compiler error in simple module

Hi I am trying to write a simple module that sort of emulates the ps command. I am trying to get the comm string for the task, using get_task_comm, but the make command keeps telling me that the symbol get_task_comm is not defined. I have included linux/sched.h. This is the header file, which I believe has the prototype definition for the function. Am I missing something? Thanks Hanumant