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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Thu, 13 Jun 2024 14:47:44 -0700
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Malcolm McLean <malcolm.arthur.mclean@gmail.com> writes:
> On 13/06/2024 01:33, Keith Thompson wrote:
>> printf is a variadic function, so the types of the arguments after
>> the format string are not specified in its declaration.  The printf
>> function has to *assume* that arguments have the types specified
>> by the format string.  This:
>>      printf("%d\n", foo);
>> (probably) has undefined behavior if foo is of type size_t.
>>
> And isn't that a nightmare?

Not at all.  Compilers commonly diagnose mismatches when the format
string is a string literal, as it most commonly is.  The format
specifier for size_t is "%zu", since C99.

>> There is no implicit conversion to the expected type.  Note that
>> the format string doesn't have to be a string literal, so it's
>> not always even possible for the compiler to check the types.
>> Variadic functions give you a lot of flexibility at the cost of
>> making some type errors difficult to detect.
>> (I wrote "probably" because size_t *might* be a typedef for unsigned
>> int, and there are special rules about arguments of corresponding
>> signed and unsigned types.)
>
> We just can't have size_t variables swilling around in prgrams for
> these reasons.

We can and do.

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