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Cons - A Software Construction System


A guide and reference for version 2.2.0

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

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

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

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


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

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

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

Why Cons? Why not Make?

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

Build complexity

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

Build reproducibility

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

Variant builds

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


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

Keeping it simple

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

Perl scripts

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

Hello, World!

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

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

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

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

Construction environments

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

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

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

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

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

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

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

Automatic and complete dependency analysis

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

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

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

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

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

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

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

Automatic global build sequencing

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

Building large trees--still just as simple

A hierarchy of build scripts

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

The Build command

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

Build qw(

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

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

Build qw(

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

Build qw(

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

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

Conscript_chdir 1;

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

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

Conscript_chdir 0;

Relative, top-relative, and absolute file names

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

Using modules in build scripts

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

use English;
require My::Module;

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

Scope of variables

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

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

The Export command

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

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

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

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

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

The Import command

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

Import qw( env INCLUDE );

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

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

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

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

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

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

Build script evaluation order

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

Build qw(

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

A Model for sharing files

Some simple conventions

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

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

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

# Construct file for Hello, World!

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


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

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

Build qw(

The world directory's Conscript file looks like this:

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

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

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

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

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

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

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

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

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

Clean, understandable, location-independent scripts

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

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

Separating source and build trees

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

Separating build and source directories using the Link command

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

Link 'build' => 'src';

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

Build qw(

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

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

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

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

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

Variant builds

Hello, World! for baNaNa and peAcH OS's

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

# Construct file for Hello, World!

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

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


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

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

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

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

Build (

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

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

Variations on a theme

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

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


MD5 cryptographic signatures

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

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

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

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

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

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

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

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

We'll demonstrate this with a simple example:

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

Notice how Cons recompiles at the appropriate times:

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

Code Repositories

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


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

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

Repository qw (

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

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

Finding the Construct file in a Repository

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

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

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

Repository source files

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

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

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

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

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

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

Repository derived files

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

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

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

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

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

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

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

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

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

Repository_Sig_Times_OK 0;

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

Local copies of files

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

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

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

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

Local qw(

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

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

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

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

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

is exactly equivalent to:

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

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

Repository dependency analysis

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

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

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

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

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

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

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

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

Repository qw(

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

Would yield a compilation command of:

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

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

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

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

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

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


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

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

Repository interaction with other Cons features

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

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

Default targets

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

% cons hello

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

% cons # does not build anything

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

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

Default '.';

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

Default qw(

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

Selective builds

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

Selective targeting

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

% cons build/hello/hello.o

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

In this example:

% cons build

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

If we do, instead:

% cons export

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

No ``special'' targets

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

% cons tests

will build all the tests.

% cons export

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

% cons build

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

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

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

% cons .

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

Build Pruning

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

% cons build +world

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

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

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

Temporary overrides

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

% cons -o over export

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

Overriding environment variables

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

The Override command

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

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

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

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

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

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

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

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

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

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

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

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

More on construction environments

Default construction variables

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

$env = new cons();

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

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

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

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

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

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

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

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

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

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

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

Interpolating construction variables

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

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

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

Default construction methods

The list of default construction methods includes the following:

The `new' constructor

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

$env = new cons(<overrides>);

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

The `clone' method

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

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

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

The `copy' method

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

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

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

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

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

The `Install' method

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

Install $env <directory>, <names>;

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

The `InstallAs' method

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

`InstallAs' works in two ways:

Single file install:

InstallAs $env TgtFile, SrcFile;

Multiple file install:

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

Or, even as:

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

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

The `Precious' method

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

Precious <files>;

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

The `Command' method

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

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

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

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

There are several pseudo variables which will also be expanded:

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

%0 Same as `%>'.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The `Objects' method

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

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

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

The `Program' method

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

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

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

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

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

The `Library' method

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

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

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

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

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

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

The `Module' method

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

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

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

The `Depends' method

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

Depends $env <target>, <dependencies>;

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

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

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

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

The `Ignore' method

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

Ignore <patterns>;

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

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

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

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

The `Salt' method

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

Salt $string;

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

Salt `uname -s`;

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

The `UseCache' method

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

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

The `SourcePath' method

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

$path = SourcePath <buildpath>;

The `ConsPath' method

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

$result = ConsPath <path>;

The `SplitPath' method

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

@paths = SplitPath <pathlist>;

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

The `DirPath' method

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

$cwd = DirPath <paths>;

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

$cwd = DirPath '.';

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

The `FilePath' method

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

$file = FilePath <path>;

The `Help' method

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

Help <helptext>;

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

Extending Cons

Overriding construction variables

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

Adding new methods

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

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

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

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

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

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

Overriding methods

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

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

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

sub new {
bless new cons(@_);

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

This functionality could be invoked as in the following example:

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

Invoking Cons

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

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

% cd libfoo/subdir
% cons -t target

is exactly equivalent to:

% cons libfoo/subdir/target

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

The command is invoked as follows:

cons <arguments> -- <construct-args>

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

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

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

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

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

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

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

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

`-d' Enable dependency debugging.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

would pass the following to cons

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

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

-c test -f DEBUG

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

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

% cons -pw export/include/foo.h

Using and writing dependency scanners

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

QuickScan is invoked as follows:


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

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

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

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

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

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

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

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

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

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

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

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


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