SYSBIOS use task module problem

Hi all:

I have a questionsabout using task module by sysbios,I

I create a empty RTSC project first, add a RTSC configruation File ,the source is:

var Task = xdc.useModule('ti.sysbios.knl.Task');
var BIOS = xdc.useModule('ti.sysbios.BIOS');
var Main = xdc.useModule('xdc.runtime.Main');
var task0Params = new Task.Params();
task0Params.instance.name = "task0";
Program.global.task0 = Task.create("&taskfxn", task0Params);

add source file main.c

#include <ti/sysbios/BIOS.h>
#include <ti/sysbios/knl/Task.h>
int main()
{
 BIOS_start();
}
void taskfxn(UArg arg0,UArg arg1)
{
}

In this status,I can build the project correctly,but if I rename the main,c as main,.cpp because I want to use c++ on my project.

the error is

<Linking>

 undefined first referenced                                                
  symbol       in file                                                     
 --------- ----------------                                                
 _taskfxn  D:\workspace1\mytask\Debug\configPkg\package\cfg\mytask_p674.o674

error: unresolved symbols remain
error: errors encountered during linking; "mytask.out" not built

 Then the build wo't be successful.I wana konw wthat's the problem and how to fix it