SQLite

** of where this might be useful is in a regular-expression matching
** function. The compiled version of the regular expression can be stored as
** metadata associated with the pattern string.  
** Then as long as the pattern string remains the same,
** the compiled regular expression can be reused on multiple
** invocations of the same function.
**
** ^The sqlite3_get_auxdata(C,N) interface returns a pointer to the metadata
** associated by the sqlite3_set_auxdata(C,N,P,X) function with the Nth argument
** value to the application-defined function.  ^N is zero for the left-most
** function argument.  ^If there is no metadata
** associated with the function argument, the sqlite3_get_auxdata(C,N) interface
** returns a NULL pointer.
**
** ^The sqlite3_set_auxdata(C,N,P,X) interface saves P as metadata for the N-th
** argument of the application-defined function.  ^Subsequent
** calls to sqlite3_get_auxdata(C,N) return P from the most recent
** sqlite3_set_auxdata(C,N,P,X) call if the metadata is still valid or
** NULL if the metadata has been discarded.

** should be called near the end of the function implementation and the
** function implementation should not make any use of P after
** sqlite3_set_auxdata() has been called.
**
** ^(In practice, metadata is preserved between function calls for
** function parameters that are compile-time constants, including literal
** values and [parameters] and expressions composed from the same.)^
**
** The value of the N parameter to these interfaces should be non-negative.
** Future enhancements may make use of negative N values to define new
** kinds of function caching behavior.
**
** These routines must be called from the same thread in which
** the SQL function is running.
*/
void *sqlite3_get_auxdata(sqlite3_context*, int N);
void sqlite3_set_auxdata(sqlite3_context*, int N, void*, void (*)(void*));

    return (void*)p->pMem->z;
  }
}

/*
** Return the auxiliary data pointer, if any, for the iArg'th argument to
** the user-function defined by pCtx.
**
** The left-most argument is 0.
**
** Undocumented behavior:  If iArg is negative then access a cache of
** auxiliary data pointers that is available to all functions within a
** single prepared statement.  The iArg values must match.
*/
void *sqlite3_get_auxdata(sqlite3_context *pCtx, int iArg){
  AuxData *pAuxData;

  assert( sqlite3_mutex_held(pCtx->pOut->db->mutex) );
#if SQLITE_ENABLE_STAT3_OR_STAT4
  if( pCtx->pVdbe==0 ) return 0;
#else
  assert( pCtx->pVdbe!=0 );
#endif
  for(pAuxData=pCtx->pVdbe->pAuxData; pAuxData; pAuxData=pAuxData->pNextAux){
    if(  pAuxData->iAuxArg==iArg && (pAuxData->iAuxOp==pCtx->iOp || iArg<0) ){
      return pAuxData->pAux;
    }
  }
  return 0;
}

/*
** Set the auxiliary data pointer and delete function, for the iArg'th
** argument to the user-function defined by pCtx. Any previous value is
** deleted by calling the delete function specified when it was set.
**
** The left-most argument is 0.
**
** Undocumented behavior:  If iArg is negative then make the data available
** to all functions within the current prepared statement using iArg as an
** access code.
*/
void sqlite3_set_auxdata(
  sqlite3_context *pCtx, 
  int iArg, 
  void *pAux, 
  void (*xDelete)(void*)
){
  AuxData *pAuxData;
  Vdbe *pVdbe = pCtx->pVdbe;

  assert( sqlite3_mutex_held(pCtx->pOut->db->mutex) );

#ifdef SQLITE_ENABLE_STAT3_OR_STAT4
  if( pVdbe==0 ) goto failed;
#else
  assert( pVdbe!=0 );
#endif

  for(pAuxData=pVdbe->pAuxData; pAuxData; pAuxData=pAuxData->pNextAux){
    if( pAuxData->iAuxArg==iArg && (pAuxData->iAuxOp==pCtx->iOp || iArg<0) ){
      break;
    }
  }
  if( pAuxData==0 ){
    pAuxData = sqlite3DbMallocZero(pVdbe->db, sizeof(AuxData));
    if( !pAuxData ) goto failed;
    pAuxData->iAuxOp = pCtx->iOp;
    pAuxData->iAuxArg = iArg;
    pAuxData->pNextAux = pVdbe->pAuxData;

**    * the corresponding bit in argument mask is clear (where the first
**      function parameter corresponds to bit 0 etc.).
*/
void sqlite3VdbeDeleteAuxData(sqlite3 *db, AuxData **pp, int iOp, int mask){
  while( *pp ){
    AuxData *pAux = *pp;
    if( (iOp<0)
     || (pAux->iAuxOp==iOp
          && pAux->iAuxArg>=0
          && (pAux->iAuxArg>31 || !(mask & MASKBIT32(pAux->iAuxArg))))
    ){
      testcase( pAux->iAuxArg==31 );
      if( pAux->xDeleteAux ){
        pAux->xDeleteAux(pAux->pAux);
      }
      *pp = pAux->pNextAux;