zzip-basics.htm.md

20. July 2002

ZZIP API Basics

The open/close API description.

Basics

The naming schem of functions in this library follow a simple rule: if you see a function with a zzip_ prefix followed by compact name representing otherwise a C library or posix function then it is a magic wrapper that can automagically handle both real files/directories or zip-contained files. This includes:

zzip_opendir opendir zzip_readdir readdir zzip_closedir closedir zzip_rewinddir rewinddir zzip_telldir telldir zzip_seekdir seekdir

The ZZIP_DIR handle can wrap both a real directory or a zip-file. Note that you can not open a virtual directory within a zip-file, the ZZIP_DIR is either a real DIR-handle of a real directory or the reference of ZIP-file but never a DIR-handle within a ZIP-file - there is no such schema of a SUB-DIR handle implemented in this library. A ZZIP_DIR does actually represent the central directory of a ZIP-file, so that each file entry in this ZZIP-DIR can possibly have a subpath prepended.

This form of magic has historic reasons as originally the magic wrappers of this library were not meant to wrap a complete subtree of a real file tree but only a single directory being wrapped with into a zip-file and placed instead. Later proposals and patches were coming in to support subtree wrapping by not only making a split between the dir-part and file-part but going recursivly up through all "/"-dirseparators of a filepath given to zzip_open and looking for zip-file there.

To open a zip-file unconditionally one should be using their respective methods that would return a ZZIP_DIR handle being the representant memory instance of a ZIP-DIR, the central directory of a zip-file. From that ZZIP-DIR one can open a compressed file entry which will be returned as a ZZIP_FILE pointer.

zzip_dir_open open a zip-file and parse the central directory to a memory shadow zzip_dir_close close a zip-file and free the memory shadow zzip_dir_fdopen acquire the given posix-file and try to parse it as a zip-file. zzip_dir_read return the next info entry of a zip-file's central directory - this would include a possible subpath

To unconditionally access a zipped-file (as the counter-part of a zip-file's directory) you should be using the functions having a zzip_file_ prefix which are the methods working on ZZIP_FILE pointers directly and assuming those are references of a zipped file with a ZZIP_DIR.

zzip_file_open open a file within a zip and prepare a zlib compressor for it - note the ZZIP_DIR argument, multiple ZZIP_FILE's may share the same central directory shadow. zzip_file_close close the handle of zippedfile and free zlib compressor of it zzip_file_read decompress the next part of a compressed file within a zip-file

From here it is only a short step to the magic wrappers for file-access - when being given a filepath to zzip_open then the filepath is checked first for being possibly a real file (we can often do that by a stat call) and if there is a real file under that name then the returned ZZIP_FILE is nothing more than a wrapper around a file-descriptor of the underlying operating system. Any other calls like zzip_read will see the realfd-flag in the ZZIP_FILE and forward the execution to the read() function of the underlying operating system.

However if that fails then the filepath is cut at last directory separator, i.e. a filepath of "this/test/README" is cut into the dir-part "this/test" and a file-part "README". Then the possible zip-extensions are attached (".zip" and ".ZIP") and we check if there is a real file under that name. If a file "this/test.zip" does exist then it is given to zzip_dir_open which will create a ZZIP_DIR instance of it, and when that was successul (so it was in zip-format) then we call zzip_file_open which will see two arguments - the just opened ZZIP_DIR and the file-part. The resulting ZZIP_FILE has its own copy of a ZZIP_DIR, so if you open multiple files from the same zip-file than you will also have multiple in-memory copies of the zip's central directory whereas otherwise multiple ZZIP_FILE's may share a common ZZIP_DIR when being opened with zzip_file_open directly - the zzip_file_open's first argument is the ZZIP_DIR and the second one the file-part to be looked up within that zip-directory.

zzip_open try the file-path as a real-file, and if not there, look for the existence of ZZIP_DIR by applying extensions, and open the file contained within that one. zzip_close if the ZZIP_FILE wraps a real-file, then call read(), otherwise call zzip_file_read() zzip_close if the ZZIP_FILE wraps a real-file, then call close(), otherwise call zzip_file_close()

Up to here we have the original functionality of the zziplib when I (Guido Draheim) created the magic functions around the work from Tomi Ollila who wrote the routines to read and decompress files from a zip archive - unlike other libraries it was quite readable and intelligible source code (after many changes there is not much left of the original zip08x source code but that's another story). Later however some request and proposals and patches were coming in.

Among the first extensions was the recursive zzip_open magic. In the first instance, the library did just do as described above: a file-path of "this/test/README" might be a zip-file known as "this/test.zip" containing a compressed file "README". But if there is neither a real file "this/test/README" and no real zip-file "this/test.zip" then the call would have failed but know the zzip_open call will recursivly check the parent directories - so it can now find a zip-file "this.zip" which contains a file-part "test/README".

This dissolves the original meaning of a ZZIP_DIR and it has lead to some confusion later on - you can not create a DIRENT-like handle for "this/test/" being within a "test.zip" file. And actually, I did never see a reason to implement it so far (open "this.zip" and set an initial subpath of "test" and let zzip_readdir skip all entries that do not start with "test/"). This is left for excercie ;-)