Having written code to extract files from a ZIP file, it's because the header is variable sized, anywhere from 22 to 65,557 bytes in size (22 bytes fixed, up to

Having written code to extract files from a ZIP file, it's because the header is variable sized, anywhere from 22 to 65,557 bytes in size (22 bytes fixed, up to 65,535 bytes for a comment). There are two ways to scan for the header, one is to seek just 22 bytes shy of the end of the file and start checking backwards (since the majority of ZIPs I've encountered do not have the comment) or, seek 65,557 bytes from the end and scan forward. This fact has been used to construct pathological ZIP files where one tool will report one list of files and another tool will list a different set of files. That's why you really need to overwrite the old header.