ZLib Inflate() failing with -3 Z_DATA_ERROR

Question

I am trying to unzip a file by calling the inflate function but it always fails with Z_DATA_ERROR even when I use the example program from the website. I am thinking that maybe the zip file I have is not supported. I have attached a picture of the zip header below.

And here is the function that I wrote to perform the unzipping. I read in the whole file at once (about 34KB) and pass it into this function. Note I have tried passing the whole zip file with the zip header as well as skipping over the zip file header and only passing the zipped data both fail with Z_DATA_ERROR when inflate() is called.

int CHttpDownloader::unzip(unsigned char * pDest, unsigned long * ulDestLen, unsigned char *  pSource, int iSourceLen)
{
int ret = 0;
unsigned int uiUncompressedBytes = 0; // Number of uncompressed bytes returned from inflate() function
unsigned char * pPositionDestBuffer = pDest; // Current position in dest buffer
unsigned char * pLastSource = &pSource[iSourceLen]; // Last position in source buffer
z_stream strm;

// Skip over local file header
SLocalFileHeader * header = (SLocalFileHeader *) pSource;
pSource += sizeof(SLocalFileHeader) + header->sFileNameLen + header->sExtraFieldLen;


// We should now be at the beginning of the stream data
/* allocate inflate state */
strm.zalloc = Z_NULL;
strm.zfree = Z_NULL;
strm.opaque = Z_NULL;
strm.avail_in = 0;
strm.next_in = Z_NULL;
ret = inflateInit2(&strm, 16+MAX_WBITS);
if (ret != Z_OK)
{
return -1;
}

// Uncompress the data
strm.avail_in = header->iCompressedSize; //iSourceLen;
strm.next_in = pSource;

do {
strm.avail_out = *ulDestLen;
strm.next_out = pPositionDestBuffer;
ret = inflate(&strm, Z_NO_FLUSH);
assert(ret != Z_STREAM_ERROR);  /* state not clobbered */
switch (ret) {
case Z_NEED_DICT:
ret = Z_DATA_ERROR;     /* and fall through */
case Z_DATA_ERROR:
case Z_MEM_ERROR:
(void)inflateEnd(&strm);
return -2;
}
uiUncompressedBytes = *ulDestLen - strm.avail_out;
*ulDestLen -= uiUncompressedBytes; // ulDestSize holds number of free/empty bytes in buffer
pPositionDestBuffer += uiUncompressedBytes;
} while (strm.avail_out == 0);

// Close the decompression stream
inflateEnd(&strm);
ASSERT(ret == Z_STREAM_END);

return 0;
}

So my question is, is the type of zip file I am reading in not supported by ZLib's inflate() function? Or is there something wrong with my CHttpDownloader::unzip() function? Thanks for any help :)

Answer 1

Inflate() was failing because it was looking for GZip headers which were not present. If you initialize the stream with:

ret = inflateInit2(&strm, -MAX_WBITS);

Passing a negative window bits value prevents inflate from checking for gzip or zlib headers and unzipping works as expected.

Answer 2

That file that begins with 50 4B 03 04 is a zip file. The zlib library does not process zip files directly. zlib can help with the compression, decompression, and crc calculations. However you need other code to process the zip file format.

You can look at contrib/minizip in the zlib distribution, or libzip.