scm_t_subr warnings

[Julian Graham <joolean@gmail.com>]

Hey Guilers,

Andy and Ludo and I were discussing this on IRC and it was suggested
that we move the question to the mailing list. I'm trying to compile
some code -- using `gcc -pedantic' -- that invokes `scm_c_make_gsubr',
and I'm getting the following warning:

  warning: ISO C forbids passing argument 5 of ‘scm_c_make_gsubr’
between function pointer and ‘void *’ [-pedantic]
  /usr/local/include/guile/2.0/libguile/gsubr.h:63:13: note: expected
‘scm_t_subr’ but argument is of type ‘struct scm_unused_struct *
(*)(struct scm_unused_struct *, struct scm_unused_struct *)’

I was confused, because I was sure that Guile defines scm_t_subr as
`SCM (*) ()', meaning that an `scm_t_subr' is of unspecified arity.
And I was right, but only at libguile compilation time. From __scm.h:

  #ifdef BUILDING_LIBGUILE
  typedef SCM (* scm_t_subr) ();
  #else
  typedef void *scm_t_subr;
  #endif

Thus the warning: ISO C lets you cast any kind of pointer to `void *'
-- except for a function pointer. Ludovic suggested that this bit of
preprocessor magic exists to support C++, in which the `()' style of
function prototyping is equivalent to `(void)'. But that leaves people
like me who want to be, well, pedantic, in a tough spot. Is there
anything we can do about this? One thing I was thinking was that we
could support the C++ case (and others) more explicitly. E.g.:

  #ifdef __cplusplus
  typedef void *scm_t_subr
  #else
  typedef SCM (* scm_t_subr) ();
  #endif

What do you think?


Regards,
Julian