Thread: passing arrays to shared object functions

I've been attempting for a while to figure out how to pass arrays of integers back and forth from a custom c-library that I'd like to plug into my server instance. Although I'm able to usefully pass single integers to a functions I've written, the best I've been able to accomplish passing in pointers and const pointers is to blow up the server. I've only attempted compiling per the instructions <a href=" http://www.postgresql.org/docs/current/static/xfunc-c.html">here</a>. I wanted to check with this list first before spending any time delving into libpq for fear that it was the wrong rabbit-hole to go looking into in the first place.

The basic idea (in pseudo-code) is to implement the following rather simple algorithm:

int*
and_function(const int* arr1, const int* arr2){
         int sz = sizeof( arr1 ); // or obtain from another param...
         // memcpy data over for arr1, arr2...
         int[sz] r_arr = (int*) palloc( VARHDRZ + sz );
         int i;
         for( i=0; i<sz; i++ ){
             r_arr[i] = arr1 & arr2;
         }
         return r_arr;
}

It seems that when I attempt to call the function with either "version 0" or "version 1" syntax, the dynamic binding confuses the server even though my
CREATE OR REPLACE FUNCTION and_function(int[]) RETURNS int[] ... is syntactically correct and the library is located appropriately.

--bash-3.00$ /opt/postgres/bin/pg_config --version
PostgreSQL 8.2.3

I apologize in advance if my problem arises from c-ignorance.

Any suggestions?

========================================================
Jason Nerothin
Programmer/Analyst IV - Database Administration
UCLA-DOE Institute for Genomics & Proteomics
Howard Hughes Medical Institute
========================================================
611 C.E. Young Drive East   | Tel: (310) 206-3907
105 Boyer Hall, Box 951570  | Fax: (310) 206-3914
Los Angeles, CA 90095. USA | Mail: jason@mbi.ucla.edu
========================================================
http://www.mbi.ucla.edu/~jason
========================================================

Jason Nerothin wrote:
> I've been attempting for a while to figure out how to pass arrays of
> integers back and forth from a custom c-library that I'd like to plug into
> my server instance. Although I'm able to usefully pass single integers to a
> functions I've written, the best I've been able to accomplish passing in
> pointers and const pointers is to blow up the server. I've only attempted
> compiling per the instructions <a href="
> http://www.postgresql.org/docs/current/static/xfunc-c.html">here</a>. I
> wanted to check with this list first before spending any time delving into
> libpq for fear that it was the wrong rabbit-hole to go looking into in the
> first place.

arrays are not passed as pointer-to-the-first-element to your c function
as they are in plain C. There are macros defined in the postgres headers
for dealing with arrays - I suggest that you look at some backend
functions that take parameters, and see how they handle things. (I think
grepping for array_concat might give you a clue of where to look...).

At least for "version 1" functions, even ints and such are not passed
as normal parameters to your function - there are macros for accessing
such parameters too.

greetings, Florian Pflug