'Cons - Software Construction System'

- NAME
- DESCRIPTION
- Introduction
- Why Cons? Why not Make?

- Build complexity
- Build reproducibility
- Variant builds
- Repositories

- Keeping it simple

- Perl scripts
- Hello, World!
- Construction environments
- Automatic and complete dependency analysis
- Automatic global build sequencing

- Building large trees-still just as simple

- A hierarchy of build scripts
- The Build command
- Relative, top-relative, and absolute file names
- Using modules in build scripts
- Scope of variables
- The Export command
- The Import command
- Build script evaluation order

- A Model for sharing files

- Some simple conventions
- Clean, understandable, location-independent scripts

- Separating source and build trees

- Separating build and source directories using the Link command

- Variant builds

- Hello, World! for baNaNa and peAcH OS's
- Variations on a theme

- Signatures

- MD5 content and build signatures
- Storing signatures in .consign files
- Using build signatures to decide when to rebuild files
- Signature example
- Source-file signature configuration
- Derived-file signature configuration
- Debugging signature calculation

- Code Repositories

- Repository
- Finding the Construct file in a Repository
- Repository source files
- Repository derived files
- Local copies of files
- Repository dependency analysis
- Repository_List
- Repository interaction with other Cons features

- Default targets
- Selective builds

- Selective targeting
- No ``special'' targets

- Build Pruning
- Temporary overrides

- Overriding environment variables
- The Override command

- More on construction environments

- Construction variable expansion
- Default construction variables
- Expanding variables in construction commands
- Expanding construction variables in file names

- Build actions

- Construction commands
- Perl expressions
- Perl code references [EXPERIMENTAL]

- Default construction methods

- The `new` constructor
- The `clone` method
- The `copy` method
- The `Install` method
- The `InstallAs` method
- The `Precious` method
- The `AfterBuild` method
- The `Command` method
- The `Objects` method
- The `Program` method
- The `Library` method
- The `Module` method
- The `Depends` method
- The `RuleSet` method
- The `DefaultRules` method
- The `Ignore` method
- The `Salt` method
- The `UseCache` method
- The `SourcePath` method
- The `ConsPath` method
- The `SplitPath` method
- The `DirPath` method
- The `FilePath` method
- The `Help` method

- Extending Cons

- Overriding construction variables
- Adding new methods
- Overriding methods

- Invoking Cons
- Using and writing dependency scanners
- SUPPORT AND SUGGESTIONS
- BUGS
- INFORMATION ABOUT CONS
- AUTHORS

# NAME

Cons - A Software Construction System

# DESCRIPTION

A guide and reference for version 2.3.0

Copyright (c) 1996-2001 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.

# Introduction

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.

## Repositories

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 application:

      % 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 relinked:

      % vi world.c
        [EDIT]
      % 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
        [EDIT]
      % 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(
            drivers/display/Conscript
            drivers/mouse/Conscript
            parser/Conscript
            utilities/Conscript
      );

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(
            parser/Conscript
            drivers/Conscript
            utilities/Conscript
      );

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

      Build qw(
            display/Conscript
            mouse/Conscript
      );

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 follows:

      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` statements:

      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(
            drivers/display/Conscript
            drivers/mouse/Conscript
            parser/Conscript
            utilities/Conscript
      );

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 build.

# 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(
            hello/Conscript
            world/Conscript
      );

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, too.

      % 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 following:

      Build qw(
            build/world/Conscript
            build/hello/Conscript
      );

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 appropriately.

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! application:

      # 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 (
            "$BUILD/hello/Conscript",
            "$BUILD/world/Conscript",
      );

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

      % 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.

# Signatures

Cons uses file signatures to decide if a derived file is out-of-date and needs rebuilding. In essence, if the contents of a file change, or the manner in which the file is built changes, the file's signature changes as well. This allows Cons to decide with certainty when a file needs rebuilding, because Cons can detect, quickly and reliably, whether any of its dependency files have been changed.

## MD5 content and build signatures

Cons uses the MD5 (Message Digest 5) algorithm to compute file signatures. The MD5 algorithm computes a strong cryptographic checksum for any given input string. Cons can, based on configuration, use two different MD5 signatures for a given file:

The content signature of a file is an MD5 checksum of the file's contents. Consequently, when the contents of a file change, its content signature changes as well.

