On Fri, Sep 01, 2023 at 07:19:13AM +0200, Phi Debian wrote: > Well after reading yet another thread regarding libc_printf() I got to > admit that even %B is crossed out, (Yet already choosen by ksh93) > > The other thread also speak about libc_printf() documentting %# as > undefined for things other than a, A, e, E, f, F, g, and G, yet the same > thread also talk about a A comming late (citing C99) in the dance, meaning > what is undefined today become defined tomorow, so %#b is no safer. >
Caution: The proposal here is for %#s (an alternative string), not %#b (which C2x wants to be similar to %#x, in that it outputs a '0b' prefix for all values except bare '0'). Yes, there is a slight risk that C may decide to define %#s. But as the Austin Group includes a member of WG14, we are able to advise the C committee that such an addition is not wise. > My guess is that printf(1) is now doomed to follow its route, keep its old > format exception, and then may be implement something like c_printf like > printf but the format string follow libc semantic, or may be a -C option to > printf(1)... Adding an option to printf is also a possibility, if there is wide-spread implementation practice to standardize. If someone wants to implement 'printf -C' right now, that could help feed such a future standardization. But it is somewhat orthogonal to the request in this thread, which is how to allow users to still access the old %b behavior even if %b gets repurposed in the future; if we can get multiple implementations to add a %#s alias now, it makes the future decisions easier (even if it is too late for Issue 8 to add any new features, or for that matter, to make any normative changes other than marking %b obsolescent as a way to be able to revisit it in the future for Issue 9). > > Well in all case %b can not change semantic in the bash script, since it is > there for so long, even if it depart from python, perl, libc, it is > unfortunate but that's the way it is, nobody want a semantic change, and on > next routers update, see the all internet falling appart :-) How many scripts in the wild actually use %b, though? And if there are such scripts, anything we can do to make it easy to do a drop-in replacement that still preserves the old behavior (such as changing %b to %#s) is going to be easier to audit than the only other currently-portable alternative of actually analyzing the string to see if it uses any octal or \c escapes that have to be re-written to portably function as a printf format argument. POSIX is not mandating %#s at this time, so much as suggesting that if implementations are willing to implement it now, it will make Issue 9 easier to reason about. -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org
