How can I determine if a file is a zip file?

Question

I need to determine if a file in my app's documents directory is a zip file. The file name cannot be used in making this determination. So I will need to be able read the MIME type or find some other property that only applies to zips.

NOTE: A solution that requires putting the entire file into memory is not ideal as files could potentially be pretty large.

Answer 1

According to http://www.pkware.com/documents/casestudies/APPNOTE.TXT, a ZIP file starts with the "local file header signature"

0x50, 0x4b, 0x03, 0x04

so it is sufficient to read the first 4 bytes to check if the file is possibly a ZIP file. A definite decision can only be made if you actually try to extract the file.

There are many methods to read the first 4 bytes of a file. You can use NSFileHandle, NSInputStream, open/read/close, ... . So this should only be taken as one possible example:

NSFileHandle *fh = [NSFileHandle fileHandleForReadingAtPath:@"/path/to/file"];
NSData *data = [fh readDataOfLength:4];
if ([data length] == 4) {
    const char *bytes = [data bytes];
    if (bytes[0] == 'P' && bytes[1] == 'K' && bytes[2] == 3 && bytes[3] == 4) {
        // File starts with ZIP magic ...
    }
}

Swift 4 version:

if let fh = FileHandle(forReadingAtPath: "/path/to/file") {
    let data = fh.readData(ofLength: 4)
    if data.starts(with: [0x50, 0x4b, 0x03, 0x04]) {
        // File starts with ZIP magic ...
    }
    fh.closeFile()
}