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