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In article <Jun.27.14.28.58.1989.250@caip.rutgers.edu> russo@caip.rutgers.edu (Mark F. Russo) writes:
>I have a question about Quintus Prolog and the latest functionality of
>the external language interface.  I know that you can install a C
>function as a predicate call in a Quintus Prolog program.  My question
>is, can you then make a query back into the prolog database from that
>external C function.

I have used Quintus Prolog version 2.0 (which may not be the latest
version) on Tektronix UTEK machines.  As far as I know, you can not
call back into prolog from a foregn language routine directly.  However,
you can achieve this effect indirectly.

In addition to foreign language interfaces, Quintus Prolog also provides
a set of IPC libraries that allow you to run multiple processes
communicating with each other.  In particular, you can have a C master
process and a Prolog servant process running at the same time, and you
can call Prolog predicates from the C process.

The following scheme would provide you with one way to indirectly
implement the kind of invocation pattern you described (a rather
painful indirection, though):

+------------+-------+        +-------+------------+
|            | IPC-1 |<======>| IPC-1 |   Prolog   |
|     C      +-------|        +-------+   Servant  |
|  Master            |        |           Process  |
|  Process   +-------+        +-------+---------+  |
|            | IPC-2 |<======>| IPC-2 |  C      |  |
+------------+-------+        +-------+ Foreign |  |
                              |        Routines |  |
                              +-----------------+--+

where IPC-1 is the system provided IPC facility, IPC-2 is a different set
of communication routines you have to write yourself.

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