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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 un-
der 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 FIT-
NESS 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 Soft-
ware Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
USA.

Introduction
Cons is a system for constructing, primarily, software, but is quite dif-
ferent 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 environ-
ments 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 glob-
ally 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 in-
structions 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 ten-
dency 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 re-
cursively 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 possibili-
ties, but these have largely served to clutter an already overextended lan-
guage. Often, builds are done in multiple passes in order to provide appro-
priate 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 be-
tween directories. Even when dependencies are working correctly, make's re-
liance 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 de-
termining when a file should be rederived. If an external library, for ex-
ample, 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. opti-
mized 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 ex-
ists 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 impor-
tant 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 sub-
sequent sections, we shall introduce Cons and show how these issues are ad-
dressed.

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 pro-
vides 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-de-
fined 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 environ-
ment. 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 equiva-
lent is used, instead. Since this version of Hello, World! requires a li-
brary, libworld.a, we have specified that any program linked in this envi-
ronment 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, lib-
world.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 depen-
dency, 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 in-
telligently 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 evi-
dent 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 in-
volved 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 Con-
struct 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 under-
stand, 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 construc-
tion 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 contain-
ing 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 gener-
ally 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 ap-
pears. 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 com-
mon, separate Perl package. Cons controls the symbol table for the package
so that, the symbol table for each script is empty, except for the Con-
struct file, which gets some of the command line arguments. All of the
variables that are set or used, therefore, are set by the script it-
self--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 be-
gins with `$'. Other variables, objects, etc. can be exported by refer-
ence--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 refer-
enced. 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 subse-
quent `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 vari-
ables directly from the superior script. Consider this example:

Import qw( env INCLUDE );

This is only legal if the parent script exported both `$env' and `$IN-
CLUDE'. 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 Con-
struct 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' com-
mand:

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 con-
ventional 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 prod-
ucts 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 prod-
ucts. 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 reorga-
nize 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 Con-
script 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-directo-
ries 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 di-
rectory. Now if you type the same command as before, you will get the fol-
lowing 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 ex-
pedient 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 under-
lying 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 ex-
ample 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! applica-
tion 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 al-
ternative 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 com-
mand 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 de-
pendency 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 gener-
ate 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 con-
stituents), 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 particu-
lar 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. Un-
related 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 effec-
tive 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 executa-
bles. 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 de-
rived 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 build-
ing 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, li-
braries, or executables), Cons will perform its normal signature calcula-
tion 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 signa-
ture 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 reposi-
tory (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 di-
rectory in a `Repository' command because Cons will remove the current di-
rectory 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 repos-
itory signatures are invalid and rebuild files unnecessarily. This behav-
ior 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 cor-
rectly 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 ex-
ample, may assume that certain generated files exist locally. Instead of
making these subsidiary scripts aware of the repository directory, the `Lo-
cal' 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 per-
formed 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 ex-
ist 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 en-
vironments are defined and onto the way the C `#include' preprocessor di-
rective 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' direc-
tories, 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 reposi-
tory 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 pre-
processor conform to its preferred search path.

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

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

Repository_List

Cons provides a `Repository_List' command to return a list of all reposi-
tory directories in their current search order. This can be used for de-
bugging, 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 fea-
tures--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 subdi-
rectories for version builds established via `Link' to a source subdirec-
tory. Cons will search for derived files in the appropriate build subdi-
rectories 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 de-
faults 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 typi-
cal behavior of building everything by default:

Default '.';

The following would add the hello and goodbye commands (in the same direc-
tory 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 de-
fault 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 prod-
uct--that Cons knows about--in the specified directory and below. For exam-
ple:

% 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 some-
one 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 sup-
pose, for example, that you have a whole series of unit tests that are as-
sociated 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, incorpo-
rated 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 suffi-
cient 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, obvi-
ously, 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 vari-
able/value pairs are used to override the values in the construction envi-
ronment 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 over-
ride could, of course, be restricted to a single directory by the appropri-
ate 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 over-
ride 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 ex-
actly 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 plat-
form. 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 envi-
ronment, in particular any method that ultimately invokes an external com-
mand will substitute these variables into the final command, as appropri-
ate. For example, the `Objects' method takes a number of source files and
arranges to derive, if necessary, the corresponding object files. For exam-
ple:

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 ap-
propriate 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 ex-
ample, 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 spec-
ified 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 en-
vironment. 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 spec-
ifies 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 com-
mand. For the utmost reproducibility, you should really arrange to set your
own execution path, in your top-level Construct file (or perhaps by import-
ing 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 construc-
tion 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 en-
vironment 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 envi-
ronment from the original. If you just want a new name for the same envi-
ronment (which may be helpful when exporting environments to existing com-
ponents), 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. Over-
rides 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 envi-
ronment 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 fol-
lows:

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 in-
stalled 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 com-
mand 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 envi-
ronment may be introduced by prefixing the name of the construction vari-
able 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 di-
rectory:

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 execu-
tion 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 brack-
ets 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 se-
quentially. If any of the commands fails, then none of the rest are exe-
cuted, 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 what-
ever platform-specific indication of success is required), then a new sig-
nature 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 con-
sidered to have failed. For example, here is a simple command which cre-
ates 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 `Com-
mand' 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 sup-
pressed 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 in-
terpolation:

# 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 ex-
panding the `CCCOM' construction variable, with `%<' and `%>' set to the
source and object files, respectively (see the `Command' method for expan-
sion 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-rela-
tive names may also be used). This variable is used by `CCCOM'. The behav-
ior 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 speci-
fied 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 ob-
ject 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-sep-
arated 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 li-
brary 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 invoca-
tions (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 li-
brary will be generated (the rest will disappear!).

The `Module' method

The `Module' method is a combination of the `Program' and `Command' meth-
ods. 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 ex-
ists (or is writable) for particular types of files. Normally, dependencies
are calculated automatically from a combination of the explicit dependen-
cies 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 `#in-
clude <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 op-
posted to the path name within a build directory. It is invoked as fol-
lows:

$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 sys-
tems, ';' on Windows NT), and returns the fully-qualified names. It is in-
voked 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 speci-
fied in the top-level Construct file.

Extending Cons
Overriding construction variables

There are several ways of extending Cons, which vary in degree of diffi-
culty. 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 en-
vironments. 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, `In-
stallScript', which installs a tcl script in a requested location, but ed-
its 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 pre-
fixing the name with `cons::'). A change made in this manner will be glob-
ally 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 mecha-
nisms.

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. Envi-
ronments 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 Con-
struct 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 di-
rectory 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 recur-
sively build everything within that directory.

+pattern Limit the Conscript files considered to just those that match
pattern, which is a Perl regular expression. Multiple `+' argu-
ments 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 de-
fined, 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 at-
tempted.

`-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 modi-
fied 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 sep-
arate the build and export directories, then you can just remove the direc-
tories).

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 in-
stalls 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 file-
names 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 sepa-
rated by the system's separator character (':' on UNIX systems, ';' on Win-
dows 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 file-
handle 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--espe-
cially 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 in-
vocation. 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 mail-
ing 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)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=cons&sektion=1&manpath=FreeBSD+Ports+15.1.quarterly>

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