3.2.6 Wrapping Another Simple C Type

If you want to wrap a simple C type which is does not fit in any of the categories above, then G-Wrap provides a simple high-level function for that purpose.

method: wrap-simple-type! (wrapset . <gw-wrapset>) . args

Add the type described by args to the list of types to be wrapped by wrapset. The arguments in args must contain the following named parameters:

#:name

a symbol that is the name for this wrapped type as used within the G-Wrap framework;

#:c-type-name

a string that is the C name of this enumerate type;

#:type-check

a list representing C expression that returns true if the given Scheme value’s type is this type; this list may contain strings (representing part of the C expression) and the scm-var symbol which will be expanded to the name of the C variable that holds the Scheme value;

#:ffspec

a symbol representing the libffi type name that describes this type best; it may be, for instance, pointer for pointer types, long for long integers, etc.; in other words, this is meant to provide run-time type information, as described in See Wrapped Types.

#:unwrap

a list representing a C statement that assigns (and converts) the value of a C variable holding a Scheme value to a C variable holding the corresponding C value; as for #:type-check, this list may only contain strings and symbols; the symbol scm-var will be expanded to the name of the C variable that holds the Scheme value, while the symbol c-var will be expanded to the C variables that will hold the corresponding C value;

#:wrap

a list representing a C statement that assigns (and converts) the value of a C variable holding a C value to a C variable holding the corresponding Scheme value; the content and interpretation of the list as the same as for #:unwrap;

#:description

an optional description of this type.

As an example, the simple scm wrapped type which lets the user wrap C functions that take or return raw SCM objects (see Guile’s SCM type in The GNU Guile Reference Manual) is defined as follows:

(wrap-simple-type! wrapset
                   #:name 'scm
                   #:c-type-name "SCM"
                   #:type-check '("1") ;; any Scheme value is correct
                   #:ffspec 'pointer
                   ;; no conversion is needed
                   #:unwrap '(c-var " = " scm-var ";\n")
                   #:wrap '(scm-var " = " c-var ";\n"))

Note that the scm type is part of the standard wrapset (see C Types Provided in the Standard Wrapset).

If wrap-simple-type! is still not good enough for what you want to do, then you may have a look at the details of G-Wrap’s code generation interface, See G-Wrap's Code Generation API.