— Scheme Procedure: make-vtable fields [print]

Create a new vtable.

fields is a string describing the fields in the structures to be created. Each field is represented by two characters, a type letter and a permissions letter, for example "pw". The types are as follows.

• p – a Scheme value. “p” stands for “protected” meaning it's protected against garbage collection.
• u – an arbitrary word of data (an scm_t_bits). At the Scheme level it's read and written as an unsigned integer. “u” stands for “uninterpreted” (it's not treated as a Scheme value), or “unprotected” (it's not marked during GC), or “unsigned long” (its size), or all of these things.
• s – a self-reference. Such a field holds the SCM value of the structure itself (a circular reference). This can be useful in C code where you might have a pointer to the data array, and want to get the Scheme SCM handle for the structure. In Scheme code it has no use.

The second letter for each field is a permission code,

• w – writable, the field can be read and written.
• r – read-only, the field can be read but not written.
• o – opaque, the field can be neither read nor written at the Scheme level. This can be used for fields which should only be used from C code.
• W,R,O – a tail array, with permissions for the array fields as per w,r,o.

A tail array is further fields at the end of a structure. The last field in the layout string might be for instance ‘pW’ to have a tail of writable Scheme-valued fields. The ‘pW’ field itself holds the tail size, and the tail fields come after it.

Here are some examples.

          (make-vtable "pw")      ;; one writable field
          (make-vtable "prpw")    ;; one read-only and one writable
          (make-vtable "pwuwuw")  ;; one scheme and two uninterpreted
          
          (make-vtable "prpW")    ;; one fixed then a tail array

The optional print argument is a function called by display and write (etc) to give a printed representation of a structure created from this vtable. It's called (print struct port) and should look at struct and write to port. The default print merely gives a form like ‘#<struct ADDR:ADDR>’ with a pair of machine addresses.

The following print function for example shows the two fields of its structure.

          (make-vtable "prpw"
                       (lambda (struct port)
                         (display "#<")
                         (display (struct-ref 0))
                         (display " and ")
                         (display (struct-ref 1))
                         (display ">")))