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.

enter image description here

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 :)

Have you tried this code with another simple zip file you've created? — Jonathon Reinhart, Sep 09 '13 at 14:35
I just tried it with a simple zip file created with WinZip it fails with the same error. — Megan, Sep 09 '13 at 14:45
See Mark Adler's answer. He is definitely right on this topic ;) Zlib is not ZIP. ZIP is archive file format built on top of zlib compression algorithm. — Nickolay Olshevsky, Sep 09 '13 at 20:29

score 21 · Accepted Answer · answered Sep 09 '13 at 18:15

21

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.

answered Sep 09 '13 at 18:15

Megan

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That is how you decompress if you first process the zip header information so that you can find the deflate stream. – Mark Adler Sep 09 '13 at 20:07
Thanks for your help. I didn't use Minizip because the file I was unzipping was in memory not written to disk. Skipping over the local file header and calling inflateinit2 with negative window bits solved the issue. I didn't quite understand the difference between a zip file and the deflate stream before but it makes more sense now. Thanks again :) – Megan Sep 11 '13 at 17:39
2

Wow how crazy is this? The docs really don't give any clue on this at all. Should be part of zlib's FAQ IMO – paulm Dec 17 '17 at 01:29
It is so crazy. – Fattie Feb 05 '19 at 15:23

score 7 · Answer 2 · answered Sep 09 '13 at 20:05

7

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.

answered Sep 09 '13 at 20:05

Mark Adler

101,978
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Looks like mixing together ZIP/zlib is like mixing FTP and SFTP ) – Nickolay Olshevsky Sep 09 '13 at 20:28
1

She is already taking this into consideration and trying to unzip the raw data in the zip archive – David Menard Jan 26 '15 at 16:31

ZLib Inflate() failing with -3 Z_DATA_ERROR

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