", "foo.tcl";

       For a small improvement in generality, the `BINDIR' variable could be passed in as an
       argument or taken from the construction environment--as `%BINDIR'.

       Overriding methods

       Instead of adding the method to the `cons' name space, you could define a new package
       which inherits existing methods from the `cons' package and overrides or adds others. This
       can be done using Perl's inheritance mechanisms.

       The following example defines a new package `cons::switch' which overrides the standard
       `Library' method. The overridden method builds linked library modules, rather than library
       archives. A new constructor is provided. Environments created with this constructor will
       have the new library method; others won't.

         package cons::switch;
         BEGIN {@ISA = 'cons'}

         sub new {
               bless new cons(@_);

         sub Library {
               my($env) = shift;
               my($lib) = shift;
               my(@objs) = Objects $env @_;
               Command $env $lib, @objs, q(
                       %LD -r %LDFLAGS %< -o %>

       This functionality could be invoked as in the following example:

         $env = new cons::switch(@overrides);
         Library $env 'lib.o', 'foo.c', 'bar.c';

Invoking Cons

       The `cons' command is usually invoked from the root of the build tree. A Construct file
       must exist in that directory. If the `-f' argument is used, then an alternate Construct
       file may be used (and, possibly, an alternate root, since `cons' will cd to Construct
       file's containing directory).

       If `cons' is invoked from a child of the root of the build tree with the `-t' argument, it
       will walk up the directory hierarchy looking for a Construct file.  (An alternate name may
       still be specified with `-f'.)  The targets supplied on the command line will be modified
       to be relative to the discovered Construct file.  For example, from a directory containing
       a top-level Construct file, the following invocation:

         % cd libfoo/subdir
         % cons -t target

       is exactly equivalent to:

         % cons libfoo/subdir/target

       If there are any `Default' targets specified in the directory hierarchy's Construct or
       Conscript files, only the default targets at or below the directory from which `cons -t'
       was invoked will be built.

       The command is invoked as follows:

         cons <arguments> -- <construct-args>

       where arguments can be any of the following, in any order:

       target    Build the specified target. If target is a directory, then recursively build
                 everything within that directory.

       +pattern  Limit the Conscript files considered to just those that match pattern, which is
                 a Perl regular expression. Multiple `+' arguments are accepted.

                 Sets name to value val in the `ARG' hash passed to the top-level Construct file.

       `-cc'     Show command that would have been executed, when retrieving from cache. No
                 indication that the file has been retrieved is given; this is useful for
                 generating build logs that can be compared with real build logs.

       `-cd'     Disable all caching. Do not retrieve from cache nor flush to cache.

       `-cr'     Build dependencies in random order. This is useful when building multiple
                 similar trees with caching enabled.

       `-cs'     Synchronize existing build targets that are found to be up-to-date with cache.
                 This is useful if caching has been disabled with -cc or just recently enabled
                 with UseCache.

       `-d'      Enable dependency debugging.

       `-f' <file>
                 Use the specified file instead of Construct (but first change to containing
                 directory of file).

       `-h'      Show a help message local to the current build if one such is defined, and exit.

       `-k'      Keep going as far as possible after errors.

       `-o' <file>
                 Read override file file.

       `-p'      Show construction products in specified trees. No build is attempted.

       `-pa'     Show construction products and associated actions. No build is attempted.

       `-pw'     Show products and where they are defined. No build is attempted.

       `-q'      Don't be verbose about Installing and Removing targets.

       `-r'      Remove construction products associated with <targets>. No build is attempted.

       `-R' <repos>
                 Search for files in repos.  Multiple -R repos directories are searched in the
                 order specified.

       `-t'      Traverse up the directory hierarchy looking for a Construct file, if none exists
                 in the current directory.  Targets will be modified to be relative to the
                 Construct file.

       `-v'      Show `cons' version and continue processing.

       `-V'      Show `cons' version and exit.

       `-wf' <file>
                 Write all filenames considered into file.

       `-x'      Show a help message similar to this one, and exit.

       And construct-args can be any arguments that you wish to process in the Construct file.
       Note that there should be a -- separating the arguments to cons and the arguments that you
       wish to process in the Construct file.

