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Editorial Team
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Asked: 2026-06-12T09:10:50+00:00

I’m trying to append a string to a file by encoding it as NSUnicodeStringEncoding

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I’m trying to append a string to a file by encoding it as NSUnicodeStringEncoding first. I’m doing this:

NSData *data = [@"data" dataUsingEncoding: NSUnicodeStringEncoding];
NSFileHandle *output = [NSFileHandle fileHandleForUpdatingAtPath:@"file"];
[output seekToEndOfFile];
[output writeData:data];

If I do this a number of times and then take a look at the file I notice that every string added has FFFE prepended to it. But when I switch from NSUnicodeStringEncoding to NSUTF8StringEncoding this prefix goes away.

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1 Answer

  1. Editorial Team

    That’s called a byte-order marker, and is put there because NSUnicodeStringEncoding doesn’t specify whether the characters are stored in big or little endian order.

    To prevent 0xFFFE or 0xFEFF from appearing at the beginning of a string, use one of NSUTF16BigEndianStringEncoding, NSUTF16LittleEndianStringEncoding, NSUTF32BigEndianStringEncoding, or NSUTF32LittleEndianStringEncoding, depending on your specific needs. (For reference: Intel and ARM processors as used by Apple are little endian.)

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