larocque@jupiter.crd.ge.com (David M. LaRocque) (09/18/90)
I'm looking for suggestions on the best technique to incorporate user-written functions into a software package I'm developing. Specifically, I have helped write a large system in C that is used by researchers to reason about events. It is a system that is used to develop other systems. As such, there needs to be a way to allow users to develop their own functions which can in turn be accessed by the system. The system was originally written in Lisp. In the Lisp version users can simply write their own functions and add them to the "Lisp world". The Lisp version can then "eval" the function name with the arguements. I wish it were that simple in C. As background, the user creates english-like rules such as: if (is-value-predicate X classification mammal) then (set-value X respiration-type air-breathing). The user creates a file of rules such as the one above. The file of rules is parsed by the yacc/lex and appropriate data structures created. For the above rule all objects that have a classification of mammals would have their respiration-type set to air-breathing. Simple functions like "is-value-predicate" and "set-value" are provided for the user. However, the user might want to do something more complicated like: if (hairy-user-calculation X air-speed) then (set-value X passed-hairy-test yes). The problem is how do I allow the user to write the C function "hairy-user-calculation" and have the system call it with the appropriate arguements. Since the file of rules is parsed using yacc/lex, what I think I need is a way for yacc/lex to grab a function pointer to the function. Another problem is that one doesn't know how many arguements a given function might use, but perhaps that could be dealt with with the varargs facility. I suppose what might be ideal is to have the user create all his functions in a separate file that is compiled into the main program. Then I think the problem becomes finding out what a function's pointer is when one only knows the name of the function. Thanks, Dave /************************************************** * larocque@crd.ge.com (518) 387-5805 * ...!crdgw1!cetus.crd.ge.com!larocque **************************************************/
dpd@mullian.ee.mu.oz.au (Denis Dowling) (09/18/90)
In article <11954@crdgw1.crd.ge.com> larocque@jupiter.crd.ge.com (David M. LaRocque) writes: >I'm looking for suggestions on the best technique to incorporate >user-written functions into a software package I'm developing. >could be dealt with with the varargs facility. [Description Deleted] >I suppose what might be ideal is to have the user create all his >functions in a separate file that is compiled into the main >program. Then I think the problem becomes finding out what a >function's pointer is when one only knows the name of the function. Have you had a look at the nlist(3) call? The synopsis is given below. NAME nlist - get entries from symbol table SYNOPSIS #include <nlist.h> int nlist(filename, nl) char *filename; struct nlist *nl; This will return the symbol table entry for the specified symbol. The nlist structure has a value field that can be cast to a function pointer. I have used this for a system similar to what you describe. I do not know of any clean way to deal with function arguments though. What is needed is a reverse form of varargs that allows the construction of an argument list for passing to a function. Anyone have any ideas? It might pay you to have a look at the recent postings in alt.sources on dynamic loading of proceedures in unix. This can allow functions to be loaded on the fly as needed and not have to be explicitly linked with the executable program. > >Thanks, Dave > >/************************************************** > * larocque@crd.ge.com (518) 387-5805 > * ...!crdgw1!cetus.crd.ge.com!larocque > **************************************************/ ______________________________________________________________________________ Denis Dowling ACSnet : dpd@mullian.mu.oz SITEE - (Elec Eng) uunet : uunet!munnari!mullian!dpd University of Melbourne internet : dpd@mullian.mu.oz.au