serial available() returns false

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Hi, I'm trying using serial to send information to a serial port.

The following is my cpp code. The result shows that ros_ser.isOpen()==true, and ros_ser.available()==false, where ros_ser is an object of serial::Serial. I've attached my device to a USB port and found the file ttyUSB0 in folder /dev.

Thanks for any help in advance.

#include "ros/ros.h"
#include <serial/serial.h>
#include <std_msgs/String.h>
#include <std_msgs/Empty.h>
#include <string>
#include <sstream>

serial::Serial ros_ser;

int main(int argc, char** argv){
 ros::init(argc, argv, "my_serial_node");
 ros::NodeHandle n;
 try
 {
     ros_ser.setPort("/dev/ttyUSB0");
     ros_ser.setBaudrate(115200);
     serial::Timeout to = serial::Timeout::simpleTimeout(1000);
     ros_ser.setTimeout(to);
     ros_ser.open();
 }
 catch (serial::IOException& e)
 {
     ROS_ERROR_STREAM("Unable to open port ");
     return -1;
 }

 if(ros_ser.isOpen()){
     ROS_INFO_STREAM("Serial Port opened");
 }else{
     return -1;
 }

 std::stringstream str;

 ros::Rate loop_rate(10);
 while(ros::ok()){
     ros::spinOnce();
     if(ros_ser.available()){
         ROS_INFO_STREAM("Sending Information...");
         str<<"myInformation"<<std::endl;
         ros_ser.write(str.str());
     }
     else ROS_INFO_STREAM("ros_ser not available");
     loop_rate.sleep();
 }
}