The build signature of a file is a combined MD5 checksum of:

the signatures of all the input files used to build the file

the signatures of all dependency files discovered by source scanners (for example, `.h` files)

the signatures of all dependency files specified explicitly via the `Depends` method)

the command-line string used to build the file

The build signature is, in effect, a digest of all the dependency information for the specified file. Consequently, a file's build signature changes whenever any part of its dependency information changes: a new file is added, the contents of a file on which it depends change, there's a change to the command line used to build the file (or any of its dependency files), etc.

For example, in the previous section, the build signature of the world.o file will include:

the signature of the world.c file

the signatures of any header files that Cons detects are included, directly or indirectly, by world.c

the text of the actual command line was used to generate world.o

Similarly, the build signature of the libworld.a file will include all the signatures of its constituents (and hence, transitively, the signatures of their constituents), as well as the command line that created the file.

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

## Storing signatures in .consign files

Before Cons exits, it stores the calculated signatures for all of the files it built or examined in .consign files, one per directory. Cons uses this stored information on later invocations to decide if derived files need to be rebuilt.

After the previous example was compiled, the .consign file in the build/peach/world directory looked like this:

      world.h:985533370 - d181712f2fdc07c1f05d97b16bfad904
      world.o:985533372 2a0f71e0766927c0532977b0d2158981
      world.c:985533370 - c712f77189307907f4189b5a7ab62ff3
      libworld.a:985533374 69e568fc5241d7d25be86d581e1fb6aa

After the file name and colon, the first number is a timestamp of the file's modification time (on UNIX systems, this is typically the number of seconds since January 1st, 1970). The second value is the build signature of the file (or ``-'' in the case of files with no build signature--that is, source files). The third value, if any, is the content signature of the file.

## Using build signatures to decide when to rebuild files

When Cons is deciding whether to build or rebuild a derived file, it first computes the file's current build signature. If the file doesn't exist, it must obviously be built.

If, however, the file already exists, Cons next compares the modification timestamp of the file against the timestamp value in the .consign file. If the timestamps match, Cons compares the newly-computed build signature against the build signature in the .consign file. If the timestamps do not match or the build signatures do not match, the derived file is rebuilt.

After the file is built or rebuilt, Cons arranges to store the newly-computed build signature in the .consign file when it exits.

## Signature example

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

## Source-file signature configuration

Cons provides a `SourceSignature` method that allows you to configure how the signature should be calculated for any source file when its signature is being used to decide if a dependent file is up-to-date. The arguments to the `SourceSignature` method consist of one or more pairs of strings:

      SourceSignature 'auto/*.c' => 'content',
                      '*' => 'stored-content';

The first string in each pair is a pattern to match against derived file path names. The pattern is a file-globbing pattern, not a Perl regular expression; the pattern <*.l> will match all Lex source files. The `*` wildcard will match across directory separators; the pattern `foo/*.c` would match all C source files in any subdirectory underneath the `foo` subdirectory.

The second string in each pair contains one of the following keywords to specify how signatures should be calculated for source files that match the pattern. The available keywords are:

content
  Use the content signature of the source file when calculating signatures of files that depend on it. This guarantees correct calculation of the file's signature for all builds, by telling Cons to read the contents of a source file to calculate its content signature each time it is run.

stored-content
  Use the source file's content signature as stored in the .consign file, provided the file's timestamp matches the cached timestamp value in the .consign file. This optimizes performance, with the slight risk of an incorrect build if a source file's contents have been changed so quickly after its previous update that the timestamp still matches the stored timestamp in the .consign file even though the contents have changed.

The Cons default behavior of always calculating a source file's signature from the file's contents is equivalent to specifying:

      SourceSignature '*' => 'content';

The `'*'` will match all source files. The `content` keyword specifies that Cons will read the contents of a source file to calculate its signature each time it is run.

A useful global performance optimization is:

      SourceSignature '*' => 'stored-content';

This specifies that Cons will use pre-computed content signatures from .consign files, when available, rather than re-calculating a signature from the the source file's contents each time Cons is run. In practice, this is safe for most build situations, and only a problem when source files are changed automatically (by scripts, for example). The Cons default, however, errs on the side of guaranteeing a correct build in all situations.

Cons tries to match source file path names against the patterns in the order they are specified in the `SourceSignature` arguments:

      SourceSignature '/usr/repository/objects/*' => 'stored-content',
                      '/usr/repository/*' => 'content',
                      '*.y' => 'content',
                      '*' => 'stored-content';

In this example, all source files under the /usr/repository/objects directory will use .consign file content signatures, source files anywhere else underneath /usr/repository will not use .consign signature values, all Yacc source files (`*.y`) anywhere else will not use .consign signature values, and any other source file will use .consign signature values.

