Correction after off-list poke: FreeBSD's /usr/bin/printf and the printf
builtin to /bin/sh both show "x61"
On Thu, Apr 15, 2021 at 7:04 PM Philip Guenther <guent...@gmail.com> wrote:
> The general question is what requirements the standard put on the printf
> utility when the format argument contains a \x or other unspecified
> backslash escape, but the example in the subject is a nice concrete
> example: what's required for or about the output of
> printf '\x61'
> ?
>
> 1003.1-2016 describes the handling of the format argument like this:
> -----
> The format operand shall be used as the format string described in
> XBD Chapter 5 (on page 121) with the following exceptions:
> -----
>
> ...followed by a list of 11 exceptions that do not cover \x. So,
> let's look at XBD Chapter 5:
> -----
> The format is a character string that contains three types of objects
> defined below:
> 1. Characters that are not "escape sequences" or "conversion
> specifications", as described below, shall be copied to the output.
>
> 2. Escape Sequences represent non-graphic characters and the
> escape character (<backslash>).
>
> 3. Conversion Specifications specify the output format of each
> argument; see below.
> -----
>
> Okay, so if it's not an escape sequence or conversion specification, it
> _shall_ be copied to the output. To jump forward to conversion
> specifications:
> -----
> Each conversion specification is introduced by the <percent-sign>
> character ('%').
> -----
>
> Okay, so \x61 isn't a conversion specification. Is it an escape
> sequence? Well, there's just a table for those, which lists the following:
> \\ \a \b \f \n \r \t and \v. There's no "other sequences starting with
> <backslash> are unspecified" statement that I can find.
>
> It therefore appears to me that
> printf '\x61'
>
> is required by the standard to output
> \x61
>
> without a following newline. Unfortunately, the systems I've
> tested (CentOS 6 and 7, MacOS, FreeBSD 12, and OpenBSD 6.9) all output an
> ascii 'a' without a following newline.
>
> Did I miss a statement about <backslash> somewhere that renders this
> behavior unspecified?
>
>
> If a wording tweak is deemed to be in order, please note that it should be
> placed or duplicated such that it also applies to the argument interpreted
> by the %b format conversion, because the same "apparently specified but no
> one behaves that way" is true of this:
> printf %b '\x61'
>
>
> Philip Guenther
>
