[ccan] [PATCH] ptrint: Module for encoding integers into void * pointers

[Rusty Russell]

David Gibson <david at gibson.dropbear.id.au> writes:
> For callbacks which need a void * context pointer in the general case,
> there are often simpler cases where an integer would suffice.  These are
> often handled by casting the integer into a pointer, rather than having
> to allocate that integer somewhere.
>
> This adds a module with some helpers for this.  It has some advantages over
> direct casts:
>   * It uses pointer arithmetic against NULL instead of casts which should
>     make it more portable, even to weird platforms with odd representations
>     for NULL.  I don't know the C standard well enough to know if it's
>     totally portalbe though.
>   * In particular it means that the truth value of the pointer
>     representation matches that of the encoded integer.
>   * The conversion functions are inlines providing more type safety than
>     raw casts.
>
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

This looks good!

One typo fix.

I wonder if an undeclared 'struct ptrint' would be better than void *
here?  Perhaps a bit more self-documenting for typesafe_cb users,
and AFAICT no change to your default void * use case.

BTW, I think it's OK if you just add modules yourself.  But of course I
should start sending my modules to the list before dropping them in
CCAN, too.

Thanks,
Rusty.
> diff --git a/ccan/ptrint/ptrint.h b/ccan/ptrint/ptrint.h
> new file mode 100644
> index 0000000..fc68a57
> --- /dev/null
> +++ b/ccan/ptrint/ptrint.h
> @@ -0,0 +1,27 @@
> +/* CC0 (Public domain) - see LICENSE file for details */
> +#ifndef CCAN_PTRINT_H
> +#define CCAN_PTRINT_H
> +
> +#include "config.h"
> +
> +#include <stddef.h>
> +
> +#include <ccan/build_assert/build_assert.h>
> +
> +static inline ptrdiff_t ptr2int(const void *p)
> +{
> +	/*
> +	 * ptrdiff_t is the right size by definition, but to avoid
> +	 * suprises we want a warning if the user can't fit at least a

surprises

Cheers,
Rusty.