Calling publisher at a specific rate

edit

As suggested by Weasfas in the comment above, you can use a ros::Timer. The following snippet should do the work:

ros::Publisher my_pub = nh.advertise<Message>("my_topic", queue);
ros::Timer timer = nh.createTimer(ros::Duration(1/100), timerCallback);
Message modified_message;

...

// timer callback. Pub lishes modified message on a specified frequency
void timerCallback(const ros::TimerEvent &) {
  my_pub.publish(modified_message);
}

...

void subCallback(Message message_in){
// do some processing of the message_in and update modified_message
...
//
}

You can also check this question regarding using throttle to republish data to another topic, either at a maximum bandwidth or maximum message rate.

@Weasfas 's answer is correct for setting a particular rate of publishing and subscribing using sleep(). Accroding to your other questions you might need to use a timer because :

1. You should not do too much things within a subscriber, imagine you receive messages at a rate of 100Hz but you take 1 sec to execute the callback, you will end up loosing data or process old data that has been stored in the callback queue. You can use a global variable to store the message data and in another function you can use the global variable and publish what's needed. About the Rate, yes it will affect both subscribers and publisher (if the publisher is within the callback). Spinning at a define rate means that your callbacks are triggered at this rate (but it will be executed as many times as messages have been stored in the queue while waiting)
2. You should use the timer to bind it with the publisher function and define the rate at 100 for calling this function that will use the global variable that gets the data from the callback. So you hsould have one callback function to store the messages and another function that is called by the timer that deals with the data and then publish it.