Re: [PATCH] style(5): No struct typedefs

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2023-07-11 12:17, Taylor R Campbell wrote:
> I propose the attached change to KNF style(5) to advise against
> typedefs for structs or unions, or pointers to them.
>
> Passing around pointers to structs and unions doesn't require the
> definition, only a forward declaration:
>
> struct vnode;
> int frobnitz(struct vnode *);

Agreed. But this is unrelated to typedefs.

> This can dramatically cut down on header file dependencies so that
> touching one header file doesn't cause the entire kernel to rebuild.
> This also makes it clearer whether you are passing around a copy of a
> struct or a pointer to a struct, which is often semantically important
> not to conceal.
>
> Typedefs are not necessary for opaque cookies to have labels, like
> `typedef void *bus_dma_tag_t;'.  In fact, using void * here is bad
> because it prevents the C compiler from checking types; bus_dma_tag_t
> is indistinguishable from audio_dai_tag_t, and from pckbc_tag_t, and
> from acpipmtimer_t, and from sdmmc_chipset_handle_t.

I don't get it. Why the "void *" stuff? That is where I think the real 
badness lies, and I agree we should not have that.

But defining something like

typedef struct bus_dma_tag *bus_dma_tag_t;

would mean we could easily change what bus_dma_tag_t actually is, 
keeping it opaque, while at the same time keeping the type checking.

Basically, getting all the benefits you mention from having it as a 
proper type, but still also keeping the ability to change what it 
actually is without any problems.

So, yes to proper forward declarations. But I don't think typedefs as 
such is a problem.

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
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