How to identify the file content as ASCII or binary

Question

How do you identify the file content as being in ASCII or binary using C++?

Answer 1

If the question is genuinely how to detect just ASCII, then litb's answer is spot on. However if san was after knowing how to determine whether the file contains text or not, then the issue becomes way more complex. ASCII is just one - increasingly unpopular - way of representing text. Unicode systems - UTF16, UTF32 and UTF8 have grown in popularity. In theory, they can be easily tested for by checking if the first two bytes are the unicocde byte order mark (BOM) 0xFEFF (or 0xFFFE if the byte order is reversed). However as those two bytes screw up many file formats for Linux systems, they cannot be guaranteed to be there. Further, a binary file might start with 0xFEFF.

Looking for 0x00's (or other control characters) won't help either if the file is unicode. If the file is UFT16 say, and the file contains English text, then every other character will be 0x00.

If you know the language that the text file will be written in, then it would be possible to analyse the bytes and statistically determine if it contains text or not. For example, the most common letter in English is E followed by T. So if the file contains lots more E's and T's than Z's and X's, it's likely text. Of course it would be necessary to test this as ASCII and the various unicodes to make sure.

If the file isn't written in English - or you want to support multiple languages - then the only two options left are to look at the file extension on Windows and to check the first four bytes against a database of "magic file" codes to determine the file's type and thus whether it contains text or not.