How to change deflate stream output format(raw, zlib, gzip) when use zlib?

Question

Zlib can output three format, I try to search the docs and zlib.h, but can't find a clear explanation about the options, anyone have any ideas?

score 3 · Accepted Answer · answered Feb 13 '19 at 14:56

3

From the zlib.h documentation of deflateInit2():

  windowBits can also be -8..-15 for raw deflate.  In this case, -windowBits
determines the window size.  deflate() will then generate raw deflate data
with no zlib header or trailer, and will not compute a check value.

  windowBits can also be greater than 15 for optional gzip encoding.  Add
16 to windowBits to write a simple gzip header and trailer around the
compressed data instead of a zlib wrapper.

answered Feb 13 '19 at 14:56

Mark Adler

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Why use `windowBits ` to alter the wrapper file format. I really found this before, but can't confirm that. – Karl Feb 14 '19 at 04:12
What is it that you can't confirm? It's written right there in the documentation. – Mark Adler Feb 14 '19 at 04:36
here is an example of creation and decompression of raw DEFLATE stream https://github.com/stokito/deflate – Sergey Ponomarev Jun 08 '20 at 20:25

score 1 · Answer 2 · answered Feb 14 '19 at 19:56

Fill in the blanks

int get_file_format(int n) {
  if      (n == 0) return 31;
  else if (n == 1) return 15;
  else if (n == 2) return -15;
  else if (n >= 9  && n <= 15) return n;  /* zlib with window size 2^9 to 2^15 */
  else if (n >= 25 && n <= 31) return n;  /* gzip with window size 2^9 to 2^15 */
  else if (n >= -15 && n <= -9)return n;  /* raw  with window size 2^9 to 2^15 */        
  else return Z_ERRNO;     
}

z_hist_sz = get_file_format(n);

ret = deflateInit2(&strm, COMPRESS_LEVEL, Z_DEFLATED, z_hist_sz ...)

How to change deflate stream output format(raw, zlib, gzip) when use zlib?

2 Answers2