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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Fri, 14 Jun 2024 19:08:13 +0200
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On 14/06/2024 00:58, Keith Thompson wrote:
> bart <bc@freeuk.com> writes:
>> On 13/06/2024 16:39, Scott Lurndal wrote:
>>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
>>>> On 13/06/2024 01:33, Keith Thompson wrote:
>>>>>

> 
> If foo is an int, for example, printf lets you decide how to print
> it (leading zeros or spaces, decimal vs. hex vs. octal (or binary
> in C23), upper vs. lower case for hex).  Perhaps "print foo" in
> your language has similar features.
> 

C23 also adds explicit width length modifiers.  So instead of having to 
guess if uint64_t is "%llu" or "%lu" on a particular platform, or using 
the PRIu64 macro, you can now use "%w64u" for uint64_t (or 
uint_least64_t if the exact width type does not exist).  I think that's 
about as neat as you could get, within the framework of printf.

> Yes, the fact that incorrect printf format strings cause undefined
> behavior, and that that's sometimes difficult to diagnose, is a
> language problem.  I don't recall anyone saying it isn't.  But it's
> really not that hard to deal with it as a programmer.

It is particularly easy if you have a decent compiler and know how to 
enable the right warning flags!

> 
> If you have ideas (other than abandoning C) for a flexible
> type-safe printing function, by all means share them.  What are your
> suggestions?  Adding `print` as a new keyword so you can use `print
> foo` is unlikely to be considered practical; I'd want a much more
> general mechanism that's not limited to stdio files.  Reasonable new
> language features that enable type-safe printf-like functions could
> be interesting.  I'm not aware of any such proposals for C.
> 

It is possible to come a long way with variadic macros and _Generic. 
You can at least end up being able to write something like :

int x = 123;
const char * s = "Hello, world!";
uint64_t u = 0x4242;

Print("X = ", x, " the string is ", s, " and u = 0x",
	as_hex(u, 6), newline);

rather than:

printf("X = %i the string is %s and u = 0x%06lx\n");


Which you think is better is a matter of opinion.


> I wouldn't mind seeing a new kind of typedef that creates a new type
> rather than an alias.  Then uint64_t could be a distinct type.
> That could cause some problems for _Generic, for example.

I too would like such a typedef.  Using it for uint64_t would cause 
problems for /existing/ uses of _Generic, but would make future uses better.