ext/misc/remember.c

   1 /*
   2 ** 2016-08-09
   3 **
   4 ** The author disclaims copyright to this source code.  In place of
   5 ** a legal notice, here is a blessing:
   6 **
   7 **    May you do good and not evil.
   8 **    May you find forgiveness for yourself and forgive others.
   9 **    May you share freely, never taking more than you give.
  10 **
  11 *************************************************************************
  12 **
  13 ** This file demonstrates how to create an SQL function that is a pass-through
  14 ** for integer values (it returns a copy of its argument) but also saves the
  15 ** value that is passed through into a C-language variable.  The address of
  16 ** the C-language variable is supplied as the second argument.
  17 **
  18 ** This allows, for example, a counter to incremented and the original
  19 ** value retrieved, atomically, using a single statement:
  20 **
  21 **    UPDATE counterTab SET cnt=remember(cnt,$PTR)+1 WHERE id=$ID
  22 **
  23 ** Prepare the above statement once.  Then to use it, bind the address
  24 ** of the output variable to $PTR (using sqlite3_bind_pointer()) and
  25 ** bind the id of the counter to $ID and run the prepared statement.
  26 **
  27 ** One can imagine doing similar things with floating-point values and
  28 ** strings, but this demonstration extension will stick to using just
  29 ** integers.
  30 */
  31 #include "sqlite3ext.h"
  32 SQLITE_EXTENSION_INIT1
  33 #include <assert.h>
  34
  35 /*
  36 **      remember(V,PTR)
  37 **
  38 ** Return the integer value V.  Also save the value of V in a
  39 ** C-language variable whose address is PTR.
  40 */
  41 static void rememberFunc(
  42   sqlite3_context *pCtx,
  43   int argc,
  44   sqlite3_value **argv
  45 ){
  46   sqlite3_int64 v;
  47   sqlite3_int64 *ptr;
  48   assert( argc==2 );
  49   v = sqlite3_value_int64(argv[0]);
  50   ptr = sqlite3_value_pointer(argv[1]);
  51   if( ptr ) *ptr = v;
  52   sqlite3_result_int64(pCtx, v);
  53 }
  54
  55 #ifdef _WIN32
  56 __declspec(dllexport)
  57 #endif
  58 int sqlite3_remember_init(
  59   sqlite3 *db,
  60   char **pzErrMsg,
  61   const sqlite3_api_routines *pApi
  62 ){
  63   int rc = SQLITE_OK;
  64   SQLITE_EXTENSION_INIT2(pApi);
  65   rc = sqlite3_create_function(db, "remember", 2, SQLITE_UTF8, 0,
  66                                rememberFunc, 0, 0);
  67   return rc;
  68 }