Add the remember(V,PTR) extension function which copies an SQL value into

an application variable.

FossilOrigin-Name: d2d30914d81022d7d4e1670caf9326524520deaf

ext/misc/remember.c

@@ -0,0 +1,68 @@
+/*
+** 2016-08-09
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+*************************************************************************
+**
+** This file demonstrates how to create an SQL function that is a pass-through
+** for integer values (it returns a copy of its argument) but also saves the
+** value that is passed through into a C-language variable.  The address of
+** the C-language variable is supplied as the second argument.
+**
+** This allows, for example, a counter to incremented and the original
+** value retrieved, atomically, using a single statement:
+**
+**    UPDATE counterTab SET cnt=remember(cnt,$PTR)+1 WHERE id=$ID
+**
+** Prepare the above statement once.  Then to use it, bind the address
+** of the output variable to $PTR and the id of the counter to $ID and
+** run the prepared statement.
+**
+** One can imagine doing similar things with floating-point values and
+** strings, but this demonstration extension will stick to using just
+** integers.
+*/
+#include "sqlite3ext.h"
+SQLITE_EXTENSION_INIT1
+#include <assert.h>
+
+/*
+**      remember(V,PTR)
+**
+** Return the integer value V.  Also save the value of V in a
+** C-language variable whose address is PTR.
+*/
+static void rememberFunc(
+  sqlite3_context *pCtx,
+  int argc,
+  sqlite3_value **argv
+){
+  sqlite3_int64 v;
+  sqlite3_int64 ptr;
+  assert( argc==2 );
+  v = sqlite3_value_int64(argv[0]);
+  ptr = sqlite3_value_int64(argv[1]);
+  *((sqlite3_int64*)ptr) = v;
+  sqlite3_result_int64(pCtx, v);
+}
+
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+int sqlite3_remember_init(
+  sqlite3 *db, 
+  char **pzErrMsg, 
+  const sqlite3_api_routines *pApi
+){
+  int rc = SQLITE_OK;
+  SQLITE_EXTENSION_INIT2(pApi);
+  rc = sqlite3_create_function(db, "remember", 2, SQLITE_UTF8, 0,
+                               rememberFunc, 0, 0);
+  return rc;
+}