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() with a
  25 ** pointer type of "carray" and bind the id of the counter to $ID and
  26 ** run the prepared statement.
  27 **
  28 ** This implementation of the remember() function uses a "carray"
  29 ** pointer so that it can share pointers with the carray() extension.
  30 **
  31 ** One can imagine doing similar things with floating-point values and
  32 ** strings, but this demonstration extension will stick to using just
  33 ** integers.
  34 */
  35 #include "sqlite3ext.h"
  36 SQLITE_EXTENSION_INIT1
  37 #include <assert.h>
  38
  39 /*
  40 **      remember(V,PTR)
  41 **
  42 ** Return the integer value V.  Also save the value of V in a
  43 ** C-language variable whose address is PTR.
  44 */
  45 static void rememberFunc(
  46   sqlite3_context *pCtx,
  47   int argc,
  48   sqlite3_value **argv
  49 ){
  50   sqlite3_int64 v;
  51   sqlite3_int64 *ptr;
  52   assert( argc==2 );
  53   v = sqlite3_value_int64(argv[0]);
  54   ptr = sqlite3_value_pointer(argv[1], "carray");
  55   if( ptr ) *ptr = v;
  56   sqlite3_result_int64(pCtx, v);
  57 }
  58
  59 #ifdef _WIN32
  60 __declspec(dllexport)
  61 #endif
  62 int sqlite3_remember_init(
  63   sqlite3 *db,
  64   char **pzErrMsg,
  65   const sqlite3_api_routines *pApi
  66 ){
  67   int rc = SQLITE_OK;
  68   SQLITE_EXTENSION_INIT2(pApi);
  69   rc = sqlite3_create_function(db, "remember", 2, SQLITE_UTF8, 0,
  70                                rememberFunc, 0, 0);
  71   return rc;
  72 }