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-<section> <date> 1. June 2000 </date>
-<h2> ZIP File Access </h2> Using Zipped Files Transparently
-
-<!--border-->
-
-<section>
-<h3>The Typedef</h3>
-
-<P>
- The typedef <code>ZZIP_FILE</code> can serve as a replacement
- for a normal file descriptor. As long as it is only used
- for reading a file, the zzlib-user can actually replace
- the posix functions <code>open/read/close</code>
- by their counterparts from the
- <a href="zziplib.html">zziplib library</a>:
- <code>zzip_open/zzip_read/zzip_close</code>.
-</P>
-<P>
- As long as the filename path given to <code>zzip_open</code>
- refers to a real file in the filesystem, it will almost
- directly forward the call to the respective posix <code>open</code>
- call. The returned file descriptor is then stored in
- a member-variable of the <code>ZZIP_FILE</code> structure.
-</P>
-<P>
- Any subsequent calls to <code>zzip_read</code> will then
- be forwarded to the posix <code>read</code> call on the
- memorized file descriptor. The same about <code>zzip_close</code>
- which will call the posix <code>close</code> function and then
- <code>free</code> the <code>ZZIP_FILE</code> structure.
-</P>
-<P>
- The real benefit of the
- <a href="zziplib.html">zziplib library</a>
- comes about when the filename argument does actually refer
- to a file that is zipped in a zip-archive. It happens that
- even both a real file and a zipped file can live under the
- same pathname given to the <code>zzip_open</code> call,
- whereas the real file is used in preference.
-</P>
-
-</section><section>
-<h3>Zipped File</h3>
-
-<P>
- Suppose you have subdirectory called '<tt>test/</tt>'. In
- this directory is just one file, called '<tt>README</tt>'.
- Calling the <code>zzip_open</code> function with an
- argument of '<i>optional-path/</i> <tt>test/README</tt>',
- then it will open that file for subsequent reading with
- <code>zzip_read</code>. In this case the real (<i>stat'able</i>)
- file is opened.
-</P>
-<P>
- Now you can go to the '<tt>test/</tt>' directory and zip up
- the files in there by calling
- <nobr><tt>`zip ../test.zip *`</tt></nobr>.
- After this, you can delete the '<tt>test/</tt>' directory and
- the call to <code>zzip_open</code> will still succeed.
- The reason is that the part of the path saying
- '<tt>test/README</tt>' will be replaced by sth. like
- '<tt>test.zip:README</tt>' - that is the real file '<tt>test.zip</tt>'
- is opened and searched for a contained file '<tt>README</tt>'.
-</P>
-<P>
- Calling <code>zzip_read</code> on the zipped '<tt>README</tt>' file
- will return the very same data as if it is a real file in a
- real directory. If the zipped file is compressed it will be
- decompressed on the fly.
-</P>
-
-</section><section>
-<h3>Zip Directory</h3>
-
-<P>
- The same applies to the use of <code>opendir/readdir/closedir</code>
- which can safely be replaced with their counterparts from the
- <a href="zziplib.html">zziplib library</a> - again their prototype
- follows the scheme of the original calls, just prepend <tt>zzip_</tt>
- to the function calls and <tt>ZZIP_</tt> to the struct-typedefs.
-</P>
-<P>
- To call <code>zzip_opendir</code> on a real directory will then
- return a <code>ZZIP_DIR</code> whose member-variable
- <code>realdir</code> points to the actual <code>DIR</code>-structure
- returned by the underlying posix <code>opendir</code>-call.
-</P>
-<P>
- If a real directory '<tt>test</tt>' does not exist, then the
- <code>zzip_opendir</code> will try to open a file '<tt>test.zip</tt>'
- with a call to <code>zzip_dir_open</code>.
- Subsequent calls to <code>zzip_readdir</code> will then return
- information as being obtained from the central archive directory
- of the zip-file.
-</P>
-
-</section><section>
-<h3>Differences</h3>
-
-<P>
- There are no differences between the posix calls and their counterparts
- from the <a href="zziplib.html">zziplib library</a> - well, just
- as long as the zip-file contains just the plain files from a directory.
-</P>
-<P>
- If the zip-file contains directory entries you may be prompted with
- some awkward behaviour, since in zip-file a directory happens to be
- just an empty file. Note that the posix function <code>open</code>
- may also open a directory for reading - it will only return
- <code>EISDIR</code> if the <code>open</code> mode-argument included
- write-access.
-</P>
-<P>
- What the current of version of the
- <a href="zziplib.html">zziplib library</a>
- can definitly not do: calling zzip_opendir on a directory zippend
- <em>inside</em> a zip-file.
-</P>
-<P>
- To prevent the enrollment of directories into the zip-archive, you
- can use the <tt>-D</tt> option of the <tt>zip</tt> program. That
- is in any <tt>Makefile</tt> you may want to use
- <nobr><tt>`cd $(dir) &amp;&amp; zip -D ../$(dir).zip *`</tt></nobr>.
-</P>
-
-</section><section>
-<h3>Advantages</h3>
-
-<P>
- Distribution of a set of files is much easier if it just means
- to wrap up a group of files into a zip-archive - and copy that
- zip-archive to the respective destination directory.
- Even more the files can be compressed and unlike a <tt>tar.gz</tt>
- archive there is no need to decompress the archive in temporary
- location before accessing a member-file.
-</P>
-<P>
- On the other hand, there is no chance to scatter files around
- on the disk like it could easily happen with a set of gzip'ed
- man-pages in a single `<tt>man</tt>`-directory. The reader
- application does not specifically need to know that the file
- is compressed, so that reading a script like
- `<tt>share/guile/x.x.x/ice-9/popen.scm</tt>` is done by simple
- calls to <code>zzip_read</code> which works on zip-file named
- `<tt>share/guile/x.x.x/ice-9.zip</tt>`.
-</P>
-<P>
- A version mismatch between different files in a group is now
- obvious: either the opened file belongs to the distribution
- archive, or otherwise in resides in a real directory <em>just
- next to the zip-archive that contains the original</em>.
-</P>
-
-</section><section>
-<h3>Issues</h3>
-
-<P>
- The <a href="zziplib.html">zziplib library</a> does not
- use any code piece from the <code>zip</code> programs, neither
- <em>pkzip</em> nor <em>infozip</em>, so there is no license
- issue here. The decompression is done by using the free
- <a href="http://www.gzip.org/zlib">zlib library</a> which has no special
- issues with respect to licensing.
- The rights to the <a href="zziplib.html">zziplib library</a>
- are reserved to the copyright holders, there is a public
- license that puts most the sources themselves under
- <a href="COPYING.LIB">the GNU Lesser General Public License</a>,
- so that the use of a shared library instance of the
- <a href="zziplib.html">zziplib library</a>
- has no restrictions of interest to application programmers.
- For more details and hints about static linking, check
- the <a href="copying.html">COPYING</a> information.
-</P>
-<P>
- The only issue you have with the
- <a href="zziplib.html">zziplib library</a>
- is the fact that you can only <em>read</em> the contained files.
- Writing/Compression is not implemented. Even more, a compressed
- file is not seekable at the moment although I hope that someone
- will stand up to implement that functionality someday.
-</P>
-
-</section></section>