DragonFly On-Line Manual Pages

CONS(1) Cons - Software Construction System CONS(1)

NAME
Cons - A Software Construction System

DESCRIPTION
A guide and reference for version 2.2.0

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";

# $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
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.

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 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 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

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 path.

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

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

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
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 statement:

$env = new cons();

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

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

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

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

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

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

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

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

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

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 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 `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 mentioned).

%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 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'.

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";
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.

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.

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 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 `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 present.

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 {
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' 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:

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.
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",
$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 %) %<
)
);
}
}

[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 {
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";

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.

3rd Berkeley Distribution 2.2.0 CONS(1)