How to check if a file is gzip compressed?

Question

I have a C / C++ program which needs to read in a file that may or may not be gzip compressed. I know we can use gzread() from zlib to read in both compressed and uncompressed files - however, I want to use the zlib functions ONLY if the file is gzip compressed (for performance reasons).

So is there any way to programatically detect or check if a certain file is gzipped from C / C++?

Answer 1

There is a magic number at the beginning of the file. Just read the first two bytes and check if they are equal to 0x1f8b.

Answer 2

Do you prefer false positives, false negatives, or no false results at all (there goes performance down the drain...)?

The RFC 1952: GZIP file format specification version 4.3 states the first 2 bytes (of each member and therefore) of the file are '\x1F' and '\x8B'. Use that for a first check that can result in false positives.