...
     sstr << "UPDATE T_Stock SET Stock=" << amount << " WHERE Article_ID=" << artid << " and Storehouse_ID=" << sthid << ends;
      strvar = sstr.str();

      //convert string to char*
      req_book = new char[strvar.size() + 1];
      strcpy(req_book, strvar.c_str());

      // free the stringstream object
      sstr.str("");
      sstr.clear();

      state=mysql_query(mysqlconn, req_book);       // send SQL-statement to the MYSQL-Server

...

####### Book goods receipt #######

$artid  = ???;  // value for Article_ID
$amount = ???;  // value for Amount
$sthid  = ???;  // value for Storehouse_ID

SET AUTOCOMMIT = 0;
LOCK TABLES T_Stock WRITE, T_Stocklist WRITE, T_Storehouse WRITE;

SELECT @stock:=Stock FROM T_Stock WHERE Storehouse_ID = $sthid and Article_ID = $artid;
SELECT @tstock:=Total_Stock FROM T_Stocklist WHERE Article_ID = $artid;

UPDATE T_Stock SET Stock =@stock+$amount WHERE Storehouse_ID = $sthid and Article_ID = $artid;
UPDATE T_Stocklist SET Total_Stock =@tstock+$amount WHERE Article_ID=$artid;

UNLOCK TABLES;
COMMIT;

BEGIN;
...
COMMIT;

NOTE: LOCK TABLES is not transaction-safe and will implicitly commit any active transactions before attempting to lock the tables.