Re: "undefined behavior"?

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 */