6.4.1 Creating and Manipulating SP_term_refs

Normally, C functions only have indirect access to Prolog terms via SP_term_refs. C functions may receive arguments as unconverted Prolog terms, in which case the actual arguments received will have the type SP_term_ref. Also, a C function may return an unconverted Prolog term, in which case it must create an SP_term_ref. Finally, any temporary Prolog terms created by C code must be handled as SP_term_refs.

SP_term_refs are motivated by the fact that SICStus Prolog's memory manager must have a means of reaching all live Prolog terms for memory management purposes, including such terms that are being manipulated by the user's C code. Previous releases of SICStus Prolog provided direct access to Prolog terms and the ability to tell the memory manager that a given memory address points to a Prolog term, but this approach was too low level and highly error-prone. The current design is modeled after and largely compatible with Quintus Prolog release 3.

SP_term_refs are created dynamically. At any given time, an SP_term_ref has a value (a Prolog term, initially []). This value can be examined, accessed, and updated by the support functions described in this section.

It is important to understand the rules governing the scope of SP_term_refs in conjunction with calls from Prolog to C and vice versa:

• When a C function called from Prolog returns, all SP_term_refs passed to the function or dynamically created by the function become invalid.
• When terms are passed to C as a result of calling Prolog, those terms and any SP_term_refs created since the start of the query are only valid until backtracking into the query or an enclosing one.

A new SP_term_ref is created by calling SP_new_term_ref().

An SP_term_ref can be assigned the value of another SP_term_ref by calling SP_put_term().