Gzip a String

Brandon9000 · 09-13-2012, 01:39 PM

I would like to use gzip from C++ (or C) to gzip a string. If possible, I would like to use zlib.

When I learned that I would have to use zlib to compress and uncompress, I Googled it for a few minutes and then quickly wrote a program to gzip a file and then ungzip it. However, I don't actually have any need to do that. I need to use gzip to compress and uncompress a string, not a file. I couldn't find much good documentation for using gzip on strings. Every example I find works with files.

Could someone show me a simple example?

Thanks in advance.

Brandon

Brandon9000 · 09-13-2012, 01:46 PM

I should add that when I say a "string," I mean a char[], although if I see an example that applies to C++ strings, one can easily convert back and forth.

ntubski · 09-13-2012, 09:55 PM

Quote:

Originally Posted by Brandon9000

When I learned that I would have to use zlib to compress and uncompress, I Googled it for a few minutes and then quickly wrote a program to gzip a file and then ungzip it. However, I don't actually have any need to do that. I need to use gzip to compress and uncompress a string, not a file.

Well you could take your program and remove the file IO parts

You can use zlib's utility functions compress() and uncompress().

Brandon9000 · 09-14-2012, 07:31 AM

Thanks. I was thinking that I might have to figure out deflate() and inflate(). Could you give an example? Can it be made to produce a gzip header?

pan64 · 09-14-2012, 07:53 AM

probably this helps: http://stackoverflow.com/questions/4...ffer-with-zlib

Brandon9000 · 09-14-2012, 07:54 AM

Thanks, Pan. I guess my question would be whether it's possible to create it with gzip headers instead of zlib headers.

NevemTeve · 09-14-2012, 08:38 AM

gzip doesn't have headers, the program interface of gzip is zlib

Brandon9000 · 09-14-2012, 08:40 AM

Zlib and gzip both produce headers in the encoded object and they aren't the same.

NevemTeve · 09-14-2012, 09:08 AM

Oh, sorry, you meant file headers...

Brandon9000 · 09-14-2012, 09:09 AM

Or if the encoding is in memory, of the encoded structure in memory. I Googled this thing for several hours yesterday.

NevemTeve · 09-14-2012, 09:22 AM

For a test I have compared the output of zlib.minigzip and gzip; the first three bytes of the file-headers are the same (1f 8b 08), but gzip also stores the original file name/attributes:

Code:

$ file *.gz
test.x.gz: gzip compressed data, from Unix
test.y.gz: gzip compressed data, was "test.y", from Unix, last modified: Sat May 28 08:40:50 2005

Brandon9000 · 09-14-2012, 09:24 AM

So does minigzip do compression on a memory variable and, if so, what functions does it call to do it?

NevemTeve · 09-14-2012, 09:39 AM

no it uses gzopen/gzwrite/etc

BTW: have you read this: http://www.zlib.net/zlib_faq.html#faq20

Brandon9000 · 09-14-2012, 09:50 AM

Since gzopen()/gzwrite() work with files, I am not surprised that minigzip creates the same file headers as gzip. The issue is getting zlib calls to do it.

After posting the first message here yesterday, I did come across that link and read the relevant portion. According to that, I would have to use deflate/deflateInit2. If that is what I have to do, I must say that the proper settings of the parameters to deflateInit2() to produce a gzip type header are not well documented. Something about adding 16 to windowsBits. I would really like to see an explicit example of compressing and uncompressing using zlib programmed to duplicate gzip.

NevemTeve · 09-14-2012, 09:59 AM

Why, it is in gzio.c in zlib source, check deflateInit2 in function gz_open, and writing crc in gzclose.

PS: Or, if you want to handle files in memory, fmemopen(3) might be useful.