Question

Gzip format files (created with the gzip program, for example) use the “deflate” compression algorithm, which is the same compression algorithm as what zlib uses. However, when using zlib to inflate a gzip compressed file, the library returns a Z_DATA_ERROR.

How can I use zlib to decompress a gzip file?

Answer 1

To decompress a gzip format file with zlib, call inflateInit2 with the windowBits parameter as 16+MAX_WBITS, like this:

inflateInit2(&stream, 16+MAX_WBITS);

If you don’t do this, zlib will complain about a bad stream format. By default, zlib creates streams with a zlib header, and on inflate does not recognise the different gzip header unless you tell it so. Although this is documented starting in version 1.2.1 of the zlib.h header file, it is not in the zlib manual. From the header file:

windowBits can also be greater than 15 for optional gzip decoding. Add
32 to windowBits to enable zlib and gzip decoding with automatic header
detection, or add 16 to decode only the gzip format (the zlib format will
return a Z_DATA_ERROR). If a gzip stream is being decoded, strm->adler is
a crc32 instead of an adler32.