MacPorts Internals:

The MacPorts system is composed of three TCL libraries:
	macports - public API
	port - portfile parsing and execution
	pextlib - necessary C extensions to TCL

    These three TCL libraries are grouped as TCL "packages". They allow
for complex versioning and have a provide/require model of use, making
them well suited for maintaining a backwards compatible ports system.

    The "macports" TCL Library provides a public API into the MacPorts
system. Any external application wishing to manipulate a port can
load the "macports" TCL library. 'port', the included command line port
utility, uses only the exported "macports" API.

    The "macports" API provides very simple primitives for dealing with
ports (ie, mportexec/mportopen/mportinit). Also, the API has very little
knowledge of the contents of a portfile; instead, it relies entirely upon
the "port" TCL library. By keeping the high level API simple and generic,
a revision of the underlying ports system will not necessarily require
a revision of the high level "macports" API.

    TCL packages allow for complex versioning with a strong requires/provides
model; this means that a revision of the API at any level will not break
backwards compatibility.

    When parsing a port, the "macports" library loads the Portfile into a
sub-interpreter. The "port" library and all port-specific code is run
entirely within this sub-interpreter, and thus can never pollute the TCL
space of other ports, the "macports" library, or the calling application.

    The "macports" library is also responsible for loading user specified
options into the sub-interpreter to be evaluated by the "ports" library. In
that case it sets the variable name in the sub-interpreter and adds the
option to the sub-interpreter's global array user_options(). User options
are passed as part of the call to mportopen.

    The "port" TCL package provides all the primitives required for a mac
portfile to be parsed, queried, and its actions executed. It also provides
a single procedure call that the "macports" TCL package uses to kick off
execution: "eval_targets".

    The port TCL package is loaded by every Portfile with the following line
at the start of the Portfile:
	PortSystem <version required>
    The version should be set to the version of the port API required to build
your port. Currently the only version supplied is 1.0.

    Because a port is not actually parsed, but in fact executed in the TCL
interpreter, every port option must be a TCL procedure. The ports library
supplies all these procedures, all of which are generated at run-time using
the "options" procedure in portutil.tcl.

    The "pextlib" TCL library provides a variety of otherwise unavailable TCL
procedures, such as an interface to flock(2) and mkstemp(3).
