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2014-12-30-the-mysterious-byte-order-mark.md

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The mysterious byte order mark

I bumped into a strange problem with reading a text file recently. The file described the layout of a Nonogram for a program my father was working on:

N 5 5
Z 5 0 1 0 2 1 0 1 0 1 0
K 3 0 1 1 0 1 0 1 1 0 1 2 0

And to read the first line, he used something like this:

char ch;
int i1;
int i2;
ifstream ifs("Goobix 5.nono");
ifs >> ch >> i1 >> i2;

However, this didn't work, the ints didn't get read successfully. I then tried

char ch;
ifstream ifs("Goobix 5.nono");
while (ifs.get(ch))
    cout << ch;

to not ignore whitespace, resulting in this:

Garbled output

Behold, three unwanted characters! What are they? Let's cast them to int to find out more:

char ch;
ifstream ifs("Goobix 5.nono");
while (ifs.get(ch))
    cout << ch << " (int: " << int(ch) << ")\n";

and we get

Program output

Negative, eh? Strange. Other people have seen similar things, so apparently these aren't normal ASCII characters. To find out more, I've looked at the text file with a hex editor:

Hex editor

The file starts with 0xef 0xbb 0xbf. That's googleable! and leads to byte order marks (BOM). The BOM indicates endianness and encoding of the text file; in the case of 0xef 0xbb 0xbf, it is a UTF-8 encoded file. To get rid of the BOM, we can just save the file with ANSI encoding:

Encoding menu

Now, the file behaves as expected when being read:

Program output

Encodings in C++ (and elsewhere!) can be daunting. Good readings I have found include these:

Edit (February 2, 2015): Since I've written this, I've read Dive Into Python 3 by Mark Pilgrim, and the chapter about strings has made encodings so much clearer to me, so it is most recommended reading.