Ejemplo 4: Una fila, C

 #include "postgres.h"
 #include "fmgr.h"
 PG_FUNCTION_INFO_V1(digitoVer);

 Datum digitoVer(PG_FUNCTION_ARGS) {
    int rut = PG_GETARG_INT32(0);
    text *ret;
    int M=0, S=1;
    for (; rut; rut = rut / 10) 
        S = (S + rut % 10 * (9 - M++ % 6)) % 11;
    ret = (text *) palloc(VARHDRSZ + 1);
    VARATT_SIZEP(ret) = VARHDRSZ + 1;
    sprintf(VARDATA(ret), "%c", S ? S + '0' - 1 : 'K');
    PG_RETURN_TEXT_P(ret);
 }