Read gzip file line-by-line

[high_flyer]

This code never gets call IMHO, or only once, if _busPos is initialized with something other than zero which is greater than _busSize when it gets initialized:

if (_bufPos >= _bufSize) {
fillBuffer();
if (_bufSize == 0) return 0;
}

if (_bufPos >= _bufSize) {
fillBuffer();
if (_bufSize == 0) return 0;
}

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Since in here, _bufPos is always set to 0:

void CompressedFile::fillBuffer()
{
    _bufOffset += _bufSize;
    _bufSize = gzread(_file, _buffer, BUFSIZE);
    _bufPos = 0; //<--- always 0.
}

void CompressedFile::fillBuffer()
{
    _bufOffset += _bufSize;
    _bufSize = gzread(_file, _buffer, BUFSIZE);
    _bufPos = 0; //<--- always 0.
}

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Is there a reason you are reading char after char and not a whole line using QIDevice::readLine()?
Because this is what takes so long...