when is the callback function called

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Hi all,

I am slightly confused about when the callback functions are called from ROS Subscriber. Taking the following code as example:

#include "ros/ros.h"
#include <geometry_msgs/Vector3.h>

void callback_fn(const geometry_msgs::Vector3::ConstPtr& ticks)
{
}

int main(int argc, char **argv)
{
    ros::init(argc,argv,"test");
    ros::NodeHandle n;
    ros::Subscriber re_msg = n.subscribe("re_ticks",1000,callback_fn);
    ros::spin();
}

So my question is whenever ros::spin() is called, does it call all the callback functions or only those for which the value of the message on the corresponding topic has changed? So in the above example, Is callback_fn called every time ros::spin() is executed or only when the message on the topic "re_ticks" has changed?

Thanks in advance.
Naman Kumar