Path: csiph.com!eternal-september.org!feeder.eternal-september.org!nntp.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: is_binary_file()
Date: Mon, 08 Dec 2025 14:43:58 -0800
Organization: None to speak of
Lines: 33
Message-ID: <871pl4fvbl.fsf@example.invalid>
References: <10gvvh8$1vv6e$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain
Injection-Date: Mon, 08 Dec 2025 22:43:59 +0000 (UTC)
Injection-Info: dont-email.me; posting-host="47bb24738c82d1d1e25e7d99eaef8eeb"; logging-data="495109"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1969ybe8gqHN8LDjtT7ERoS"
User-Agent: Gnus/5.13 (Gnus v5.13)
Cancel-Lock: sha1:RE605cf4QmrXnffLeNq+xpONsRc= sha1:32eyVCbpbcbyxOQjurGhfggyIWk=
Xref: csiph.com comp.lang.c:395730

Michael Sanders <porkchop@invalid.foo> writes:
[...]

For yet another set of unreliable hueristics for guessing whether a file
is text or binary, you can take a look at Perl's built-in "-T" and "-B"
operators.

        The "-T" and "-B" tests work as follows. The first block
        or so of the file is examined to see if it is valid
        UTF-8 that includes non-ASCII characters. If so, it's a
        "-T" file. Otherwise, that same portion of the file is
        examined for odd characters such as strange control codes
        or characters with the high bit set. If more than a third
        of the characters are strange, it's a "-B" file; otherwise
        it's a "-T" file. Also, any file containing a zero byte
        in the examined portion is considered a binary file. (If
        executed within the scope of a use locale which includes
        "LC_CTYPE", odd characters are anything that isn't a
        printable nor space in the current locale.) If "-T" or
        "-B" is used on a filehandle, the current IO buffer is
        examined rather than the first block. Both "-T" and "-B"
        return true on an empty file, or a file at EOF when testing
        a filehandle. Because you have to read a file to do the "-T"
        test, on most occasions you want to use a "-f" against the
        file first, as in "next unless -f $file && -T $file".

It's not clear how big a "block" is.  For an empty file, both -T
and -B are true.  I don't know whether there are other cases where
both are true, or where both are false.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */