获取zlib中未压缩数据的大小？

Question

I'm creating something that includes a file upload service of sorts, and I need to store data compressed with zlib's compress() function. I send it across the internet already compressed, but I need to know the uncompressed file size on the remote server. Is there any way I can figure out this information without uncompress()ing the data on the server first, just for efficiency? That's how I'm doing it now, but if there's a shortcut I'd love to take it.

By the way, why is it called uncompress? That sounds pretty terrible to me, I always thought it would be decompress...

Answer 1

The zlib format doesn't have a field for the original input size, so I doubt you will be able to do that without simulating a decompression of the data. The gzip format has a "input size" (ISIZE) field, that you could use, but maybe you want to avoid changing the compression format or having the clients sending the file size.

But even if you use a different format, if you don't trust the clients you would still need to run a more expensive check to make sure the uncompressed data is the size the client says it is. In this case, what you can do is to make the uncompress-to-/dev/null process less expensive, making sure zlib doesn't write the output data anywhere, as you just want to know the uncompressed size.