is partial gz decompression possible?

Question

For working with images that are stored as .gz files (my image processing software can read .gz files for shorter/smaller disk time/space) I need to check the header of each

Answer 1

A Deflate stream can have multiple blocks back to back. But you can always decompress just the number of bytes you want, even if it's part of a larger block. The zlib function gzread takes a length arg, and there are various other ways to decompress a specific amount of plaintext bytes, regardless of how long the full stream is. See the zlib manual for a list of functions and how to use them.

It's not clear if you want to modify just the headers. (You mention recompressing the whole file, but option B doesn't recompress anything). If so, write headers in a separate Deflate block so you can replace that block without recompressing the rest of the image. Use Z_FULL_FLUSH when you call the zlib deflate function to write the headers. You probably don't need to record the compressed length of the headers anywhere; I think it can be computed when reading them to figure out which bytes to replace.

If you aren't modifying anything, recompressing the whole file makes no sense. You can seek and restart decompression from the start after finding headers you like...