Question

I am trying to decompress some zlib data using the dataByDecompressingData: method from ObjectiveZlib example code, which I have had some help converting to be used with Objective-c instead of c++. Code below

- (NSData*) dataByDecompressingData:(NSData*)data{
    Byte* bytes = (Byte*)[data bytes];
    NSInteger len = [data length];
    NSMutableData *decompressedData = [[NSMutableData alloc] initWithCapacity:COMPRESSION_BLOCK];
    Byte* decompressedBytes = (Byte*) malloc(COMPRESSION_BLOCK);

    z_stream stream;
    int err;
    stream.zalloc = (alloc_func)0;
    stream.zfree = (free_func)0;
    stream.opaque = (voidpf)0;

    stream.next_in = bytes;
    err = inflateInit(&stream);
    CHECK_ERR(err, @"inflateInit");

    while (true) {
        stream.avail_in = len - stream.total_in;
        stream.next_out = decompressedBytes;
        stream.avail_out = COMPRESSION_BLOCK;
        err = inflate(&stream, Z_NO_FLUSH);
        [decompressedData appendBytes:decompressedBytes length:(stream.total_out-[decompressedData length])];
        if(err == Z_STREAM_END)
            break;
        CHECK_ERR(err, @"inflate");
    }

    err = inflateEnd(&stream);
    CHECK_ERR(err, @"inflateEnd");

    free(decompressedBytes);
    return decompressedData;
}

Once this runs I then get the Z_BUF_ERROR, I have read here which I know is referring to ASIHTTPReqest but I figure might be using the same zlib classes supplied by xcode to decompress, and they said its an error with the size of the Buffer not being able to handle the decompression of the file due to space.

I am not totally sure if this is correct, they offered two solutions to fix, the first would be to split the packet… thats not an option imo, the other thing to do would be to increase the buffer size.. I was wondering A, how could I do this? B, is there a better third option?

any help would be greatly appreciated.

Answer 1

Please refer to http://www.zlib.net/zlib_how.html

Z_BUF_ERROR will be explained further below, but suffice it to say that this is simply an indication that inflate() could not consume more input or produce more output. inflate() can be called again with more output space or more available input, which it will be in this code.

Some of the errors I see:

1/ everytime you set stream.next_in, you should set stream.avail_in, too. Note that you shouldn’t change stream.avail_in when you are not giving next chunk of input. Setting the initial avail_in before calling inflateInit might be a good idea.

2/ Using inflateInit2(&stream, -MAX_WBITS) might be a good idea. Note that the normal version of inflateInit doesn’t check data for compression type (gzip or zlib) and if it chooses gzip, your decompression will fail.

(should work, written without testing)

-(NSData*)decompressData:(NSData*)compressedData {
    z_stream stream;

    stream.zalloc = Z_NULL;
    stream.zfree = Z_NULL;
    stream.opaque = Z_NULL;

    //we set the input, no need to change the values later
    stream.next_in = (Bytef *) [compressedData bytes];
    stream.avail_in = compressedData.length;  

    int result = inflateInit2(&stream, -MAX_WBITS);

    if (result != Z_OK) {
        return nil;
    }

    NSMutableData* uncompressedData = [NSMutableData data];
    //make buffer big enough - 128KB
    int bufferLength = 128 * 1024;
    char *buffer = malloc(bufferLength);

    while (true) {
        stream.next_out = buffer;
        stream.avail_out = bufferLength;

        //calling with Z_FINISH because we already have all the input
        result = inflate(&stream, Z_FINISH);

        if (result != Z_OK && result != Z_STREAM_END) {
             inflateEnd(&stream);
             free(buffer);
             return nil;
        }

        [uncompressedData appendBytes:buffer length:(bufferLength - stream.avail_out)];

        if (result == Z_STREAM_END) {
            break;
        }
    }


    inflateEnd(&stream);
    free(buffer);

    return uncompressedData;
}