Discarding Data

The Prolog data created during setting up the call and calling Prolog can in most cases be discarded right after the call. See figure [*] for an example.

.BF .F void PL_mark 1 bktrk_buf Mark the global and trail stacks in bktrk_buf. .F void PL_bktrk 1 bktrk_buf Undo all changes in the runtime stacks since a snapshot has been made in buffer using PL_mark(). Changes to the heap are not affected. .EF

It is not necessary to call PL_bktrk() for each PL_mark(). The user should ensure that PL_bktrk() is never called with a buffer that is created after a buffer to which PL_bktrk() has been called. Thus PL_mark(b1) ... PL_mark(b2) ... PL_bktrk(b1) is valid, but it is not allowed to call PL_bktrk(b2) after this sequence.

Figure: Calling Prolog
\begin{figure}\begin{boxed} \begin{code} int count_atoms() { term t; int atoms;... ...l stack data created */ \par return atoms; } \end{code}\end{boxed} \end{figure}