In the last year I’ve spent six months digging into a PostgreSQL project. PostgreSQL is one of the most interesting and fast developed database management systems. However it is still a little bit behind the major commercial competitors, it is without any doubt, the most advanced OpenSource and Free DBMS. On the great features of PostgreSQL is the documentation of its architecture, that is maintained updated constantly by its vivid and active community. Although this community provides one of the best (the fastest that I’ve every experienced as a single user) developer on-line support system, at that time (and still now!) I had serious difficulties to find out information about on of the most used and useful coding extensions: custom types. Some of you may had to work with PostgreSQL extensions like PostGIS (the simplest and most effective spatial extension that integrates spatial information into PostgreSQL) and wanted to extend or to provide similar functionalities so easy to use and comfortable. I was mentioning the user-friendly constructors for spatial objects. Custom Types for defining spatial objects are defined in the SQL language like that

POINT('37','-97');

(for more information, take a look to this link)

This type of syntax is only possible (as far as I know) if the type is defined using a C function, and for instance compiled into a .so shared object library provided to PostgreSQL. C PostgreSQL custom types are defined by two functions that translate the low level description of the type into SQL usable constructors. Here it follows the code  to define constructors for a Currency type, and attached into this file, there is the source code for other operations like equals, greater and exchange rate calculation (the source of this piece of code is PostgreSQL C Reference Documentation).

/*
**  Name: fcur_in()
**
**        Converts an fcur value from external form
**	  to internal form.
*/

PG_FUNCTION_INFO_V1(fcur_in);

Datum fcur_in(PG_FUNCTION_ARGS)
{
    char  * src     = PG_GETARG_CSTRING(0);
    char  * workStr = (char *)palloc( strlen( src ));
    char  * units   = NULL;
    char  * name    = NULL;
    char  * xrate   = NULL;
    fcur  * result  = NULL;
    char  * endPtr  = NULL;

    strcpy( workStr, src );

    units = strtok( workStr, "(" );
    xrate = strtok( NULL, "/)" );
    name  = strtok( NULL, ")" );

    result = (fcur *)palloc( sizeof( fcur ));

    memset( result, 0x00, sizeof( fcur ));

    result->fcur_units = strtod( units, &endPtr );

    if( xrate )
    {
	result->fcur_xrate = strtod( xrate, &endPtr );
    }
    else
    {
	result->fcur_xrate = 1.0;
    }

    if( name )
    {
	strncpy( result->fcur_name,
		 name,
		 sizeof( result->fcur_name ));
    }
    else
    {
	strncpy( result->fcur_name,
		 unknownCurrencyName,
		 sizeof( result->fcur_name ));
    }

    PG_RETURN_POINTER( result );
}

/*
**  Name: fcur_out()
**
**        Converts an fcur value from internal form
**	  to external form.
*/

PG_FUNCTION_INFO_V1(fcur_out);

Datum fcur_out(PG_FUNCTION_ARGS)
{
    fcur  * src  = (fcur *)PG_GETARG_POINTER( 0 );
    char  * result;
    char    work[16+1+sizeof(src->fcur_name)+16];

    sprintf( work, "%g(%g/%s)",
	     src->fcur_units,
	     src->fcur_xrate,
	     src->fcur_name );

    result = (char *)palloc( strlen( work ) + 1 );

    strcpy( result, work );

    PG_RETURN_CSTRING( result );

}