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+/*
+** 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 using sqlite3_bind_pointer() with a
+** pointer type of "carray" and bind the id of the counter to $ID and
+** run the prepared statement.
+**
+** This implementation of the remember() function uses a "carray"
+** pointer so that it can share pointers with the carray() extension.
+**
+** 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_pointer(argv[1], "carray");
+ if( ptr ) *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;
+}