Printf function error stm32f103ze keil debugger not working

Hello. I am trying to interface MCBSTM32 with GP22. The main function involves printf function. The code is compiling without error. But the debugger is not working in presence of printf function. I have tested with simple integer/ float print. Debugger is not working. When I comment the printf functions the debugger is working fine. I have tried with increasing the heap size to 400 in startup_stm32f10x_hd.s but still it is not working. the stack size is 400. I am using keil version 510. Help solicited.