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Date: Tue, 10 Oct 2017 16:56:54 -0400
From: Rich Felker <>
Subject: Re: [PATCH] stdio: implement fopencookie(3)

On Tue, Oct 10, 2017 at 08:51:17PM +0200, Jens Gustedt wrote:
> Hello William,
> On Tue, 10 Oct 2017 18:03:56 +0000 William Pitcock
> <> wrote:
> > The fopencookie(3) function allows the programmer to create a custom
> > stdio implementation, using four hook functions which operate on a
> > "cookie" data type.
> I know it is not your fault, but the naming conventions in this new
> interface are realy bad design.
> > +typedef struct {
> > +	ssize_t (*read)(void *cookie, char *buf, size_t size);
> > +	ssize_t (*write)(void *cookie, const char *buf, size_t size);
> > +	int (*seek)(void *cookie, off_t *offset, int whence);
> > +	int (*close)(void *cookie);
> > +} cookie_io_functions_t;
> > +FILE *fopencookie(void *cookie, const char *mode, cookie_io_functions_t io_funcs);
> The members may clash with macro names. E.g an implementation would be
> allowed to overload "close" with a macro. This is not possible if the
> implementation would want to use this interface here at the same time.
> User code could legitimately want to use a macro "seek" for its own
> purpose.
> Could you at least avoid to use user-space names as function
> parameters? Here you should just omit cookie, buf, size, offset,
> whence, mode and io_funcs. I think in musl parameters in prototypes
> usually don't have names. If you think that we should have them (they
> sort of document the interface) you should put them into a reserved
> namespace with leading underscore or so, or at least prefix them with
> cookie_

I agree with most of the principles here (esp. how bad the public
interface of this function is), but there's not a whole lot that can
be done. Your one request is reasonable and in fact mandatory for musl
header policy: we do not use parameter named at all in prototypes. So
it should read just:

FILE *fopencookie(void *, const char *, cookie_io_functions_t);

Also note that while standard functions in POSIX can additionally be
defined as function-like macros, they can't be object-like macros, so
(*read), etc. are safe due to the parentheses.


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