The SCM Type
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Guile represents all Scheme values with the single C type SCM. For an introduction to this topic, see Dynamic Types.

 -- C Type: SCM
     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 SCM, so you should only use macros and functions to work with SCM values.  Values are converted between C data types and the SCM type with utility functions and macros.

 -- C Type: scm_t_bits
     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.

 -- C Type: scm_t_signed_bits
     This is a signed integral type of the same size as scm_t_bits.

 -- C Macro: scm_t_bits SCM_UNPACK (SCM X)
     Transforms the SCM value X into its representation as an integral type.  Only after applying SCM_UNPACK it is possible to access the bits and contents of the SCM value.

 -- C Macro: SCM SCM_PACK (scm_t_bits X)
     Takes a valid integral representation of a Scheme object and transforms it into its representation as a SCM value.