@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.


@node The SCM Type
@section The SCM Type

Guile represents all Scheme values with the single C type @code{SCM}.
For an introduction to this topic, @xref{Dynamic Types}.

@deftp {C Type} SCM
@code{SCM} is the user level abstract C type that is used to represent
all of Guile's Scheme objects, no matter what the Scheme object type is.
No C operation except assignment is guaranteed to work with variables of
type @code{SCM}, so you should only use macros and functions to work
with @code{SCM} values.  Values are converted between C data types and
the @code{SCM} type with utility functions and macros.
@end deftp
@cindex SCM data type

@deftp {C Type} scm_t_bits
@code{scm_t_bits} is an unsigned integral data type that is guaranteed
to be large enough to hold all information that is required to
represent any Scheme object.  While this data type is mostly used to
implement Guile's internals, the use of this type is also necessary to
write certain kinds of extensions to Guile.
@end deftp

@deftp {C Type} scm_t_signed_bits
This is a signed integral type of the same size as @code{scm_t_bits}.
@end deftp

@deftypefn {C Macro} scm_t_bits SCM_UNPACK (SCM @var{x})
Transforms the @code{SCM} value @var{x} into its representation as an
integral type.  Only after applying @code{SCM_UNPACK} it is possible to
access the bits and contents of the @code{SCM} value.
@end deftypefn

@deftypefn {C Macro} SCM SCM_PACK (scm_t_bits @var{x})
Takes a valid integral representation of a Scheme object and transforms
it into its representation as a @code{SCM} value.
@end deftypefn