Archives play two roles: they combine multiple documents into a
single one and they typically provide compression and sometimes
encryption or other services. Bundling multiple resources into a single
archive may greatly simplify distribution and guarantee that the
individual resources are consistent. SWI-Prolog provides archiving using
its (rather arcane) saved-state format. See resource/3
and open_resource/3.
It also provides compression by means of library(zlib).
External archives may be accessed through the process interface
provided by process_create/3,
but this has disadvantages. The one that motivated this library was that
using external processes provide no decent platform independent access
to archives. Most likely zip files come closest to platform independent
access, but there are many different programs for accessing zip files
that provide slightly different sets of options and the existence of any
of these programs cannot be guaranteed without distributing our own
bundled version. Similar arguments hold for Unix tar archives, where
just about any Unix-derives system has a tar program but except for very
basic commands, the command line options are not compatible and tar is
not part of Windows. The only format granted on Windows is .cab, but a
program to create them is not part of Windows and the .cab format is
rare outside the Windows context.
Discarding availability of archive programs, each archive program
comes with its own set of command line options and its own features and
limitations. Fortunately,
libarchive
provides a consistent interface to a wealth of compression and archiving
formats. The library library(archive) wraps this library,
providing access to archives using Prolog streams both for the archive
as a whole and the archive entries. E.g., archives may be read from
Prolog streams and each member in turn may be processed using Prolog
streams without materialising data using temporary files.
