Why does ROS overwrite my sequence number?

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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 <stdio.h>
#include <stdlib.h>
#include <time.h>

#include <ros ros.h="">
#include <sensor_msgs image.h="">

// 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<sensor_msgs::image>("/foo", 1);

    ros::AdvertiseOptions op = ros::AdvertiseOptions::create<sensor_msgs::image>("/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 <ros ros.h="">
#include <sensor_msgs image.h="">

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 ...