       Processing of construct-args can be done by any standard package like Getopt or its
       variants, or any user defined package. cons will pass in the construct-args as @ARGV and
       will not attempt to interpret anything after the --.

         % cons -R /usr/local/repository -d os=solaris +driver -- -c test -f DEBUG

       would pass the following to cons

         -R /usr/local/repository -d os=solaris +driver

       and the following, to the top level Construct file as @ARGV

         -c test -f DEBUG

       Note that `cons -r .' is equivalent to a full recursive `make clean', but requires no
       support in the Construct file or any Conscript files. This is most useful if you are
       compiling files into source directories (if you separate the build and export directories,
       then you can just remove the directories).

       The options `-p', `-pa', and `-pw' are extremely useful for use as an aid in reading
       scripts or debugging them. If you want to know what script installs export/include/foo.h,
       for example, just type:

         % cons -pw export/include/foo.h

Using and writing dependency scanners

       QuickScan allows simple target-independent scanners to be set up for source files. Only
       one QuickScan scanner may be associated with any given source file and environment.

       QuickScan is invoked as follows:


       The subroutine referenced by CODEREF is expected to return a list of filenames included
       directly by FILE. These filenames will, in turn, be scanned. The optional PATH argument
       supplies a lookup path for finding FILENAME and/or files returned by the user-supplied
       subroutine.  The PATH may be a reference to an array of lookup-directory names, or a
       string of names separated by the system's separator character (':' on UNIX systems, ';' on
       Windows NT).

       The subroutine is called once for each line in the file, with $_ set to the current line.
       If the subroutine needs to look at additional lines, or, for that matter, the entire file,
       then it may read them itself, from the filehandle SCAN. It may also terminate the loop, if
       it knows that no further include information is available, by closing the filehandle.

       Whether or not a lookup path is provided, QuickScan first tries to lookup the file
       relative to the current directory (for the top-level file supplied directly to QuickScan),
       or from the directory containing the file which referenced the file. This is not very
       general, but seems good enough--especially if you have the luxury of writing your own
       utilities and can control the use of the search path in a standard way. Finally, the
       search path is, currently, colon separated. This may not make the NT camp happy.

       Here's a real example, taken from a Construct file here:

         sub cons::SMFgen {
             my($env, @tables) = @_;
             foreach $t (@tables) {
                 $env->QuickScan(sub { /\b\S*?\.smf\b/g }, "$t.smf",
                     ["$","$t.smdb.h","$","$", "$"],
                       smfgen %( %SMF_INCLUDE_OPT %) %<

       [NOTE that the form `$env->QuickScan ...'  and `$env->Command ...' should not be
       necessary, but, for some reason, is required for this particular invocation. This appears
       to be a bug in Perl or a misunderstanding on my part; this invocation style does not
       always appear to be necessary.]

       This finds all names of the form <name>.smf in the file. It will return the names even if
       they're found within comments, but that's OK (the mechanism is forgiving of extra files;
       they're just ignored on the assumption that the missing file will be noticed when the
       program, in this example, smfgen, is actually invoked).

       A scanner is only invoked for a given source file if it is needed by some target in the
       tree. It is only ever invoked once for a given source file.

       Here is another way to build the same scanner. This one uses an explicit code reference,
       and also (unecessarily, in this case) reads the whole file itself:

         sub myscan {
             do {
                 push(@includes, /\b\S*?\.smf\b/g);
             } while <SCAN>;

       Note that the order of the loop is reversed, with the loop test at the end. This is
       because the first line is already read for you. This scanner can be attached to a source
       file by:

           QuickScan $env \myscan, "$_.smf";


       Cons is maintained by the user community.  To subscribe, send mail to cons-discuss- with body subscribe.

       Please report any suggestions through the mailing list.


       Sure to be some. Please report any bugs through the mailing list.


       Information about CONS can be obtained from the official cons web site or its mirrors listed there.

       The cons maintainers can be contacted by email at


       Originally by Bob Sidebotham. Then significantly enriched by the members of the Cons

       The Cons community would like to thank Ulrich Pfeifer for the original pod documentation
       derived from the cons.html file. Cons documentation is now a part of the program itself.