How to print pointer to function?

Segher Boessenkool
Tue Dec 17 22:49:00 GMT 2019

On Tue, Dec 17, 2019 at 05:30:01PM +0100, Vincent Lefevre wrote:
> On 2019-12-17 16:27:28 +0100, Manfred wrote:
> > It says implementation-defined, not undefined,
> IMHO, the difference matters mainly for the implementer: in short,
> he must not ignore the issue. But an implementation-defined behavior
> (or result) can be almost as useless as undefined behavior.

Thankfully GCC documents it as working as you expect.  All sane
implementations on sane platforms do, as far as I know (work, that is; I
have no idea how much they document).

Almost all useful programs are not strictly conforming anyway, so that't
alright.  It's not a bad idea to document what implementation-defined
behaviours your programs depend on, of course :-)

> I would have actually preferred undefined behavior with a diagnostic.
> I'm wondering why a programmer would want to convert a pointer to an
> integer (except as a workaround to bizarre limitations, which would
> better be solved directly by the standard).

There are many reasons.  Of course there are also many ways to abuse
this :-)

A very simple example is tagged pointers: put some value in the normally
unused high or low bits of a pointer, indicating whatever you want it to
indicate.  In C, you cannot do this in a pointer, but you *can* do it in
an uintptr_t (and after masking the tag out again, you can convert it
back to a pointer).


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