glutGet(GLUT_ELAPSED_TIME)获取值总是0

#include

int main()
{
int oldTimeSinceStart = 0;

while( 1 )
{
int timeSinceStart = glutGet(GLUT_ELAPSED_TIME);
int deltaTime = timeSinceStart - oldTimeSinceStart;
oldTimeSinceStart = timeSinceStart;

 //... stuff to update using deltaTime

}
return 0;
}
比如这样一个简单的小程序，为什么我获取到的timeSinceStart值总是0

http://www.cnblogs.com/live41/p/3393941.html