Thanks Larry for quick response. Sorry I cannot state my problem clearly. The problem is when I rerun the sqlite3_exec() , by the second hitting the url to query the database, (such as select * from users; ) , it returns nothing. the first run of Sqlite3_exec() is correct by returning a list of users rows, As I use static variable in callback to remember the position of char * ret. I google searched static variable is thread unsafe, therefore I calculate string length in the callback, it seems the problem was solved. Thanks have a good weekend.