How to compile a user defined function in SQLite - c

Question

I have an example C function that I would like to attach to a database temp.sqlite (it's from an O'Reilly book, so I know it works). I read the section in the book and sqlite.org, but they assume away that I know how and where to compile this thing with the proper settings. I'm on either Mac (with XCode) or Ubuntu.
I know enough C to alter the code to do what I want, but I have no idea what to do call it from my database temp.sqlite.
Thanks for your help! I've been churning on this!
Update: A few more hours in and I've scrapped together enough stuff from abandoned webpages to create a compile command and generate an error:
richard$ gcc -o wtavg wtavg.c -Wall -W -O2 -L/usr/local/lib -lsqlite3
wtavg.c: In function ‘wtavg_init’:
wtavg.c:63: warning: unused parameter ‘error’
Undefined symbols:
"_main", referenced from:
start in crt1.10.6.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
FWIW, here's wtavg.c, which is straight from the O'Reilly site provided in my book:
/* wtavg.c */
#include "sqlite3ext.h"
SQLITE_EXTENSION_INIT1;
#include <stdlib.h>
typedef struct wt_avg_state_s {
double total_data; /* sum of (data * weight) values */
double total_wt; /* sum of weight values */
} wt_avg_state;
static void wt_avg_step( sqlite3_context *ctx, int num_values, sqlite3_value **values )
{
double row_wt = 1.0;
int type;
wt_avg_state *st = (wt_avg_state*)sqlite3_aggregate_context( ctx,
sizeof( wt_avg_state ) );
if ( st == NULL ) {
sqlite3_result_error_nomem( ctx );
return;
}
/* Extract weight, if we have a weight and it looks like a number */
if ( num_values == 2 ) {
type = sqlite3_value_numeric_type( values[1] );
if ( ( type == SQLITE_FLOAT )||( type == SQLITE_INTEGER ) ) {
row_wt = sqlite3_value_double( values[1] );
}
}
/* Extract data, if we were given something that looks like a number. */
type = sqlite3_value_numeric_type( values[0] );
if ( ( type == SQLITE_FLOAT )||( type == SQLITE_INTEGER ) ) {
st->total_data += row_wt * sqlite3_value_double( values[0] );
st->total_wt += row_wt;
}
}
static void wt_avg_final( sqlite3_context *ctx )
{
double result = 0.0;
wt_avg_state *st = (wt_avg_state*)sqlite3_aggregate_context( ctx,
sizeof( wt_avg_state ) );
if ( st == NULL ) {
sqlite3_result_error_nomem( ctx );
return;
}
if ( st->total_wt != 0.0 ) {
result = st->total_data / st->total_wt;
}
sqlite3_result_double( ctx, result );
}
int wtavg_init( sqlite3 *db, char **error, const sqlite3_api_routines *api )
{
SQLITE_EXTENSION_INIT2(api);
sqlite3_create_function( db, "wtavg", 1, SQLITE_UTF8,
NULL, NULL, wt_avg_step, wt_avg_final );
sqlite3_create_function( db, "wtavg", 2, SQLITE_UTF8,
NULL, NULL, wt_avg_step, wt_avg_final );
return SQLITE_OK;
}

Answer 1

The user defined function should be compiled as shared library file:
gcc -shared -fPIC -o wtavg.so wtavg.c -lsqlite3
That shared library can then be loaded using an SQLite statement:
SELECT load_extension('/path/to/wt_avg.so', 'wtavg_init');
Unfortunately the version of sqlite3 provided by Apple doesn't support loading shared libraries. You can use sqlite from MacPorts instead. MacPorts programs linking against sqlite should also have the ability to load user defined functions this way.
When using SQLite inside another program however the extension loading mechanism may be disabled for security reasons. In Python for instance you have to call con.enable_load_extension(True) to enable it.