How to be 100% sure that only one callback is being executed at a time?

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Hi @schizzz8,

I have created a simple publisher at 30hz and a subscriber that simulates a process that takes 0.2 seconds (works at 5hz).

This script demonstrates that you can receive an image and do not have its value updated while you are processing it.

I'm using two variables to control the flow: new_msg and processing. I think it can help you.

It is written in python, but you can use the same logic (basically because it does not rely on ROS Python topic queue, which could be or not different of ROScpp).

Publisher (Simulating your camera publisher)

#!/usr/bin/env python

import rospy
from std_msgs.msg import Int32

def publisher():

    pub = rospy.Publisher('topic_name', Int32, queue_size=10)
    rospy.init_node('publisher')
    r = rospy.Rate(30) # 30hz

    count = 1
    msg = Int32()
    while not rospy.is_shutdown():
        msg.data = count
        count = count + 1 if count < 30 else 1
        pub.publish(msg)
        rospy.loginfo(msg)
        r.sleep()

if __name__ == "__main__":
    publisher()

Subscriber (Simulating your script that processes a given image)

#!/usr/bin/env python

import rospy
from std_msgs.msg import Int32

processing = False
new_msg = False

msg = None
def callback(data):
    global processing, new_msg, msg
    if not processing:
        new_msg = True
        msg = data

def listener():
    global processing, new_msg, msg
    rospy.init_node('subscriber')
    rospy.Subscriber("topic_name", Int32, callback)
    # just to use the sleep method
    r = rospy.Rate(5)
    while not rospy.is_shutdown():
        if new_msg:
            # set processing to True
            new_msg = False
            processing = True
            # simulate a process that takes 0.2 seconds
            rospy.loginfo(msg)
            r.sleep()
            # uncomment below to see the same value being "processed"
            # rospy.loginfo(msg)
            # r.sleep()
            # set processing to False
            processing = False

if __name__ == "__main__":
    listener()

I hope it can help you. You can see it being executed at this video (https://youtu.be/XoirEyK4J2A)

Cheers

Hi! Try something like this:

...
ros::Rate rate(<yourFrequency>);
...
while (ros::ok())
{
ros::spinOnce();
rate.sleep();
}

Also, be sure to set the right buffer length for your subscriber, in order to prevent your node to elaborate old data.