The problem about the double messages read from a rosbag file

edit

I want to write the time stamps of rosbag in a .txt document. There, I use two kinds of methods to do that:

1. Reference of C++ API link text.

2. Use the callback fuction to subscribe the topics from the rosbag played in another terminal.

But I meet a strange problem about the quantity of time stamps between two methods. The first method will output almost the double quantity of time stamps while that of the second one is right.

The rosbag I choose is from the datase EuRoC MH_easy_01.bag. It is about the indoor UAV for vision and INS (vio) SLAM. I check the inforamtion of it and there are 3682 messages for two cameras.

path:        MH_01_easy.bag
version:     2.0
duration:    3:06s (186s)
start:       Jun 25 2014 03:02:59.81 (1403636579.81)

end:         Jun 25 2014 03:06:06.70 (1403636766.70)
size:        2.5 GB
messages:    47283
compression: none [2456/2456 chunks]
types:       geometry_msgs/PointStamped [c63aecb41bfdfd6b7e1fac37c7cbe7bf]
             sensor_msgs/Image          [060021388200f6f0f447d0fcd9c64743]
             sensor_msgs/Imu            [6a62c6daae103f4ff57a132d6f95cec2]
topics:      /cam0/image_raw    3682 msgs    : sensor_msgs/Image         
             /cam1/image_raw    3682 msgs    : sensor_msgs/Image         
             /imu0             36820 msgs    : sensor_msgs/Imu           
             /leica/position    3099 msgs    : geometry_msgs/PointStamped

For the fist method, I get almost double messages (7364) and for the second one I get the same number of rosbag file. And the double number has a strange phenomenon that each message has a copy. So, it has the double number.

My environment is Ubuntu 16.05 and ROS Kinetic. Two method codes are in the following:

The first one of C++ API

#include <ros/ros.h>
#include <rosbag/bag.h>
#include <rosbag/view.h>
#include <sensor_msgs/Image.h>
#include <sensor_msgs/Imu.h>

#include <boost/foreach.hpp>

#define foreach BOOST_FOREACH

#include <string>
#include <vector>
#include <stdio.h>

int main (int argc, char ** argv)
{
    //Initiliaze the node and bag
    ros::init(argc,argv,"read");
    ros::NodeHandle n;
    rosbag::Bag bag;
bag.open("/home/cyx/dataset/MH_01_easy.bag");
// bag.open("data.bag");

ROS_INFO("Open the bag");

FILE *fpOut0, *fpOut1, *fpout2;
fpOut0=fopen("cam0_MH.txt", "w");
fpOut1=fopen("cam1_MH.txt", "w");
fpout2=fopen("imu_MH.txt", "w");

//Topics name in the rosbag
std::vector<std::string> topics;
topics.push_back(std::string("/cam0/image_raw"));
topics.push_back(std::string("/cam1/image_raw"));
topics.push_back(std::string("/imu0"));

rosbag::View view(bag, rosbag::TopicQuery(topics));

ROS_INFO("Start to read");
//read the time stamp and write them to .txt
foreach(rosbag::MessageInstance const m, view)
{
    sensor_msgs::Image::ConstPtr img0 = m.instantiate<sensor_msgs::Image>();
    sensor_msgs::Image::ConstPtr img1 = m.instantiate<sensor_msgs::Image>();
    sensor_msgs::Imu::ConstPtr imu = m.instantiate<sensor_msgs::Imu>();

    if(img0!=NULL)
    {
        fprintf(fpOut0, "%.9lf\n", img0->header.stamp.toSec());
        // printf("t0 = %.9lf\n", img0->header.stamp.toSec());
    }

    if(img1!=NULL)
    {
        fprintf(fpOut1, "%.9lf\n", img1->header.stamp.toSec());
        // printf("t1 = %.9lf\n", img1->header.stamp.toSec());
    }

    if(imu!=NULL)
    {
        fprintf(fpout2, "%.9lf\n", imu->header.stamp.toSec());
    }

}

bag.close();

return 1;

}

And the second one of callback function

#include <ros/ros.h>
#include <rosbag/bag.h>
#include <sensor_msgs/Imu.h>
#include <sensor_msgs/Image.h>

#include <iostream>
#include <stdio.h>

FILE *fp0, *fp1, *fp2;

void ...