## Derived-file signature configuration

Cons provides a `SIGNATURE` construction variable that allows you to configure how signatures are calculated for any derived file when its signature is being used to decide if a dependent file is up-to-date. The value of the `SIGNATURE` construction variable is a Perl array reference that holds one or more pairs of strings, like the arguments to the `SourceSignature` method.

The first string in each pair is a pattern to match against derived file path names. The pattern is a file-globbing pattern, not a Perl regular expression; the pattern `*.obj' will match all (Win32) object files. The `*` wildcard will match across directory separators; the pattern `foo/*.a' would match all (UNIX) library archives in any subdirectory underneath the foo subdirectory.

The second string in each pair contains one of the following keywords to specify how signatures should be calculated for derived files that match the pattern. The available keywords are the same as for the `SourceSignature` method, with an additional keyword:

build
  Use the build signature of the derived file when calculating signatures of files that depend on it. This guarantees correct builds by forcing Cons to rebuild any and all files that depend on the derived file.

content
  Use the content signature of the derived file when calculating signatures of files that depend on it. This guarantees correct calculation of the file's signature for all builds, by telling Cons to read the contents of a derived file to calculate its content signature each time it is run.

stored-content
  Use the derived file's content signature as stored in the .consign file, provided the file's timestamp matches the cached timestamp value in the .consign file. This optimizes performance, with the slight risk of an incorrect build if a derived file's contents have been changed so quickly after a Cons build that the file's timestamp still matches the stored timestamp in the .consign file.

The Cons default behavior (as previously described) for using derived-file signatures is equivalent to:

      $env = new cons(SIGNATURE => ['*' => 'build']);

The `*` will match all derived files. The `build` keyword specifies that all derived files' build signatures will be used when calculating whether a dependent file is up-to-date.

A useful alternative default `SIGNATURE` configuration for many sites:

      $env = new cons(SIGNATURE => ['*' => 'content']);

In this configuration, derived files have their signatures calculated from the file contents. This adds slightly to Cons' workload, but has the useful effect of ``stopping'' further rebuilds if a derived file is rebuilt to exactly the same file contents as before, which usually outweighs the additional computation Cons must perform.

For example, changing a comment in a C file and recompiling should generate the exact same object file (assuming the compiler doesn't insert a timestamp in the object file's header). In that case, specifying `content` or `stored-content` for the signature calculation will cause Cons to recognize that the object file did not actually change as a result of being rebuilt, and libraries or programs that include the object file will not be rebuilt. When `build` is specified, however, Cons will only ``know'' that the object file was rebuilt, and proceed to rebuild any additional files that include the object file.

Note that Cons tries to match derived file path names against the patterns in the order they are specified in the `SIGNATURE` array reference:

      $env = new cons(SIGNATURE => ['foo/*.o' => 'build',
                                    '*.o' => 'content',
                                    '*.a' => 'cache-content',
                                    '*' => 'content']);

In this example, all object files underneath the foo subdirectory will use build signatures, all other object files (including object files underneath other subdirectories!) will use .consign file content signatures, libraries will use .consign file build signatures, and all other derived files will use content signatures.

## Debugging signature calculation

Cons provides a `-S` option that can be used to specify what internal Perl package Cons should use to calculate signatures. The default Cons behavior is equivalent to specifying `-S md5` on the command line.

The only other package (currently) available is an `md5::debug` package that prints out detailed information about the MD5 signature calculations performed by Cons:

      % cons -S md5::debug hello
      sig::md5::srcsig(hello.c)
              => |52d891204c62fe93ecb95281e1571938|
      sig::md5::collect(52d891204c62fe93ecb95281e1571938)
              => |fb0660af4002c40461a2f01fbb5ffd03|
      sig::md5::collect(52d891204c62fe93ecb95281e1571938,
                        fb0660af4002c40461a2f01fbb5ffd03,
                        cc   -c %< -o %>)
              => |f7128da6c3fe3c377dc22ade70647b39|
      sig::md5::current(||
                     eq |f7128da6c3fe3c377dc22ade70647b39|)
      cc -c hello.c -o hello.o
      sig::md5::collect()
              => |d41d8cd98f00b204e9800998ecf8427e|
      sig::md5::collect(f7128da6c3fe3c377dc22ade70647b39,
                        d41d8cd98f00b204e9800998ecf8427e,
                        cc  -o %> %<  )
              => |a0bdce7fd09e0350e7efbbdb043a00b0|
      sig::md5::current(||
                     eq |a0bdce7fd09e0350e7efbbdb043a00b0|)
      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.

