A question about multithread

[shiyuwei]

I have completed a telephone accounting program.In this program,I adopt multithread to inspect several serial ports.Now I found a problem.if the program run a long time,the three worker threads I wrote in the program stop to work.if I use function GetExitCodeThread to test threads at the moment, STILL_ACTIVE returned.Why? What's wrong? Can you give me some advice? Thank you for your help.
sincerely yours
shiyuwei