Connection character set be changed silently?

My programm connect to MYSQL server(4.1.12), set it's character set by "set NAMES gbk", the variables when client programm started is character_set_client gbk character_set_connection gbk character_set_databasegbk characte

Client connection character set be changed after a long while?

My programm connect to MYSQL server, adn set it's charactor set by "set NAMES gbk", after a long while, like one night, I found the client charactor set have been changed to latin, it seems related to some "timeout" value. why the characotor set would be changed? My programm use MYSQL C API.