## Repository

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 (
            /usr/experiment/repository
            /usr/product/repository
      );

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 files:

      % pico world.c
        [EDIT]
      % 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 list.)

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
        [EDIT]
      % 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 signature.)

## 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 happens:

      % 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(
            hello
      );

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 construction 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 command:

      % 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(
            /u1
            /u2
      );

      $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

In order to shorten the command lines as much as possible, Cons will remove `-I` flags for any directories, locally or in the repositories, which do not actually exist. (Note that the `-I` flags are not included in the MD5 signature calculation for the target file, so the target will not be recompiled if the compilation command changes due to a directory coming into existence.)

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 any C source code that you plan to modify:

      #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 path.

Consequently, when using repository trees in Cons, always use angle-brackets for included files in any C source (.c or .h) files that you plan to modify locally:

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

Code that will not change can still safely use double quotes on #include lines.

## Repository_List

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 stuff:

      @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(
            hello
            goodbye
      );

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 build/hello.

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 tree.

# 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 this:

      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

As previously mentioned, a construction environment is an object that has a set of keyword/value pairs and a set of methods, and which is used to tell Cons how target files should be built. This section describes how Cons uses and expands construction environment values to control its build behavior.

## Construction variable expansion

Construction variables from a construction environment are expanded by preceding the keyword with a `%` (percent sign):

      Construction variables:
            XYZZY => 'abracadabra',

      The string:  "The magic word is:  %XYZZY!"
      expands to:  "The magic word is:  abracadabra!"

A construction variable name may be surrounded by `{` and `}` (curly braces), which are stripped as part of the expansion. This can sometimes be necessary to separate a variable expansion from trailing alphanumeric characters:

      Construction variables:
            OPT    => 'value1',
            OPTION => 'value2',

      The string:  "%OPT %{OPT}ION %OPTION %{OPTION}"
      expands to:  "value1 value1ION value2 value2"

Construction variable expansion is recursive--that is, a string containing `%-`expansions after substitution will be re-expanded until no further substitutions can be made:

      Construction variables:
            STRING => 'The result is:  %FOO',
            FOO    => '%BAR',
            BAR    => 'final value',

      The string:  "The string says:  %STRING"
      expands to:  "The string says:  The result is:  final value"

If a construction variable is not defined in an environment, then the null string is substituted:

      Construction variables:
            FOO => 'value1',
            BAR => 'value2',

      The string:  "%FOO <%NO_VARIABLE> %BAR"
      expands to:  "value1 <> value2"

A doubled `%%` will be replaced by a single `%`:

      The string:  "Here is a percent sign:  %%"
      expands to:  "Here is a percent sign: %"

## Default construction variables

When you specify no arguments when creating a new construction environment:

      $env = new cons();

Cons creates a reference to a new, default construction environment. This contains a number of construction variables and some methods. At the present writing, the default construction variables on a UNIX system are:

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

And on a Win32 system (Windows NT), the default construction variables are (unless the default rule style is set using the DefaultRules method):

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

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:

      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. The substitution rules will be discussed in detail in the next section.

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 environment.

The `CFLAGS`, `LDFLAGS`, and `ARFLAGS` variables all supply a place for passing options to the compiler, loader, and archiver, respectively.

The `INCDIRPREFIX` and `INCDIRSUFFIX` variables specify option strings to be appended to the beginning and end, respectively, of each include directory so that the compiler knows where to find .h files. Similarly, the `LIBDIRPREFIX` and `LIBDIRSUFFIX` variables specify the option string to be appended to the beginning of and end, respectively, 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.

## Expanding variables in construction commands

Within a construction command, construction variables will be expanded according to the rules described above. In addition to normal variable expansion from the construction environment, construction commands also expand the following pseudo-variables to insert the specific input and output files in the command line that will be executed:

%>
  The target file name. In a multi-target command, this expands to the first target mentioned.)

%0
  Same as `%>`.

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

%<
  The full set of input file names. 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 directory:

        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`.

There are additional `%` elements which affect the command line(s):

%[ %]
  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 tgt.in:

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

  This will execute:

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

%( %)
  Cons includes the text of the command line in the MD5 signature for a build, so that targets get rebuilt if you change the command line (to add or remove an option, for example). Command-line text in between `%(` and `%)`, however, will be ignored for MD5 signature calculation.

