Re: portability question

[Owen Taylor <otaylor redhat com>]

On Tue, 2003-12-16 at 14:47, Sheldon Simms wrote:

> What I meant by assumption of arbitrary pointer/int
> interchangeability was things like this (from
> gtype.c):
> 
>  static inline TypeNode*
>  lookup_type_node_I (register GType utype)
>  {
>    if (utype > G_TYPE_FUNDAMENTAL_MAX)
>      return (TypeNode*) (utype & ~TYPE_ID_MASK);
>    else
>      return static_fundamental_type_nodes[utype >>
> G_TYPE_FUNDAMENTAL_SHIFT];
>  }
> 
> Since GType is actually an unsigned long and there is
> no guarantee in the
> C standard that an arbitrary pointer value can be
> stored in a value of
> unsigned integer type. This could be done portably
> like this:
> 
>  typedef uintptr_t GType;
>  ...
>  static inline TypeNode*
>  lookup_type_node_I (register GType utype)
>  {
>    if (utype > G_TYPE_FUNDAMENTAL_MAX)
>      return (TypeNode*)(void *)(utype &
> ~TYPE_ID_MASK);
>    else
>      return static_fundamental_type_nodes[utype >>
> G_TYPE_FUNDAMENTAL_SHIFT];
>  }
> 
> Because a uintptr_t is guaranteed to be able to hold a
> void * and the double
> cast makes sure that any necessary conversions between
> a void * and a
> TypeNode * are done.

You'll find that uintptr_t is vastly less portable than
what we are doing now. It's a C99'ism...

Part of the GLib porting process is making sure that the
size of the GType integral type is the same as the size of
a pointer. Luckily for the ease of the porting process,
all common platforms have long and pointers with the
same size.

We're playing a bunch of "non-portable" tricks with GType
because the speed and compactness of GType is really important.

Note that we're assuming as well:
 
  - No pointers are found in the first 1024 bytes of memory
  - Pointers to TypeNode structures are aligned on 4-byte
    boundaries.

Regards,
					Owen