I have noticed that the ROS Publishers seem to overwrite the sequence number set by me in the message header (std_msgs/Header). This only happens if the message is one known by ROS (such as sensor_msgs/Image) but not if it is one I defined myself.
What happens is that I set the header.seq field in e.g. the sensor_msgs/Image message to something, but during publishing, ROS overwrites this number with a counter which is apparently kept in the ros::Publisher class itself. So, when I subscribe to the message, sequence numbers of sensor_msgs/Image always start with 0 and count up by one from here, no matter what I set in the header before my call to ros::Publisher::publish().
I dug a little deeper and found that I can prevent this by using ros::AdvertiseOptions.
ros::NodeHandle n;
ros::AdvertizeOptions op = ros::AdvertiseOptions::create<sensor_msgs::Image>("/foo", 1, &connected, &disconnected, ros::VoidPtr(), NULL);
op.has_header = false;
ros::Publisher pub = n.advertise(op);
The callbacks unfortunately have to be defined, I was unable to just give NULL...
void connected(const ros::SingleSubscriberPublisher&) {}
void disconnected(const ros::SingleSubscriberPublisher&) {}
This way, my sequence numbers persist and are not overwritten by the ros::Publisher::publish() call later.
My question is, why does ROS not trust me to set the sequence number as I want it?
My test publisher:
#include
#include
#include
#include
#include
// dummy callbacks
void connected(const ros::SingleSubscriberPublisher&) {}
void disconnected(const ros::SingleSubscriberPublisher&) {}
int main(int argc, char** argv) {
srand ( time(NULL) );
ros::init(argc, argv, "test_pub");
ros::NodeHandle n;
ros::Publisher pub_messed_up = n.advertise("/foo", 1);
ros::AdvertiseOptions op = ros::AdvertiseOptions::create("/bar", 1, &connected, &disconnected, ros::VoidPtr(), NULL);
op.has_header = false;
ros::Publisher pub_ok = n.advertise(op);
ros::Rate r(1);
while(ros::ok()) {
sensor_msgs::Image msg;
msg.header.seq = rand();
msg.header.stamp = ros::Time::now();
msg.header.frame_id = "foo";
msg.height = msg.header.seq;
pub_messed_up.publish(msg);
pub_ok.publish(msg);
r.sleep();
}
}
My test subscriber:
#include
#include
void callback( const sensor_msgs::ImageConstPtr& msg ) {
ROS_INFO("BAD: image seq is: %10u, should be %10u", msg->header.seq, msg->height);
}
void callback2( const sensor_msgs::ImageConstPtr& msg ) {
ROS_INFO("OK : image seq is: %10u, should be %10u", msg->header.seq, msg->height);
}
int main(int argc, char** argv) {
ros::init(argc, argv, "test_sub");
ros::NodeHandle n;
ros::Subscriber sub = n.subscribe("/foo", 1, &callback);
ros::Subscriber sub2 = n.subscribe("/bar", 1, &callback2);
ros::spin();
}
Output:
[ INFO] [1360774481.858622442]: OK : image seq is: 180403487, should be 180403487 [ INFO] [1360774481.858737733]: BAD: image seq is: 3, should be 180403487 [ INFO] [1360774481.958441959]: OK : image seq is: 1993875195, should be 1993875195 [ INFO] [1360774481.958503343]: BAD: image seq is: 4, should be 1993875195 [ INFO] [1360774482.058392020]: OK : image seq is: 458683548, should be 458683548 [ INFO] [1360774482.058461137]: BAD: image seq is: 5, should be 458683548 [ INFO] [1360774482.158405818]: OK : image seq is: 146467386, should be 146467386 [ INFO] [1360774482.158460950]: BAD: image seq is: 6, should be 146467386 [ INFO] [1360774482.258369550]: OK : image seq is: 615549610, should be 615549610 [ INFO] [1360774482.258405823]: BAD: image seq is: 7, should be 615549610 [ INFO] [1360774482.358434563]: OK : image seq is: 598694760, should be 598694760 [ INFO] [1360774482.358496809]: BAD: image seq is: 8, should be 598694760 [ INFO] [1360774482.458443763]: OK : image seq is: 1102210696, should be 1102210696 [ INFO] [1360774482.458504035]: BAD: image seq is: 9, should be 1102210696
Originally posted by Max Pfingsthorn on ROS Answers with karma: 111 on 2013-02-13
Post score: 11