  Internally, Cons uses `%(` and `%)` around include and library directory options (`-I` and `-L` on UNIX systems, `/I` and `/LIBPATH` on Windows NT) to avoid rebuilds just because the directory list changes. Rebuilds occur only if the changed directory list causes any included files to change, and a changed include file is detected by the MD5 signature calculation on the actual file contents.

## Expanding construction variables in file names

Cons expands construction variables in the source and target file names passed to the various construction methods according to the expansion rules described above:

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

This allows for flexible configuration, through the construction environment, of directory names, suffixes, etc.

# Build actions

Cons supports several types of build actions that can be performed to construct one or more target files. Usually, a build action is a construction command--that is, a command-line string that invokes an external command. Cons can also execute Perl code embedded in a command-line string, and even supports an experimental ability to build a target file by executing a Perl code reference directly.

A build action is usually specified as the value of a construction variable:

      $env = new cons(
            CCCOM         => '%CC %CFLAGS %_IFLAGS -c %< -o %>',
            LINKCOM       => '[perl] &link_executable("%>", "%<")',
            ARCOM         => sub { my($env, $target, @sources) = @_;
                                     # code to create an archive
                                    }
      );

A build action may be associated directly with one or more target files via the `Command` method; see below.

## Construction commands

A construction command goes through expansion of construction variables and `%-` pseudo-variables, as described above, to create the actual command line that Cons will execute to generate the target file or files.

After substitution occurs, strings of white space are converted into single blanks, and leading and trailing white space is eliminated. It is therefore currently not possible to introduce variable length white space in strings passed into a command.

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.

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', 'foo.in', 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 string:

      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.

## Perl expressions

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 code, 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";
            close(FILE);
            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.

## Perl code references [EXPERIMENTAL]

Cons supports the ability to create a derived file by directly executing a Perl code reference. This feature is considered EXPERIMENTAL and subject to change in the future.

A code reference may either be a named subroutine referenced by the usual `\&` syntax:

      sub build_output {
            my($env, $target, @sources) = @_;
            print "build_output building $target\n";
            open(OUT, ">$target");
            foreach $src (@sources) {
                if (! open(IN, "<$src")) {
                    print STDERR "cannot open '$src': $!\n";
                    return undef;
                }
                print OUT, <IN>;
            }
            close(OUT);
            return 1;
      }
      Command $env 'output', \&build_output;

or the code reference may be an anonymous subroutine:

      Command $env 'output', sub {
            my($env, $target, @sources) = @_;
            print "building $target\n";
            open(FILE, ">$target");
            print FILE "hello\n";
            close(FILE);
            return 1;
      };

To build the target file, the referenced subroutine is passed, in order: the construction environment used to generate the target; the path name of the target itself; and the path names of all the source files necessary to build the target file.

The code reference is expected to generate the target file, of course, but may manipulate the source and target files in any way it chooses. The code reference must return a false value (`undef` or `0`) if the build of the file failed. Any true value indicates a successful build of the target.

Building target files using code references is considered EXPERIMENTAL due to the following current limitations:

Cons does not print anything to indicate the code reference is being called to build the file. The only way to give the user any indication is to have the code reference explicitly print some sort of ``building'' message, as in the above examples.

Cons does not generate any signatures for code references, so if the code in the reference changes, the target will not be rebuilt.

Cons has no public method to allow a code reference to extract construction variables. This would be good to allow generalization of code references based on the current construction environment, but would also complicate the problem of generating meaningful signatures for code references.

Support for building targets via code references has been released in this version to encourage experimentation and the seeking of possible solutions to the above limitations.

# 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 following:

      %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 undisturbed.

## 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 foo/bar).

## 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 `AfterBuild` method

The `AfterBuild` method evaluates the specified perl string after building the given file or files (or finding that they are up to date). The eval will happen once per specified file. `AfterBuild` is called as follows:

      AfterBuild $env 'foo.o', qq(print "foo.o is up to date!\n");

The perl string is evaluated in the `script` package, and has access to all variables and subroutines defined in the Conscript file in which the `AfterBuild` method is called.

## The `Command` method

The `Command` method is a catchall method which can be used to arrange for any build action to be executed to update the target. For this command, a target file and list of inputs is provided. In addition, a build action is specified as the last argument. The build action is typically a command line or lines, but may also contain Perl code to be executed; see the section above on build actions for details.

The `Command` method is called as follows:

      Command $env <target>, <inputs>, <build action>;

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.

