simultaneous open queries

Hi,

I am currently trying to get by head around the restrictions relating to
simultaneous queries in a single threaded program that uses the SWI Prolog C
API.

We are told in the docs [0] that "a foreign context can have at most one
active query" and that "we are "not allowed to open multiple queries and
start generating solutions for each".

So it seems that we are limited (at least without threading?) to one
simultaneous query per calling context. Now, a calling context is an
instance of module_t; the type used to refer to a Prolog module.

I am having difficulty understanding what exactly this means in the
sense of a calling context. Does it mean that in order for simultaneous
queries to be open, the "callers" must be defined in different modules?
I can't seem to find any documentation describing the role of a calling
context (other than it is a module).

Asked differently, is it possible to have simultaneous queries open in a
single threaded C application at all?

Perhaps we can use the outcomes of this discussion to improve the
documentation.

[0] http://www.swi-prolog.org/pldoc/man?section=foreign-create-query

-- 
Best Regards
Edd Barrett

http://www.theunixzoo.co.uk