Question

I would like to maintain interoperability between every other application on the planet (including web applications) when compressing text. Since qCompress and qUncompress seem to go against the grain, I’m trying to use zlib directly from my Qt application.

I will accept the simplest (most minimal) answer that shows me how to use the zlib library with a QByteArray directly OR modify the output of qCompress so that it can be used outside of a Qt application.

Here’s my embarrassing attempt:

QByteArray tdata = QString("Oh noes!").toUtf8();
QByteArray cdata;
uLongf len = 12 + 1.002*tdata.length();
compress(&cdata, &len, &tdata, tdata.length());

And the error:

error: cannot convert ‘QByteArray*’ to ‘Bytef*’ for argument ‘1’ to ‘int compress(Bytef*, uLongf*, const Bytef*, uLong)’

Then I tried using QByteArray::constData()

compress(cdata.constData(), &len, &tdata, tdata.length());

But got the following error:

error: invalid conversion from ‘const char*’ to ‘Bytef*’

I have no idea what a Bytef is so I start looking in the zlib sources to investigate. But all I can find for this is in QtSources/src/3rdparty/zlib/zconf.h

# define Bytef                 z_Bytef

So now I’m just lost.

Answer 1

Based on this note in qUncompress, I think it’s pretty easy.

Note: If you want to use this function to uncompress external data that was compressed using zlib, you first need to prepend a four byte header to the byte array containing the data. The header must contain the expected length (in bytes) of the uncompressed data, expressed as an unsigned, big-endian, 32-bit integer.

So you can probably just compress it like this:

QByteArray tdata = QString("Oh noes!").toUtf8();
QByteArray compressedData = qCompress(tdata);
compressedData.remove(0, 4);