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6.20.5.2 Foreign Variables

Pointers to variables in the current address space may be looked up dynamically using dynamic-pointer.

— Scheme Procedure: dynamic-pointer name dobj
— C Function: scm_dynamic_pointer (name, dobj)

Return a “wrapped pointer” for the symbol name in the shared object referred to by dobj. The returned pointer points to a C object.

Regardless whether your C compiler prepends an underscore ‘_’ to the global names in a program, you should not include this underscore in name since it will be added automatically when necessary.

For example, currently Guile has a variable, scm_numptob, as part of its API. It is declared as a C long. So, to create a handle pointing to that foreign value, we do: 

     (use-modules (system foreign))
     (define numptob (dynamic-pointer "scm_numptob" (dynamic-link)))
     numptob
     ⇒ #<pointer 0x7fb35b1b4688>

(The next section discusses ways to dereference pointers.)

A value returned by dynamic-pointer is a Scheme wrapper for a C pointer.

— Scheme Procedure: pointer-address pointer
— C Function: scm_pointer_address pointer

Return the numerical value of pointer. 

          (pointer-address numptob)
          ⇒ 139984413364296 ; YMMV
— Scheme Procedure: make-pointer address [finalizer]

Return a foreign pointer object pointing to address. If finalizer is passed, it should be a pointer to a one-argument C function that will be called when the pointer object becomes unreachable.

— Scheme Procedure: pointer? obj

Return #t if obj is a pointer object, #f otherwise.

— Scheme Variable: %null-pointer

A foreign pointer whose value is 0.

— Scheme Procedure: null-pointer? pointer

Return #t if pointer is the null pointer, #f otherwise.

For the purpose of passing SCM values directly to foreign functions, and allowing them to return SCM values, Guile also supports some unsafe casting operators.

— Scheme Procedure: scm->pointer scm

Return a foreign pointer object with the object-address of scm.

— Scheme Procedure: pointer->scm pointer

Unsafely cast pointer to a Scheme object. Cross your fingers!