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Re: reading compressed files
| Re: reading compressed files [message #46941] |
Tue, 10 January 2006 09:43 |
Norbert Hahn
Messages: 46
Registered: May 2003
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"Klaus Scipal" <ks@ipf.tuwien.ac.at> wrote:
> Hi
>
> I have to read compressed files with IDL. As I can not read the files using
> the compress keyword (I don't know why, but if I open the file using the
> compress keyword I do not get what I expect)
There are several methods to compress a file. IDL used GZIP to compress/
inflate a file using the freely available ZLIB library. Unfortunately
the zip compression used on computers with MS Windows uses a different
algorithm to compress the data. PKWARE Inc. is the company that implemented
that compression.
> I spawn a winzip command, unzip
> the file first and read it in with the normal IDL commands. In principal
> everything works fine, at least if the compressed file is not corrupted,
So you definately have PKzip compressed data. You may look at the file with
some ascii editor, the first two letters should be PK then. A gzip compressed
file starts with unprintable characters, the hex code (magic number) is
0x1f8b.
> i.e. winzip can not extract the content of the archive because of some
> missing blocks. As I have to read thousands of files I wonder if there is a
> simple way to check if a compressed file is ok.
zip repair is freely available to repair a slightly damaged PKzip compressed
file. zipfix is another utility to repair damaged zip files. To check for
damage zip files must be read and the checksum must be computed for each
block and compared the the checksum stored with the block.
Norbert
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