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 section above on construction variable expansion for 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 present.

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 section above on construction variable expansion for 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 present.

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 section above on construction variable expansion for 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 `RuleSet` method

The `RuleSet` method returns the construction variables for building various components with one of the rule sets supported by Cons. The currently supported rule sets are:

msvc
  Rules for the Microsoft Visual C++ compiler suite.

unix
  Generic rules for most UNIX-like compiler suites.

On systems with more than one available compiler suite, this allows you to easily create side-by-side environments for building software with multiple tools:

        $msvcenv = new cons(RuleSet("msvc"));
        $cygnusenv = new cons(RuleSet("unix"));

In the future, this could also be extended to other platforms that have different default rule sets.

## The `DefaultRules` method

The `DefaultRules` method sets the default construction variables that will be returned by the `new` method to the specified arguments:

      DefaultRules(CC     => 'gcc',
                   CFLAGS => '',
                   CCCOM  => '%CC %CFLAGS %_IFLAGS -c %< -o %>');
      $env = new cons();
      # $env now contains *only* the CC, CFLAGS,
      # and CCCOM construction variables

Combined with the `RuleSet` method, this also provides an easy way to set explicitly the default build environment to use some supported toolset other than the Cons defaults:

        # use a UNIX-like tool suite (like cygwin) on Win32
        DefaultRules(RuleSet('unix'));
        $env = new cons();

Note that the `DefaultRules` method completely replaces the default construction environment with the specified arguments, it does not simply override the existing defaults. To override one or more variables in a supported `RuleSet`, append the variables and values:

      DefaultRules(RuleSet('unix'), CFLAGS => '-O3');
      $env1 = new cons();
      $env2 = new cons();
      # both $env1 and $env2 have 'unix' defaults
      # with CFLAGS set to '-O3'

## 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 {
            shift;
            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.

name=<val>
  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`
  Make the build quiet. Multiple `-q` options may be specified.

  A single `-q` options suppress messages about Installing and Removing targets.

  Two `-q` options suppress build command lines and target up-to-date messages.

`-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.

`-S` <pkg>
  Use the sig::<pkg> package to calculate. Supported <pkg> values include ``md5'' for MD5 signature calculation and ``md5::debug'' for debug information about MD5 signature calculation.

  If the specified package ends in <::debug>, signature debug information will be printed to the file name specified in the `CONS_SIG_DEBUG` environment variable, or to standard output if the environment variable is not set.

`-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.

  Internally, `cons` will change its working directory to the directory which contains the top-level Construct file and report:

        cons: Entering directory `top-level-directory'

  This message indicates to an invoking editor (such as emacs) or build environment that Cons will now report all file names relative to the top-level directory. This message can not be suppressed with the `-q` option.

`-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, although the same scanner may (and should) be used for multiple files of a given type.

A QuickScan scanner is only ever invoked once for a given source file, and it is only invoked if the file is used by some target in the tree (i.e., there is a dependency on the source file).

QuickScan is invoked as follows:

      QuickScan CONSENV CODEREF, FILENAME [, PATH]

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.

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",
                              $env->{SMF_INCLUDE_PATH});
              $env->Command(["$t.smdb.cc","$t.smdb.h","$t.snmp.cc",
                             "$t.ami.cc", "$t.http.cc"], "$t.smf",
                            q(smfgen %( %SMF_INCLUDE_OPT %) %<));
          }
      }

The subroutine above 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).

[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.]

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

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";

This final example, which scans a different type of input file, takes over the file scanning rather than being called for each input line:

      $env->QuickScan(
          sub { my(@includes) = ();
              do {
                 push(@includes, $3)
                     if /^(#include|import)\s+(\")(.+)(\")/ && $3
              } while <SCAN>;
              @includes
          },
          "$idlFileName",
          "$env->{CPPPATH};$BUILD/ActiveContext/ACSCLientInterfaces"
      );

# SUPPORT AND SUGGESTIONS

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

Please report any suggestions through the cons-discuss@gnu.org mailing list.

# BUGS

Sure to be some. Please report any bugs through the bug-cons@gnu.org mailing list.

# INFORMATION ABOUT CONS

Information about CONS can be obtained from the official cons web site http://www.dsmit.com/cons/ or its mirrors listed there.

The cons maintainers can be contacted by email at cons-maintainers@gnu.org

# AUTHORS

Originally by Bob Sidebotham. Then significantly enriched by the members of the Cons community cons-discuss@gnu.org.

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.